Fusarium-Derived Secondary Metabolites with Antimicrobial Effects

Fungal microbes are important in the creation of new drugs, given their unique genetic and metabolic diversity. As one of the most commonly found fungi in nature, Fusarium spp. has been well regarded as a prolific source of secondary metabolites (SMs) with diverse chemical structures and a broad spectrum of biological properties. However, little information is available concerning their derived SMs with antimicrobial effects. By extensive literature search and data analysis, as many as 185 antimicrobial natural products as SMs had been discovered from Fusarium strains by the end of 2022. This review first provides a comprehensive analysis of these substances in terms of various antimicrobial effects, including antibacterial, antifungal, antiviral, and antiparasitic. Future prospects for the efficient discovery of new bioactive SMs from Fusarium strains are also proposed.


Introduction
Antimicrobial agents play a significant role in the treatment of infectious diseases caused by pathogenic microorganisms with various modes of action. Since the fortuitous discovery of penicillin in 1928, hundreds of antibiotics have been approved for clinical use. However, some of these drugs have become less efficacy or unavailability simultaneously owing to the development of antimicrobial resistance (AMR), in which a pathogenic microbe evolves a survival mechanism that protects the drug target by modification or replacement, or degradation or modification of the antibiotic to render it harmless, such as MRSA (methicillin-resistant Staphylococcus aureus), multidrug-resistant S. aureus (MDRS), VREF (vancomycin-resistant Enterococcus faecium), CRKP (cephalosporin-resistant Klebsiella pneumoniae) [1]. Antimicrobial resistance has become an increasing threat to human health and is widely considered to be the next global pandemic [2]. Therefore, it is an urgent need for the discovery of new antimicrobial drugs with novel structural scaffolds and new modes of action.
Microorganisms are well recognized as a prolific source of biomolecules with diverse chemical structures and various biological properties. Microbial natural products have been, to date, our most successful defense against infectious disease. As one of the most commonly isolated filamentous fungi in terrestrial and marine environments, Fusarium spp. possess the potential capability to biosynthesize structurally diverse secondary metabolites (SMs), including alkaloids, peptides, amides, terpenoids, quinones, pyranones, and miscellaneous compounds [3]. Up to now, however, no document highlighting Fusarium-derived SMs with antimicrobial effects has been reported. With the aim to enrich our knowledge, this review comprehensively summarizes the occurrence of these antimicrobial substances, including antibacterials, antifungals, antivirals, and antiparasitics.
As of December 2022, the Dictionary of Natural Products (DNP) database listed 783 Fusarium-derived SMs, many of them also occurring in other microbial genera. By extensive literature search, as many as 185 antimicrobial SMs (1-185) had been discovered from Fusarium strains and are, respectively, introduced in terms of various antimicrobial

Anti-Gram-Positive Bacterial SMs
Fifty Fusarium-derived SMs (1-50, Figure 1) had been characterized and displayed various bactericidal effects on Gram-positive strains, such as Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, multidrug-resistant S. aureus, Mycobacterium tuberculosis, Bacillus subtilis, etc. Fusariumins C (1) and D (2) are two new polyketides produced by an endophytic strain F. oxysporum ZZP-R1 from coastal plant Rumex midair Makino displayed medium effect on S. aureus with MIC (minimum inhibitory concentration) values of 6.25 and 25.0 µM, respectively [4]. Two triterpene sulfates (3 and 4) isolated from F. compactum exhibited weak activity toward S. aureus and Streptococcus strains in the range of 6-50 µg/mL [5]. Enniatins (5-10), a group of antibiotics commonly synthesized by various Fusarium strains, are six-membered cyclic depsipeptides formed by the union of three

Anti-Gram-Positive Bacterial SMs
Fifty Fusarium-derived SMs (1-50, Figure 1) had been characterized and displayed various bactericidal effects on Gram-positive strains, such as Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, multidrug-resistant S. aureus, Mycobacterium tuberculosis, Bacillus subtilis, etc. Fusariumins C (1) and D (2) are two new polyketides produced by an endophytic strain F. oxysporum ZZP-R1 from coastal plant Rumex midair Makino displayed medium effect on S. aureus with MIC (minimum inhibitory concentration) values of 6.25 and 25.0 µM, respectively [4]. Two triterpene sulfates (3 and 4) isolated from F. compactum exhibited weak activity toward S. aureus and Streptococcus strains in the range of 6-50 µg/mL [5]. Enniatins (5-10), a group of antibiotics commonly synthesized by various Fusarium strains, are six-membered cyclic depsipeptides formed by the union of three

Both Anti-Gram-Positive and Anti-Gram-Negative Bacterial SMs
Seventeen Fusarium-derived SMs (65-81, Figure 3) were shown to have both anti-Grampositive and anti-Gram-negative activity. Seven naphthoquinones (65-71) demonstrated moderate activities against an array of Gram-positive and Gram-negative bacteria, such as B. megaterium, B. subtilis, C. perfringens, E. coli, methicillin-resistant S. aureus, P. aeruginosa, S. aureus, and S. pyogenes [13,21,30,31]. The mechanism of action (MoA) study indicated that compounds 66 and 71 could stimulate the oxygen consumption of bacterial cells and induce cyanide-insensitive oxygen consumption, which results in the generation of superoxide anion and hydrogen peroxide [32]. Compounds 72-75 were polycyclic terpenoids, respectively, produced by three Fusarium strains [33][34][35]. Compound 72 had significant activity against S. aureus and P. aeruginosa with a MIC value of 6.3 µg/mL, and 73 showed moderate activities against Salmonella enteritidis and Micrococcus luteus with MIC values of 6.3 and 25.2 µg/mL, respectively, while 74 showed a broad spectrum of antibacterial activity and 75 exhibited moderate antibacterial activities against S. aureus and E. coli with the same MIC value of 16 µg/mL. Two xanthine oxidase inhibitory cerebrosides (76 and 77) were identified and purified from the culture broth of Fusarium sp. IFB-121 and showed strong antibacterial activities against B. subtilis, E. coli, and P. fluorescens with MICs of less than 7.8 µg/mL [36]. Enniatins J 1 (78) and J 3 (79) were two hexadepsipeptides with an array of antibacterial activity toward C. perfringens, E. faecium, E. coli, S. dysenteriae, S. aureus, Y. enterocolitica, and lactic acid bacteria except for B. adolescentis [37]. Halymecin A (80) was produced by a marine-derived Fusarium sp. FE-71-1 and exhibited a moderate inhibitory effect on E. faecium, K. pneumoniae, and P. vulgaris with the MIC value of 10 µg/mL [38]. Fusaequisin A (81) was isolated from rice cultures of F. equiseti SF-3-17 and found to have moderate antimicrobial activity against S. aureus NBRC 13,276 and P. aeruginosa ATCC 15,442 [39].

Antifungal Secondary Metabolites
Invasive fungal infections are very common in immunocompromised patients (such as acquired immune deficiency syndrome and organ transplantation) and have become a global problem resulting in 1.7 million deaths every year [40][41][42]. Furthermore, the overuse of antifungal agents increases opportunistic pathogen resistance, which had been listed as one of the dominant threats by the World Health Organization in 2019. Therefore, the urgent need for new antimycotics with novel targets is undeniable. Till the end of 2022, twenty-seven antifungal SMs (82-108, Figure 4) had been discovered from Fusarium strains. Compounds 82-84 are three anti-C. albicans glycosides belong to the papulacandin class [43,44]. The MoA study suggested that compound 82 is an inhibitor of glutamine synthetase (GS) enzyme for (l,3)-β-glucan biosynthesis [43]. CR377 (85) was a new α-furanone derivative from an endophytic Fusarium sp. CR377 and showed a similar antifungal effect on C. albicans with nystatin [45]. Compounds 86 and 87 were two zearalenone analogs and exhibited weak activity against Cryptococcus neoformans [46]. Neofusapyrone (88) produced by a marine-derived Fusarium sp. FH-146 displayed moderate activity against A. clavatus F318a with a MIC value of 6.25 µg/mL [47]. Six cyclic depsipeptides 89-94 had been isolated from several Fusarium strains and found to have significant inhibitory activities against pathogenic fungi, such as C. albicans [48], C. glabrata, C. krusei, V. ceratosperma, and A. fumigates [49]. Cyclosporin A (91) has long been recognized as an immunosuppressant agent and could inhibit the growth of sensitive fungi after their germination [50,51]. Parnafungins A-D (95-98) were isoxazolidinone-containing natural products and demonstrated broad-spectrum antifungal activity with no observed activity against bacteria. The targeted pathway of these alkaloids was determined to be the mRNA 3'-cleavage and polyadenylation process [52,53]. One N-hydroxypyridine derivative (99) showed antifungal activity against C. albicans and Penicillium chrysogenum with MICs of 16 and 8 µg/mL, respectively [54]. Indole acetic acid (100) exhibited activity against the fluconazole-resistant C. albicans (MIC = 125 µg/mL) [55]. Fusaribenzamide A (101) possessed a significant anti-C. albicans activity with MIC of 11.9 µg/disc compared to nystatin (MIC = 4.9 µg/disc) [56]. Three pyridone derivatives (102-104) displayed significant activities against multidrug-sensitive S. cerevisiae 12geneΔ0HSR-iERG6, and the MoA study indicated that these substances have a potent inhibitory effect on NADH-cytochrome C oxidoreductase [57]. Compounds 105-107 were derived from strain F. oxysporum N17B, and the former (105 and 106) showed selective fungistatic activity against Aspergillus fumigatus, and the latter (107) had selective potent activity against C. albicans through inhibition of phosphatidylinositol 3-kinase [58]. Culmorin (108) displayed remarkable antifungal activity against both marine (S. marina, M. pelagica) and medically relevant fungi (A. fumigatus, A. niger, C. albicans, T. mentagrophytes) [59,60].

Antiparasitic Secondary Metabolites
Parasitic diseases caused by protozoa, helminths and ectoparasites affect millions of people each year and result in substantial morbidity and mortality, particularly in tropical regions [95]. Therefore, new antiparasitic agents are urgently needed to treat and control

Conclusions
In summary, the genus Fusarium is one of the excellent producers of antimicrobial SMs, some of which have great potential in new drug development, such as anti-Grampositive bacterial terpenes 38, 42 and 43, anti-Gram-negative lucilactaenes 52-54, antifungal papulacandin 82 and pyridones 102-104, antiviral enniatins 135-137, and antiparasitic integracides 161-164, etc. In the past two decades, however, the rate of discovery of novel SMs from Fusarium has constantly been decreasing [3]. Fortunately, a growing number of evidence suggest that the potential of Fusarium spp. to make novel SMs is still immense since most of their SM biosynthetic gene clusters (BGCs) are inactive or un-awakened under traditional fermentation and culture conditions [107]. More and more cryptic BGCs responsible for the biosynthesis of novel SMs have been disclosed by various bio-informative tools and approaches and efficiently activated using genome mining strategies, such as BGC heterogeneous expression [108], promoter engineering [109] and gene transcriptional regulation [110]. In addition, more efforts should be made to analyze and interpret the action mechanisms of Fusarium-derived leading compounds, which have similar or more potent antimicrobial effects compared to positive controls.

Supplementary Materials:
The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/molecules28083424/s1, Table S1: Detail information for Fusarium-derived anti-Gram-positive bacterial SMs; Table S2: Detail information for Fusarium-derived anti-Gram-negative bacterial SMs; Table S3: Detail information for Fusarium-derived both anti-Gram-positive and anti-Gram-negative bacterial SMs; Table S4: Detail information for Fusarium-derived antifungal SMs; Table  S5: Detail information for Fusarium-derived both antibacterial and antifungal SMs; Table S6: Detail information for Fusarium-derived antiviral SMs; Table S7: Detail information for Fusarium-derived antiparasitic SMs.

Conflicts of Interest:
The authors declare no conflict of interest.