Secondary Metabolites from Marine-Derived Bacteria with Antibiotic and Antibiofilm Activities against Drug-Resistant Pathogens

The search for new antibiotics against drug-resistant microbes has been expanded to marine bacteria. Marine bacteria have been proven to be a prolific source of a myriad of novel compounds with potential biological activities. Therefore, this review highlights novel and bioactive compounds from marine bacteria reported during the period of January 2016 to December 2021. Published articles containing novel marine bacterial secondary metabolites that are active against drug-resistant pathogens were collected. Previously described compounds (prior to January 2016) are not included in this review. Unreported compounds during this period that exhibited activity against pathogenic microbes were discussed and compared in order to find the cue of the structure–bioactivity relationship. The results showed that Streptomyces are the most studied bacteria with undescribed bioactive compounds, followed by other genera in the Actinobacteria. We have categorized the structures of the compounds in the present review into four groups, based on their biosynthetic origins, as polyketide derivatives, amino acid derivatives, terpenoids, as well as compounds with mixed origin. These compounds were active against one or more drug-resistant pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus epidermidis (MRSE), vancomycin-resistant Enterococci (VRE), multidrug-resistant Mycobacterium tuberculosis (MDR-TB), and amphotericin B-resistant Candida albicans. In addition, some of the compounds also showed activity against biofilm formation of the test bacteria. Some previously undescribed compounds, isolated from marine-derived bacteria during this period, could have a good potential as lead compounds for the development of drug candidates to overcome multidrug-resistant pathogens.


Introduction
One of the most significant global concerns regarding health issues is the emergence and rapid spread of drug-resistant pathogens that have acquired new resistance mechanisms, leading to antimicrobial resistance [1]. Currently, the pan-or multidrug-resistant (MDR) microbes are responsible for about 40-60% of the infection cases in many low-to middle-income countries, such as Indonesia, Brazil, and Russia [2]. It is predicted that antibiotic resistance in these countries will rise four to seven times faster than in other Two undescribed angucycline-type glycosides, namely stremycins A (4) and B (5) (Figure 1), were isolated from the culture extract of Streptomyces pratensis strain NA-ZhouS1, which was obtained from a marine sediment in Zhoushan, East China Sea [13]. The structures of both compounds were determined using high-resolution time-of-flight mass spectrometry (HR-TOF-MS), 1D, and 2D NMR techniques. Compounds 4 and 5 have the same glycone moiety, comprising β-olivose-(4→1)-β-olivomycose-(3→1)-4-O-carbamoyl-β-amicetose, but slightly differ only in the tetracyclic benz[α]anthracene skeleton with the hydroxyl group on C-4 in 5. Both compounds were isolated from 25 L of a metalstress medium culture (liquid gauze's medium containing 20 g of soluble starch, 1 g of KNO3, 0.5 g of K2HPO4, 0.5 g of MgSO4·7H2O, 0.01 g of FeSO4·7H2O, 35 g of sea salt per liter at pH 7.4 and supplemented with 100 μM of NiCl2·6H2O) and incubated at 28 °C for 10 days. Nickel was selected due to its capacity to activate cryptic gene clusters, which induce the production of bioactive metabolites that were not detected in the medium without metal. Both compounds showed antibacterial activity with a MIC value of 16 μg/mL against P. aeruginosa, methicillin-resistant S. aureus (MRSA), K. pneumoniae, and E. coli. Two undescribed angucycline-type glycosides, namely stremycins A (4) and B (5) (Figure 1), were isolated from the culture extract of Streptomyces pratensis strain NA-ZhouS1, which was obtained from a marine sediment in Zhoushan, East China Sea [13]. The structures of both compounds were determined using high-resolution time-of-flight mass spectrometry (HR-TOF-MS), 1D, and 2D NMR techniques. Compounds 4 and 5 have the same glycone moiety, comprising β-olivose-(4→1)-β-olivomycose-(3→1)-4-O-carbamoylβ-amicetose, but slightly differ only in the tetracyclic benz[α]anthracene skeleton with the hydroxyl group on C-4 in 5. Both compounds were isolated from 25 L of a metal-stress medium culture (liquid gauze's medium containing 20 g of soluble starch, 1 g of KNO 3 , Mar. Drugs 2023, 21, 50 4 of 26 0.5 g of K 2 HPO 4 , 0.5 g of MgSO 4 ·7H 2 O, 0.01 g of FeSO 4 ·7H 2 O, 35 g of sea salt per liter at pH 7.4 and supplemented with 100 µM of NiCl 2 ·6H 2 O) and incubated at 28 • C for 10 days. Nickel was selected due to its capacity to activate cryptic gene clusters, which induce the production of bioactive metabolites that were not detected in the medium without metal. Both compounds showed antibacterial activity with a MIC value of 16 µg/mL against P. aeruginosa, methicillin-resistant S. aureus (MRSA), K. pneumoniae, and E. coli. When tested against B. subtilis, both compounds showed MIC values of 8-16 µg/mL. Therefore, the variation in the aglycone part does not significantly alter antibacterial bioactivities.
The marine sediment-derived actinomycete, Streptomyces sp. 182SMLY, yielded polycyclic anthraquinone compounds, namely N-acetyl-N-demethylmayamycin (6) and streptoanthraquinone A (7) (Figure 1). Compounds 6 and 7 were isolated from the fermentation of the bacterium at 28 • C for 7 days in the Gause's liquid medium (20 g/L starch, 1.0 g/L KNO 3 , 0.5 g/L K 2 HPO 4 , 0.5 g/L MgSO 4 ·7H 2 O, 0.5 g/L NaCl, and 0.01 g/L FeSO 4 ·7H 2 O). The structure of 6 was established using 1D and 2D NMR, HRESIMS techniques. The relative configurations of the stereogenic carbons of the amino sugar moiety were determined by NOESY correlations, while their absolute configurations were established by comparison of the experimental and calculated electronic circular dichroism (ECD) spectra. The antibiotic activities of 6 and 7 were tested against methicillin-resistant S. aureus ATCC 43300 and E. coli ATCC 25922, however, 7 was not active. Compound 6 showed inhibition activity only against MRSA with a MIC value of 20.0 µM. The positive control, norfloxacin, exhibited MIC values of 62.6 µM against both E. coli and MRSA [14].
Two unreported resistoflavin derivatives, chlororesistoflavins A (8) and B (9) (Figure 1), were obtained from the culture extract of Streptomyces sp. strain EG32, isolated from a sediment collected from the northern coast of Egypt (Mediterranean Sea) and cultured in the Waksman liquid medium (20.0 g glucose, 5.0 g peptone, 5.0 g beef extract, 3.0 g yeast extract, 3.0 g CaCO 3 , 5.0 g NaCl, 50% seawater, 50% DI H 2 O, 10 L in total) at 27 • C for 7 days. The structures of both compounds were established by extensive 1D and 2D NMR and HR-ESI-TOF-MS spectral analysis. Intriguingly, even though 8, 9, and resistoflavin, all possessing only one stereogenic carbon (C-11b), displayed the same sign of optical rotation (levorotatory) and shared the same biosynthetic origin, the ECD spectrum of 8 exhibited a strong Cotton effect near the n-π* transition of the carbonyl at its C-10, which is antipodal to the ECD spectra of resistoflavin and 9. The authors hypothesized that a chlorine atom imposes severe allylic-1,3 strain in 8, distorting the ring from the ideal conformation found in 9 and resistoflavin, thus altering the first-sphere contributions to the Cotton effect. By using molecular mechanics (MM) and density functional theory (DFT) calculations of both 8 and 9, they have found that 8 contained a cyclohexenone ring that has a severely distorted conformation from that of 9 and resistoflavin by steric influence of the bulky, electron-rich chlorine atom on C-11 and the carbonyl group at C-10. The conformational ring bending of the cyclohexenone in 8, when compared to 9, is interpreted as forcing the disposition of the heavy chlorine substituent from a negative-contributing quadrant to a positive-contributing quadrant. Therefore, the authors concluded that the absolute configuration of C-11b in 8 is the same as that of C-11b in 9 and resistoflavin, i.e., R configuration.
Compound 8 showed comparable inhibitory activity to resistoflavin (MIC = 0.25 µg/mL) but eight times stronger than 9 (MIC = 2.0 µg/mL) against MRSA. The positive control, ciprofloxacin, showed a MIC value of 0.2 µg/mL against MRSA. Interestingly, the position of the chlorine substituent on the resistoflavin scaffold was found to affect the activity of its analogs. For instance, when the chlorine atom is on C-11, as in 8, its activity is comparable to that of the parent compound, i.e., resistoflavin. However, when the chlorine atom is on C-4, as in 9, its activity is reduced when compared to resistoflavin [15].
The liquid culture of Nonomuraea sp. strain MM565M-173N2, isolated from a deep-sea sediment collected in Japan trench at a depth of 329 m off the Sanriku coast, furnished sealutomicins A-D (10-13) (Figure 1). Compounds 10-13 were obtained from 220 L of the fermented production medium containing 0.75% glycerin, 0.75% cotton seed meal, 0.25% L-glutamate, 0.15% NaCl, and 1.8% Daigo's Artificial Seawater SP (pH value 7.4 before sterilization). The fermentation was performed at 27 • C for 7 days on a rotary shaker (180 rpm). The structures of 10-13 were established by extensive analysis of their 1D and 2D NMR and HRESIMS spectra. In the case of 10, the nuclear Overhauser effect (NOE) data revealed the relative configurations as 16S*, 17R*, 24S*, 25S*. Since its circular dichroism (CD) spectrum is nearly identical to that of a related compound, dynemicin A, its absolute stereochemistry was established as 16S, 17R, 24S, 25S. However, the absolute configuration of C-28 of the side chain could not be determined. The relative configurations of the stereogenic carbons of 11-13 were established by NOE data, whereas their absolute configurations were determined by comparison of their CD spectra with that of 10. Since 11-13 were products from the Bergman cyclization of 10, which retain the stereochemistry at C-16, C-24, and C-25, the absolute structures of 11-13 were determined.
Antibacterial activity-guided isolation of a marine sediment-derived bacterium, Streptomyces sp. MBTI36, yielded an undescribed glycosylated 6,8,9-trihydroxy-3,4dihydroanthracen-1 (2H)-one derivative, chromomycin A 9 (14) (Figure 1). Analysis of the 16S rRNA of the bacterium showed 99.9% sequence similarity to Streptomyces microflavus NBRC13062. Compound 14 was isolated from the fermentation of the bacterium at 28 • C for 14 days without shaking in a total of 20 L of the liquid GTYB medium (each liter of the medium contains 10 g of glucose, 2 g of tryptone, 1 g of yeast extract, and 1 g of beef extract in 1 L of artificial seawater). The structure of 14 was established based on 1D and 2D NMR and HRESIMS spectral analysis as well as a comparison of its 1 H and 13 C NMR data with the previously reported chromomycins Ap, A 2 , and A 3 .
S. aureus ATCC43300 was selected to evaluate its resistance development against ciprofloxacin and 14. A steady increase in MIC was observed for ciprofloxacin during the passage experiment in S. aureus ATCC43300, with a 128-fold change (MIC = 32 µg/mL) when compared with the initial MIC value (MIC = 0.25 µg/mL). On the contrary, a 2-fold increase in the MIC (from 0.13 to 0.25 µg/mL) was observed for S. aureus ATCC43300 after 21 passages. Therefore, during the passage experiment, MIC values for 14 did not increase more than 4-fold from its initial MIC value, confirming that there was no resistance development during the 21 passages [17].

Naphthoquinones
The undescribed napyradiomycins, i.e., napyradiomycin B7a (15), napyradiomycin B7b (16), and napyradiomycin D1 (17) (Figure 2), were isolated from the EtOAc extract of the culture of Streptomyces sp. strain CA-271078 (which showed 99.34% similarity with Streptomyces aculeolatus NBRC 14824(T)), fermented at 28 • C in 3 L of the R358 medium for 6 days. The structures of 15, 16, and 17 were established by the interpretation of HR-ESI-TOF-MS, 1D, and 2D NMR spectra. The relative stereochemistry of both compounds was determined based on NOESY and ROESY correlations of the key protons as well as the values of coupling constants of some protons. The absolute configurations of the stereogenic carbons in 15 and 16 were assumed to be the same as those of all the previously reported napyradiomycins in the B series on the basis of their common biosynthetic origin as well as the same signs of their specific rotations. The structure of napyradiomycins can be recognized by the semi-naphthoquinone chromophore, a prenyl unit attached to C-4a, which is cyclized to form a tetrahydropyran ring in most cases, and a monoterpenoid subunit attached to C-10a. Compounds 15 and 17 displayed moderate inhibitory activity against MRSA, with MIC values of 48 and 12-24 μg/mL, respectively. In addition, both compounds also inhibited the growth of Mycobacterium tuberculosis H37Ra, with MIC values of 12-24 and 24-48 μg/mL, respectively, while 16 exhibited a MIC value > 64 μg/mL against both MRSA and M. tuberculosis. However, the assay did not include any positive control in the experimental design. The results suggested that the absolute configurations at C-3 of 15 and 16 have a significant influence on the antibacterial activity [18].
Bioassay-guided fractionation of the EtOAc extracts of solid agar cultures of Micromonospora sp. RJA4480, isolated from a marine sediment obtained at −85 m in Barkley Sound, British Columbia, that exhibited potent in vitro inhibition of MRSA and E. coli, led to the identification of four undescribed macrolides containing a naphthoquinone core, 3amino-27-demethoxy-27-hydroxyrifamycin S (18), 3-aminorifamycin S (19), sporalactam A (20), and sporalactam B (21) (Figure 2) as the antibacterial componentes. Although no 3-amino ansa macrolide has been reported as a natural product, 19 has been previously obtained by semisynthesis [19].
The structure of 18 was established by interpretation of HR-ESI-TOF-MS, 1D, and 2D NMR spectral data. Curiously, the proton and carbon signals in the 1 H and 13 C NMR spectra of 18, recorded in deuterated dimethyl sulfoxide (DMSO-d6), were doubled. The single-crystal X-ray diffraction analysis of 18 did not only determine its absolute structure as 12S, 20S, 21S, 22R, 23R, 24R, 25S, 26S, 27S but also revealed that 18 existed in two conformations, thus causing doubled signals in the 1 H and 13 C NMR spectra. The structures of 20 and 21 were established by HRESIMS data and extensive analysis of their 1D and 2D NMR spectra. Interestingly, unlike the 1 H and 13 C NMR spectra of 18, the signals in the 1 H The structures of 15, 16, and 17 were established by the interpretation of HR-ESI-TOF-MS, 1D, and 2D NMR spectra. The relative stereochemistry of both compounds was determined based on NOESY and ROESY correlations of the key protons as well as the values of coupling constants of some protons. The absolute configurations of the stereogenic carbons in 15 and 16 were assumed to be the same as those of all the previously reported napyradiomycins in the B series on the basis of their common biosynthetic origin as well as the same signs of their specific rotations. The structure of napyradiomycins can be recognized by the semi-naphthoquinone chromophore, a prenyl unit attached to C-4a, which is cyclized to form a tetrahydropyran ring in most cases, and a monoterpenoid subunit attached to C-10a. Compounds 15 and 17 displayed moderate inhibitory activity against MRSA, with MIC values of 48 and 12-24 µg/mL, respectively. In addition, both compounds also inhibited the growth of Mycobacterium tuberculosis H37Ra, with MIC values of 12-24 and 24-48 µg/mL, respectively, while 16 exhibited a MIC value > 64 µg/mL against both MRSA and M. tuberculosis. However, the assay did not include any positive control in the experimental design. The results suggested that the absolute configurations at C-3 of 15 and 16 have a significant influence on the antibacterial activity [18].
Bioassay-guided fractionation of the EtOAc extracts of solid agar cultures of Micromonospora sp. RJA4480, isolated from a marine sediment obtained at −85 m in Barkley Sound, British Columbia, that exhibited potent in vitro inhibition of MRSA and E. coli, led to the identification of four undescribed macrolides containing a naphthoquinone core, 3-amino-27-demethoxy-27-hydroxyrifamycin S (18), 3-aminorifamycin S (19), sporalactam A (20), and sporalactam B (21) (Figure 2) as the antibacterial componentes. Although no 3-amino ansa macrolide has been reported as a natural product, 19 has been previously obtained by semisynthesis [19].
The structure of 18 was established by interpretation of HR-ESI-TOF-MS, 1D, and 2D NMR spectral data. Curiously, the proton and carbon signals in the 1 H and 13 C NMR spectra of 18, recorded in deuterated dimethyl sulfoxide (DMSO-d6), were doubled. The singlecrystal X-ray diffraction analysis of 18 did not only determine its absolute structure as 12S, 20S, 21S, 22R, 23R, 24R, 25S, 26S, 27S but also revealed that 18 existed in two conformations, thus causing doubled signals in the 1 H and 13 C NMR spectra. The structures of 20 and 21 were established by HRESIMS data and extensive analysis of their 1D and 2D NMR spectra. Interestingly, unlike the 1 H and 13 C NMR spectra of 18, the signals in the 1 H and 13  with those of the reference compounds, 22 and 23, that lack only the amino substituent on C-3, it was found that the 3-amino substituent significantly enhances the potency of these antibiotics against MRSA, E. coli, and M. tuberculosis [20].
The previously reported 7,8-dideoxygriseorhodin C (24) (Figure 2) was obtained from the EtOAc extract of Streptomyces sp. strain 1425S.R.1a.1, isolated from a body tissue homogenate of a gastropod mollusk, Truncatella guerinii, collected in Cebu, Philippines, and was cultured in R2A broth (0.2% yeast extract, 1% malt extract, 0.2% glucose, and supplemented with 2% NaCl). Although the 13 C NMR spectrum of 24 was similar to that of 7,8-dideoxygriseorhodin C in the previous report, in this work, the authors have unambiguously assigned all the 1 H and 13 C NMR chemical shift values of 24 by 1D and 2D NMR spectral analysis. Since the absolute configuration at C-6 and C-6a of 7,8dideoxygriseorhodin C had not been established in the previous report, Miller et al. [21] have attempted to determine the absolute configurations of these stereogenic carbons in 24 by comparison of the calculated and experimental ECD spectra. However, this method could only determine the absolute configuration of C-6 as 6S with certainty, while the absolute configuration of C-6a was ambiguous. Finally, a combination of the 3D modeling and the correlations observed in the ROESY spectrum have allowed the authors to establish the absolute configuration of C-6a as 6aS. Compound 24 inhibited the growth of S. aureus ATCC ® 43300™ MRSA, which is resistant to oxacillin and methicillin, with a MIC value of 0.08-0.12 µg/mL. The positive control, oxacillin, showed MIC values of 1.59-6.24 µg/mL. Treatment of ATCC ® 43300™ MRSA strain with a combination of 24 and oxacillin at 1xMIC showed the reduction of the individual MICs (MIC of 24 = 0.01-0.02 µg/mL; MIC of oxacillin = 0.02-0.298 µg/mL). Moreover, the combination index (CI) for the combination of 24 and oxacillin at 1xMIC was 0.12-0.24, indicating a synergistic effect between 24 and oxacillin [21].
Mersaquinone (25) (Figure 2), an unreported tetracene derivative, was isolated from the extract of a marine-derived Streptomyces sp. EG1, obtained from a sediment sample that was collected from the North Coast of the Mediterranean Sea, Egypt, and cultured in the Waksman medium at 28 • C for 7 days. The structure of 25 was established based on an extensive analysis of HRESIMS, IR, 1D, and 2D NMR spectra. Compound 25 inhibited the growth of the methicillin-resistant S. aureus (MRSA) strain TCH1516, with a MIC value of 3.36 µg/mL. The positive control, ciprofloxacin, showed a MIC value of 0.93 µM [22].

Macrolides
An undescribed anthracimycin congener, anthracimycin B (26) (Figure 3), was obtained together with the previously reported anthracimycin (27), from the EtOAc extract of the solid culture (R5A agar) of a marine-derived Streptomyces cyaneofuscatus M-169, which was isolated from a gorgonian coral (Order Gorgonacea) collected at 1500 m depth in the Avilés submarine Canyon, Cantabrian Sea [23]. The structure of 26 was determined by ESI-TOF-MS and 1D and 2D NMR spectral analysis as well as by comparison of its NMR data with those of 27 [24]. Since the optical rotation of 26 has the same sign (levorotatory) and similar magnitude of rotation to those of 27 whose absolute structure was established by X-ray diffraction by Jang et al. [24], the authors concluded that absolute configurations of the stereogenic carbons in 26 were the same as those of 27. Mar. Drugs 2023, 21, x FOR PEER REVIEW 9 of 28 tatory) and similar magnitude of rotation to those of 27 whose absolute structure was established by X-ray diffraction by Jang et al. [24], the authors concluded that absolute configurations of the stereogenic carbons in 26 were the same as those of 27.  Two unreported macrocyclic polyketides containing dodecahydropyranotrioxacyclooctadecine dione (28) and trioxo-octadecahydro-1H-benzo[o]tetraoxacyclopentacosine carboxylate (29) (Figure 3) were isolated from the EtOAc extract of the agar nutrient culture of a heterotrophic Gamma-proteobacterium Shewanella algae MTCC 12715, associated with an intertidal red alga, Hypnea valentiae, which was collected from the Gulf of Mannar region in the southeast coast of India. The structures of 28 and 29 were established based on extensive analysis of HRMS, 1D, and 2D NMR spectra. The relative configurations of the stereogenic carbons of the macrocyclic rings were established based on NOESY correlations and supported with molecular mechanistic studies by MM2 force-field calculations by Chem3D Pro (ver-12.0).
Compounds 28 and 29 displayed potential antibacterial activity against methicillinresistant S. aureus MRSA (≥20 mm) and vancomycin-resistant E. faecalis VREfs (≥25 mm, 30 µg on disc), whereas the standard antibiotic disc of streptomycin (30 µg) displayed a smaller clearance zone around the disc. In the same manner, 28 and 29 exhibited greater activity (MIC of 3-5 µg/mL) against drug-resistant pathogenic bacteria causing nosocomial infections than chloramphenical (≥6.25 µg/mL) in the microdilution method. The structural attributions and the correlations with bioactivity through various physico-chemical parameters of 28 and 29, adopted from ChemDraw Ultra ver-12.0 /ACD Chemsketch ver-8.0 databases, revealed that the optimum logarithmic value of the octanol-water coefficient (log Pow), in conjunction with greater electronic parameters and less steric bulkiness of 28 and 29, could be fundamental for bioactivity. Moreover, in silico docking studies of 28 and 29 exhibited significant inhibition at the allosteric site of the penicillin-binding protein (PBP2a) in S. aureus, which supports their in vitro activity profile [25].
Two undescribed bafilomycins, 21,22-en-bafilomycin D (30) and 21,22-en-9hydroxybafilomycin D (31) (Figure 3) were obtained from a methanol extract of a GYM solid culture of Streptomyces sp. HZP-2216E, isolated from a fresh seaweed, Ulva pertusa, which was collected from the Turtle Islet in the South China Sea close to Shanwei City (Guangdong), China. The structure of the compounds was established by extensive analysis of HRMS and 1D and 2D NMR spectra [26]. The geometries of the double bonds were established from the NOE information, while the relative configurations of the stereogenic carbons were determined by NOE information as well as a close similarity of the carbon chemical shift values in the 13 C NMR spectra of the macrocyclic portion of 30 and bafilomycin D (32) [27]. On the other hand, the hydroxyl group on C-23 was deduced as βoriented since 30 could be prepared from bafilomycin D (32) via double bond migration in DMSO solution. In the case of 31, the α-orientation of the hydroxyl group on C-9 was based on a large coupling constant (J = 10.3 Hz) between H-8 and H-9. Compounds 30 and 31 showed growth inhibitory activity against MRSA with MIC values of 12.5 µg/mL for both compounds, while gentamicin, a positive control, showed inhibition with a MIC value of 0.36 µg/mL [26]. Compound 32 was also isolated, together with the undescribed 23-Obutyrylbafilomycin D (33), from the same bacterium cultured in a 2216E solid medium. give four niphimycin congeners, niphimycins C−E (34)(35)(36), and 17-O-methylniphimycin (37) as well as 38 ( Figure 3). The planar structures of 34-36 were established by HRESIMS, 1D, and 2D NMR spectral analysis. By using J-based configurational analysis, the relative configurations of C-2/C-3, C-6/C-7, and C-28/C-29 were established as anti, anti, and anti, respectively. On the other hand, the relative configurations of C-7/C-9/C-10/C-11 were elucidated as syn/anti/syn by application of Kishi's universal NMR database, while the relative configurations of C-17−C-21 of the pyran ring were determined by 1 H-1 H coupling constants and ROESY correlations. Additionally, the full absolute configurations were predicted from the ketoreductase (KR) domain analysis of the npm gene cluster. It is worth mentioning that the authors have observed that during the isolation process, the previously described 38 [28], also isolated from the extract, could transform to 37 in the MeOH solution in the presence of a trace of acetic acid, suggesting that 37 was an artifact arising from ketalization of 38 with MeOH during the isolation process [29].
Compounds 34-37 were tested against representative members of ESKAPE pathogens. Compounds 34 and 38, with only one malonyl group, showed good antimicrobial activity against methicillin-resistant S. epidermidis (MRSE) and S. aureus (MRSA), with MIC values of 4-32 µg/mL, but modest activity against vancomycin-resistant E. faecalis and E. faecium (VRE), with a MIC value of 64 µg/mL. The similar potency of 34 and 38 suggested that the position of the malonyl substituent is not crucial for antibacterial activity. On the contrary, antibacterial activity of 35 and 36, which contain two malonyl groups, was two-fold less than that of 34 and 38, suggesting that more malonyl groups in the compounds cause a reduction of antibacterial activity. Moreover, 37 was as active as 34 and 38, indicating that antibacterial activity was not significantly affected by the ketalization of the hemiketal ring [29].
The extract of the marine actinomycete, Streptomyces althioticus MSM3, isolated from samples of the seaweed Ulva sp., which was collected in the Cantabrian Sea (Northeast Atlantic Ocean) and cultured in R5A liquid medium at 28 • C at 250 rpm for 6 days, furnished an undescribed desertomycin G (39) (Figure 3), a glycosylated macrolide containing a primary amine group. The planar structure of 39 was established by extensive analysis of HRMS, 1D, and 2D NMR spectra, as well as by comparison of its NMR data with those of the previously described desertomycin A [29]. Compound 39 exhibited strong inhibitory activity against various Gram-positive multidrug-resistant clinical pathogens, such as M. tuberculosis, S. Aureus, S. Pneumoniae, S. Pyogenes, Clostridium perfringens, C. Urealyticum, E. Faecalis, E. Faecium, and moderate antibiotic activity against relevant Gram-negative clinical pathogens, such as Bacteroides fragilis, Haemophilus influenzae, and Neisseria meningitidis [30].

Spirotetronates
Spirotetronates are a class of compounds consisting of tetronic acid spiro linked with cyclohexane or cyclohexene moiety and can be divided into two sub-classes: Sub-class I, which consists of spirotetronates connected with a macrocycle, and sub-class II, which contains spirotetronates and macrocycle with an integration of decalin and sometimes oligosaccharide chains [31]. The biosynthesis of spirotetronates of both sub-classes starts with the attachment of acetyl and/or propanoyl CoA to the acyl carrier protein (ACP) to form a chain elongation in type I PKS (PKS I) ( Figure 4). In sub-class I, the elongation of the chain is stopped and condensed by a glyceryl-ACP unit after reaching the appropriate length to form tetronic acid (i) then undergoes acetylation-elimination to form a butanolide (ii). Intramolecular Diels-Alder (IMDA) reaction connects butanolide with a diene portion. The biosynthesis of sub-class II is quite similar to that of sub-class I, but the elongation of the chain in sub-class II incorporates decalin. In addition, glycosylation adds sugar moiety to the structure. the chain is stopped and condensed by a glyceryl-ACP unit after reaching the appropriate length to form tetronic acid (i) then undergoes acetylation-elimination to form a butanolide (ii). Intramolecular Diels-Alder (IMDA) reaction connects butanolide with a diene portion. The biosynthesis of sub-class II is quite similar to that of sub-class I, but the elongation of the chain in sub-class II incorporates decalin. In addition, glycosylation adds sugar moiety to the structure.  The results indicated that the type of sugar moieties on C-9 was important for their bioactivities. Nitro sugars showed superior anti-MRSA compared to their amino sugar counterparts. The addition of sugar and C-17, as well as a change of C-23 α,β-unsaturated carbonyl moiety with a formyl group (CHO), have no effect on the anti-MRSA activity [33].
Abyssomicins are a family of antimicrobial compounds which are generally composed of C19 spirotetronates and often contain a four-or five-membered ring system within their architectures. A majority of abyssomicin analogs exist as monomers, and the only known dimer was abyssomicin J. In order to reanalyze the HPLC dataset of the metabolic profile of Streptomyces koyangensis SCSIO 5802, isolated from a sediment collected in the Northern South China Sea, Huang et al. [34] performed a large-scale (60 L) fermentation of the strain SCSIO 5802. Extraction and HPLC separation resulted in the isolation of monomeric abyssomicin congeners with undescribed frameworks (neoabyssomicins) and two undescribed abyssomicin dimers, named neoabyssomicins F (50) and G (51) (Figure 6). The structures of both compounds were established by HRMS, 1D, and 2D NMR spectral analysis. Key HMBC correlations indicated that the two monomers were connected via a C-9−S−C-9 thioether bond to form a dimer. In the case of 50, X-ray crystallographic analysis Seven undescribed spirotetronate glycosides, microsporanates A-F (43)(44)(45)(46)(47)(48), and tetrocarcon P (49) ( Figure 5) were isolated from the acetone extract of the mycelium and of the culture broth absorbed on the XAD-16 resin of Micromonospora harpali SCSIO GJ089, isolated from a sediment sample obtained from the northern South China Sea, and cultured in N4 medium (soluble starch 15 g/L, fish peptone 8 g/L, bacterial peptone 5 g/L, monomers were connected via a C-9−S−C-9′ thioether bond to form a dimer. In the case of 50, X-ray crystallographic analysis confirmed the structure and determined the absolute configurations for all stereogenic carbons. Both 50 and 51 showed growth inhibitory activity against three clinical isolates of MRSA (MRSA-shhs A1, MRSA-699, and MRSA-1862) with the same MIC value of 16 μg/mL. Ampicillin (MIC values of 64, 4, 0.125 μg/mL) and vancomycin (MIC values of 0.5, 0.125, and 0.125 μg/mL) were used as positive controls [34].

Amino acid Derivatives
A number of secondary metabolites produced by bacteria are originated from amino acids. These metabolites can be classified into several structural groups as follows.

Simple Amino Acid Derivatives
p-Terphenyls originate from a condensation of two precursors, which are derived from L-Phe/L-Tyr and L-Trp, which is catalyzed by a tridomain nonribosomal peptide synthetase to yield polyporic acid (PPA). Subsequent reduction and dehydration of PPA give terphenyl triol, a p-terphenyl scaffold [35].
By using the MS/MS-based Global Natural Products Social (GNPS) molecular networking analysis, two antibacterial p-terphenyl derivatives, nocarterphenyls D (52)

Amino Acid Derivatives
A number of secondary metabolites produced by bacteria are originated from amino acids. These metabolites can be classified into several structural groups as follows.

Simple Amino Acid Derivatives
p-Terphenyls originate from a condensation of two precursors, which are derived from L-Phe/L-Tyr and L-Trp, which is catalyzed by a tridomain nonribosomal peptide synthetase to yield polyporic acid (PPA). Subsequent reduction and dehydration of PPA give terphenyl triol, a p-terphenyl scaffold [35].
By using the MS/MS-based Global Natural Products Social (GNPS) molecular networking analysis, two antibacterial p-terphenyl derivatives, nocarterphenyls D (52) and E (53) ( Figure 6) were isolated from the EtOAc extract of a liquid culture (1% soluble starch, 1% glucose, 0.5% peptone, 0.2% yeast extract powder, 1% glycerinum, and 0.25% corn flour, pH = 7.0, in seawater) of the marine-derived actinomycete, Nocardiopsis sp. HDN154086, isolated from a sediment sample that was collected from the South China Sea. The structure of 52 was elucidated by 1D and 2D NMR and HRMS spectral analysis and was confirmed by single X-ray diffraction analysis using MoKα radiation. The structure of 52 consists of a p-terphenyl scaffold with the central benzene ring fused with a 2,2 -bithiazole moiety. On the other hand, the structure of 53 consists of a p-terphenylquinone with a methyl proprionate linked to a quinone ring through a sulfur atom [36].

Linear Peptides
Linear peptides can be formed by a direct condensation of two or more amino acid residues to form amide linkages or can be modified to form pyrazinone or diketopiperazine ring.
An undescribed lipopeptide, nesfactin (54) (Figure 7), was obtained from the culture extract of an actinomycete, Nesterenkonia sp. MSA31, isolated from the marine sponge Fasciospongia cavernosa, which was collected from the southwest coast of India [37]. The struc-ture of 54 was elucidated by interpretation of the fragmentation patterns from the liquid chromatography-mass spectrometry (LC-MS/MS). Compound 54 was found to inhibit virulence phenotypes, including the production of hemolysin, protease, lipase, phospholipase, esterase, elastase, rhamnolipid, alginate, and pyocyanin, as well as motility of P. aeruginosa strain FSPA02 which is resistant to cefipime, ceftazidime, aztreonam, gentamycin, ampicillin, kanamycin, vancomycin, tetracycline, and streptomycin. By using high-performance thin layer chromatography (HP-TLC) and the reporter assay using CV026.G, it was found that 54 also inhibited the quorum sensing molecule, N-acyl-homoserine lactones (AHL), extracted from the culture supernatants of P. aeruginosa. Additionally, 54 also inhibited a biofilm formation in P. aeruginosa, as observed in a test tube, microtiter plate as well as confocal image analysis. Molecular docking studies showed that the LasR protein had binding energy-4.5 kcal/mol. It has two hydrogen-bonding interactions (Ala59 and Lys34 with binding distance of 2.8 and 3.5 Ằ) and one hydrophobic interaction (Ala58:4.01381) against 54 [38].

Cyclic Peptides
Two unreported cyclic lipotetrapeptides, named bacilotetrins A (55) and B (56) (Figure 7), were obtained from the culture extract of B. subtilis 109GGC020, isolated from a sediment sample collected from the Gageocho reef, Republic of Korea. The structures of 55 and 56 were elucidated by extensive analysis of HRESIMS, 1D, and 2D NMR spectra. The absolute configurations of the stereogenic carbons of the amino acids were established by hydrolysis of the lipotetrapeptides, followed by derivatization with Marfey's reagent and chiral HPLC analysis. The absolute configuration of C-3 of 3-hydroxy fatty acids in both compounds was established as R by direct comparison of their specific rotation values with those of the previously reported 3-hydroxy fatty acids [39].
The liquid culture extract of Streptomyces sp. IMB094, isolated from a marine sediment at a depth of 40 m in Heishijiao Bay, Dalian, China, yielded two unreported actinomycin analogs containing a tetracyclic 5H-oxazolo[4,5-b]phenoxazine chromophore, named neo-actinomycins A (57) and B (58) (Figure 8). The structures of the compounds were established by extensive analysis of HRESIMS, 1D, and 2D NMR spectra. The absolute configurations of amino acids were determined using advanced Marfey's method after acid hydrolysis of 57 and 58. The presence of the phenoxazine ring system

Cyclic Peptides
Two unreported cyclic lipotetrapeptides, named bacilotetrins A (55) and B (56) (Figure 7), were obtained from the culture extract of B. subtilis 109GGC020, isolated from a sediment sample collected from the Gageocho reef, Republic of Korea. The structures of 55 and 56 were elucidated by extensive analysis of HRESIMS, 1D, and 2D NMR spectra. The absolute configurations of the stereogenic carbons of the amino acids were established by hydrolysis of the lipotetrapeptides, followed by derivatization with Marfey's reagent and chiral HPLC analysis. The absolute configuration of C-3 of 3-hydroxy fatty acids in both compounds was established as R by direct comparison of their specific rotation values with those of the previously reported 3-hydroxy fatty acids [39].
The liquid culture extract of Streptomyces sp. IMB094, isolated from a marine sediment at a depth of 40 m in Heishijiao Bay, Dalian, China, yielded two unreported actinomycin analogs containing a tetracyclic 5H-oxazolo[4,5-b]phenoxazine chromophore, named neo-actinomycins A (57) and B (58) (Figure 8). The structures of the compounds were established by extensive analysis of HRESIMS, 1D, and 2D NMR spectra. The absolute configurations of amino acids were determined using advanced Marfey's method after acid hydrolysis of 57 and 58. The presence of the phenoxazine ring system was proposed to originate from condensation between the previously reported actinomycin D (59) with α-ketoglutarate and pyruvate, respectively, as depicted in Figure 9. The hypothesis of the biosynthesis of 57 and 58 was supported by adding α-ketoglutaric acid and pyruvic acid (1 mg/mL) after the cultivation of Streptomyces sp. IMB094, followed by the detection of a 12-fold increase in the production of 57 in α-ketoglutaric acid-supplemented cultures compared to the unsupplemented control, while the yield of both 57 and 58 increased about 6-fold, 24 h after pyruvic acid was added into the cultures.

Alkaloids
Four undescribed indole alkaloids, streptoindoles A-D (64-67) (Figure 10), were isolated from the EtOAc extract of Streptomyces sp. ZZ1118, isolated from a gut sample of a marine shrimp (Penaeus sp.) in the Zhoushan archipelago, Zhejiang, China, and cultured on the rice medium. Compounds 64 and 65 are enantiomeric bis-indole alkaloids and were separated by HPLC using a chiral column (ChiralCel OJ-RH). The planar structures of 64-67 were elucidated by extensive analysis of 1D and 2D NMR as well as HRESIMS data. While the absolute configurations of the stereogenic carbons of 64 and 65 were established by comparison of the calculated and experimental ECD spectra, the absolute configuration of the only chiral carbon (C-15) in 66 was determined by comparison of the experimental and calculated optical rotations. Moreover, the structure of 67 was also confirmed by X-ray analysis [42].
Compound 65 exhibited three times stronger antibacterial activity against MRSA (MIC value of 7 µg/mL) than 64 and 67 (MIC values of 25 µg/mL), while 66 showed no activity at a concentration of 50 µg/mL. The positive control, vancomycin, showed a MIC value of 0.75 µg/mL against MRSA [42].
Two undescribed chlorinated bis-indole alkaloids, dienomycin (68) and 6-OMe-7 ,7dichorochromopyrrolic acid (69) (Figure 10 Interestingly, 68 showed stronger activity than vancomycin against MRSA GDE4P037P (isolated from pig), with a MIC value of 1 µg/mL compared to 8 µg/mL for vancomycin [43]. separated by HPLC using a chiral column (ChiralCel OJ-RH). The planar structures of 64-67 were elucidated by extensive analysis of 1D and 2D NMR as well as HRESIMS data. While the absolute configurations of the stereogenic carbons of 64 and 65 were established by comparison of the calculated and experimental ECD spectra, the absolute configuration of the only chiral carbon (C-15) in 66 was determined by comparison of the experimental and calculated optical rotations. Moreover, the structure of 67 was also confirmed by Xray analysis [42].  [42].
Two undescribed chlorinated bis-indole alkaloids, dienomycin (68) and 6-OMe-7′,7″dichorochromopyrrolic acid (69) (Figure 10), were obtained from the butanone and acetone extracts of a supernatant and a mycelia cake of Streptomyces sp. SCSIO 11791, isolated The undescribed indolizinium alkaloid, streptopertusacin A (70) (Figure 10), was obtained from the GYM solid culture extract of Streptomyces sp. HZP-2216E, isolated from a fresh seaweed, Ulva pertusa, which was collected at the Turtle islet in the South China Sea. The planar structure of 70 was established by extensive analysis of HRESIMS, 1D, and 2D NMR spectral data. The relative configurations of the stereogenic carbons in 70 were established through analysis of proton coupling constants and NOE data. The absolute configuration of the stereogenic carbon of its amino acid moiety (C-20) was determined by Marfey's method after acid hydrolysis of 70, while the absolute configurations of C-1, C-2, and C-3 were established by comparison of the experimental and calculated ECD spectra. Compound 70 showed moderate inhibitory activity against MRSA with a MIC value of 40 µg/mL. The positive control, gentamicin, showed a MIC value of 0.36 µg/mL [26].
By using LC-MS-principal component analysis (PCA)-based metabolomics and molecular networking approaches, two antibacterial pyrrole-derived alkaloids, phallusialides A (71) and B (72) (Figure 11), were obtained from the acetone extract of a bacterium Micromonospora sp. WMMA-2495, isolated from a tunicate, Phallusia nigra, which was collected in the Florida Keys, and cultured in the artificial medium ASW-A (20 g soluble starch, 10 g glucose, 5 g peptone, 5 g yeast extract, 5 g CaCO 3 per liter of artificial seawater) supplemented with NaCl and KBr. The structures of the compounds were elucidated by extensive analysis of HRESIMS, 1D, and 2D NMR data. The relative configurations of the stereogenic carbons of 71 were established by a combination of NOE studies, coupling constant analyses, extensive molecular modeling, DFT calculations, and ROESY correlations. Since a comparison of the experimental and calculated ECD spectra allowed the determination of the absolute configuration of only C-4 , the absolute structure of 71 was based on the established relative configurations of the stereogenic carbons relative to C-4 whose absolute configuration was unambiguously determined. The absolute configuration of the stereogenic carbons in 72 was established by comparison of the Cotton effects observed in its ECD spectrum with those in 71. Compounds 71 and 72 displayed antibacterial activity against MRSA (ATCC #33591), with a MIC value of 32 µg/mL (vancomycin, the positive control, showed MIC = 0.25 µg/mL). Comparing the structural feature of 71 and 72 with the co-isolated analogs that did not exhibit antibacterial activity revealed that halogenation at C-4 of the pyrrole moiety is crucial to antibacterial activity of 71 and 72, and the extra sugar moieties in the co-isolated analogs could modulate antibacterial activity [44].

Terpenoid Derivatives
Two undescribed halimane-type diterpenoids, micromonohalimanes A (73) and B (74) (Figure 11), were isolated from the culture extract of Micromonospora sp. WMMC-218, which was obtained from the ascidian, Symplegma brakenhielmi, collected in Florida at Stanblum State Park, USA, and cultured in ASW-medium. The structure of 74 was established by HRESIMS, 1D, and 2D NMR spectral analysis. The relative configuration of 74 was established by ROESY, 1D-double-pulsed-field-gradient-spin-echo (DPFGSE)-NOE, and DFT studies. The DFT studies of 74 revealed that stereochemical configurations of 74 were identical to those found in micromonohalimane A (73), which was co-isolated and whose absolute structure was determined by X-ray analysis. Compound 74 displayed antibacterial activity against MRSA (ATCC #33591), with a MIC value of 40 μg/mL, while 73 showed a MIC value greater than 200 μg/mL. Vancomycin was used as a positive control and showed a MIC value of 1 μg/mL. Moreover, 74 was found to be a bacteriostatic agent since the MRSA was able to grow in the LB plate in the presence of 74 [45].
Three unreported anti-MRSA diterpenoids, viz. 18-acetyl-cyclooctatin (75), 5,18-dedihydroxy-cyclooctatin (76), and 5-dehydroxy-cyclooctatin (77) (Figure 11) were isolated from the culture of Streptomyces sp. ZZ820, obtained from a soil sample, which was collected from a sea coastal in the East China Sea close to Zhoushan Archipelago, Zhejiang, China, and cultured in the SGYC liquid medium. The planar structures of 75-77 were elucidated by interpretation of HRESIMS, 1D, and 2D NMR spectra. The absolute configuration of C-5 in 75 was determined by Mosher's method, while the absolute structures of 75 and 76 were established by comparison of their calculated and experimental ECD spectra. Since the structure of 77 differs from that of 76 in that Me-18 in 76 was replaced by a hydroxymethyl group in 77, and both compounds had positive optical rotation values and similar ECD curves, it was assumed that 77 had the same absolute structure as 76. Compounds 75-77 showed moderate anti-MRSA activity with MIC values of 27.45, 24.11, and 29.39 μg/mL, respectively. The positive control, gentamicin, showed a MIC value of 0.91 μg/mL [46].

Terpenoid Derivatives
Two undescribed halimane-type diterpenoids, micromonohalimanes A (73) and B (74) (Figure 11), were isolated from the culture extract of Micromonospora sp. WMMC-218, which was obtained from the ascidian, Symplegma brakenhielmi, collected in Florida at Stanblum State Park, USA, and cultured in ASW-medium. The structure of 74 was established by HRESIMS, 1D, and 2D NMR spectral analysis. The relative configuration of 74 was established by ROESY, 1D-double-pulsed-field-gradient-spin-echo (DPFGSE)-NOE, and DFT studies. The DFT studies of 74 revealed that stereochemical configurations of 74 were identical to those found in micromonohalimane A (73), which was co-isolated and whose absolute structure was determined by X-ray analysis. Compound 74 displayed antibacterial activity against MRSA (ATCC #33591), with a MIC value of 40 µg/mL, while 73 showed a MIC value greater than 200 µg/mL. Vancomycin was used as a positive control and showed a MIC value of 1 µg/mL. Moreover, 74 was found to be a bacteriostatic agent since the MRSA was able to grow in the LB plate in the presence of 74 [45].
Three unreported anti-MRSA diterpenoids, viz. 18-acetyl-cyclooctatin (75), 5,18dedihydroxy-cyclooctatin (76), and 5-dehydroxy-cyclooctatin (77) (Figure 11) were isolated from the culture of Streptomyces sp. ZZ820, obtained from a soil sample, which was collected from a sea coastal in the East China Sea close to Zhoushan Archipelago, Zhejiang, China, and cultured in the SGYC liquid medium. The planar structures of 75-77 were elucidated by interpretation of HRESIMS, 1D, and 2D NMR spectra. The absolute configuration of C-5 in 75 was determined by Mosher's method, while the absolute structures of 75 and 76 were established by comparison of their calculated and experimental ECD spectra. Since the structure of 77 differs from that of 76 in that Me-18 in 76 was replaced by a hydroxymethyl group in 77, and both compounds had positive optical rotation values and similar ECD curves, it was assumed that 77 had the same absolute structure as 76. Compounds 75-77 showed moderate anti-MRSA activity with MIC values of 27.45, 24.11, and 29.39 µg/mL, respectively. The positive control, gentamicin, showed a MIC value of 0.91 µg/mL [46].

Miscellaneous
An undescribed glycerol 1-hydroxy-2,5-dimethyl benzoate (78) (Figure 12) was isolated from the culture extract of Verrucosispora sp. strain MS100047, obtained from a deep-sea sediment sample collected at the depth 2733 m in the South China Sea, and cultured in the VER01 liquid medium. The structure of 78 was determined by analysis of HRESIMS and 1D and 2D NMR spectra. However, the absolute configuration of C-9 was not determined. Compound 78 showed selective activity against MRSA with a MIC value of 12.5 µg/mL [47].

Miscellaneous
An undescribed glycerol 1-hydroxy-2,5-dimethyl benzoate (78) (Figure 12) was isolated from the culture extract of Verrucosispora sp. strain MS100047, obtained from a deepsea sediment sample collected at the depth 2733 m in the South China Sea, and cultured in the VER01 liquid medium. The structure of 78 was determined by analysis of HRESIMS and 1D and 2D NMR spectra. However, the absolute configuration of C-9 was not determined. Compound 78 showed selective activity against MRSA with a MIC value of 12.5 μg/mL [47]. An unreported 2,2′-pyridine containing methyl sulfonyl and carboxaldehyde oxime moieties, named maipomycin A (79) (Figure 12), was obtained from a rare actinomycete, Kibdelosporangium phytohabitans XY-R10, isolated from the root sediments of a mangrove plant, Kandelia candel (L.) Druce, collected from Mai Po Inner Deep Bay Ramsar Site, Hong Kong, China, and cultured in SGTPY medium (17 g sea salts, 5 g starch, 5 g glucose, 1 g tryptone, 1 g peptone, and 1 g yeast extract dissolved in 1 L of distilled water). The structure of 79 was established by analysis of HRESIMS, 1D, and 2D NMR spectra and confirmed by a single-crystal X-ray analysis. Compound 79 did not show antibacterial activity against a panel of Gram-positive and Gram-negative bacteria, except for a weak antibacterial activity against the reference strains and clinical isolate of Actinobacter baumannii (MIC = 128 mg/mL). On the contrary, 79 showed an effective inhibitory activity against the Gram-negative bacteria biofilm formation in a dose-dependent manner from 2 to 64 mg/mL. The percentage of biofilm biomass at a minimum biofilm inhibitory concentration (MBIC) was reduced by 84.3% for A. baumannii, and 82.6% for P. aeruginosa compared to the control. Confocal microscopy analysis showed an increase in the roughness coefficient and surface-to-biovolume ratio of the biofilm, suggesting the heterogeneity and incompleteness of biofilm development caused by 79. Interestingly, although 79 exhibited weak antibacterial activity, it could efficiently potentiate colistin An unreported 2,2 -pyridine containing methyl sulfonyl and carboxaldehyde oxime moieties, named maipomycin A (79) (Figure 12), was obtained from a rare actinomycete, Kibdelosporangium phytohabitans XY-R10, isolated from the root sediments of a mangrove plant, Kandelia candel (L.) Druce, collected from Mai Po Inner Deep Bay Ramsar Site, Hong Kong, China, and cultured in SGTPY medium (17 g sea salts, 5 g starch, 5 g glucose, 1 g tryptone, 1 g peptone, and 1 g yeast extract dissolved in 1 L of distilled water). The structure of 79 was established by analysis of HRESIMS, 1D, and 2D NMR spectra and confirmed by a single-crystal X-ray analysis. Compound 79 did not show antibacterial activity against a panel of Gram-positive and Gram-negative bacteria, except for a weak antibacterial activity against the reference strains and clinical isolate of Actinobacter baumannii (MIC = 128 mg/mL). On the contrary, 79 showed an effective inhibitory activity against the Gram-negative bacteria biofilm formation in a dose-dependent manner from 2 to 64 mg/mL. The percentage of biofilm biomass at a minimum biofilm inhibitory concentration (MBIC) was reduced by 84.3% for A. baumannii, and 82.6% for P. aeruginosa compared to the control. Confocal microscopy analysis showed an increase in the roughness coef-ficient and surface-to-biovolume ratio of the biofilm, suggesting the heterogeneity and incompleteness of biofilm development caused by 79. Interestingly, although 79 exhibited weak antibacterial activity, it could efficiently potentiate colistin against A. Baumannii. A combination of colistin with 79 resulted in the reduction of the MIC of colistin 4-8 folds. Moreover, 79 also showed a synergistic effect with colistin against A. Baumannii but only an additive effect for anti-biofilm activity [48].
The extract of Streptomyces sp. ZZ741, isolated from a mud sample in Jintang Island, Zhoushan, China, and cultured on a rice medium, furnished ten undescribed glutarimide antibiotics, streptoglutarimides A−J (80-89) ( Figure 12). The planar structures of 80-89 were elucidated based on their HRESIMS data and extensive analysis of their 1D and 2D NMR spectra. The absolute configurations of C-11 and C-13 in 80 were determined as 11R, 13R by X-ray diffraction analysis and confirmed by comparison of their experimental and calculated ECD spectra. The absolute configurations of C-11 and C-13 in 81 were determined as 11R, 13S by comparison of their experimental and calculated ECD spectra. Therefore, 80 and 81 are diastereomers. Compound 82 has only one stereogenic carbon (C-13), and its absolute configuration was established as 13S, also by comparison of their experimental and calculated ECD spectra. The absolute configurations of the stereogenic carbons in 83-85 were determined by comparison their experimental and calculated ECD spectra as well as by 13 C chemical shift calculations. The absolute configurations of the stereogenic carbons in 86-88 were determined by Mosher's method and by comparison of their experimental and calculated ECD spectra, while the absolute configurations of the stereogenic carbons in 89 were established by NOE correlations and comparison of their experimental and calculated ECD spectra.
The unreported N-isoprenoid bromo-phenazinone, marinocyanin A (90) (Figure 13), was isolated from the culture extracts of the strains CNS-284 (isolated from a sediment sample collected at a depth of 34 m in Palau) and CNY-960 (isolated from a sediment sample collected in the Solomon Islands), while the undescribed marinocyanins B-F (91-95) ( Figure 13) were isolated from the culture extract of the strain CNS-284. Both strains were identified as members of MAR4 clade within the Streptomycetaceae. The planar structures of 90-95 were elucidated by HRESIMS, 1D, and 2D NMR spectral analysis. The structure of 90 was confirmed by X-ray crystallographic analysis. The absolute configurations of the stereogenic carbons in 93 and 95 were not determined due to the limited amount of the compounds and their stability [50].
Compound 90 strongly inhibited the growth of amphotericin-resistant Candida albicans, with a MIC value of 0.95 µM. The structure-activity relationship indicated that the terpenoid ring system was important for the activity. Modification in the ring system significantly reduced the potencies of the marinocyanins [50].
The unreported N-isoprenoid bromo-phenazinone, marinocyanin A (90) (Figure 13), was isolated from the culture extracts of the strains CNS-284 (isolated from a sediment sample collected at a depth of 34 m in Palau) and CNY-960 (isolated from a sediment sample collected in the Solomon Islands), while the undescribed marinocyanins B-F (91-95) ( Figure 13) were isolated from the culture extract of the strain CNS-284. Both strains were identified as members of MAR4 clade within the Streptomycetaceae. The planar structures of 90-95 were elucidated by HRESIMS, 1D, and 2D NMR spectral analysis. The structure of 90 was confirmed by X-ray crystallographic analysis. The absolute configurations of the stereogenic carbons in 93 and 95 were not determined due to the limited amount of the compounds and their stability [50].   The structure-activity relationship indicated that the terpenoid ring system was important for the activity. Modification in the ring system significantly reduced the potencies of the marinocyanins [50].

Conclusions
Marine-derived bacteria have been acknowledged as an important source of compounds with unique structures and potential bioactivities. Moreover, the biomass of marine bacteria can be scaled up to an unlimited level in the laboratory for secondary metabolite production. The structures of antimicrobial compounds derived from marine bacteria reported from January 2016 to December 2021 consisted mostly of polyketides, amino acid derivatives, terpenoid derivatives, and also derivatives of mixed origins ( Figure 15). As can be seen in Table 1, Streptomyces species constitute a major source of bioactive compounds. However, other bacterial taxa, such as Nonomuraea sp., Micromonospora sp. Shewanella sp., Bacillus sp., are also found to produce potential compounds with interesting activity against drug-resistant pathogens. Thus, marine-derived bacteria are an excellent source for further exploration to search for novel antimicrobial and antibiofilm compounds against multi-drug resistant pathogens.

Conclusions
Marine-derived bacteria have been acknowledged as an important source of compounds with unique structures and potential bioactivities. Moreover, the biomass of marine bacteria can be scaled up to an unlimited level in the laboratory for secondary metabolite production. The structures of antimicrobial compounds derived from marine bacteria reported from January 2016 to December 2021 consisted mostly of polyketides, amino acid derivatives, terpenoid derivatives, and also derivatives of mixed origins ( Figure 15). As can be seen in Table 1, Streptomyces species constitute a major source of bioactive compounds. However, other bacterial taxa, such as Nonomuraea sp., Micromonospora sp. Shewanella sp., Bacillus sp., are also found to produce potential compounds with interesting activity against drug-resistant pathogens. Thus, marine-derived bacteria are an excellent source for further exploration to search for novel antimicrobial and antibiofilm compounds against multi-drug resistant pathogens.