Equisetin Restores Colistin Sensitivity against Multi-Drug Resistant Gram-Negative Bacteria

The overuse of antibiotics and the scarcity of new drugs have led to a serious antimicrobial resistance crisis, especially for multi-drug resistant (MDR) Gram-negative bacteria. In the present study, we investigated the antimicrobial activity of a marine antibiotic equisetin in combination with colistin against Gram-negative bacteria and explored the mechanisms of synergistic activity. We tested the synergistic effect of equisetin in combination with colistin on 23 clinical mcr-1 positive isolates and found that 4 µg/mL equisetin combined with 1 µg/mL colistin showed 100% inhibition. Consistently, equisetin restored the sensitivity of 10 species of mcr-1 positive Gram-negative bacteria to colistin. The combination of equisetin and colistin quickly killed 99.9% bacteria in one hour in time-kill assays. We found that colistin promoted intracellular accumulation of equisetin in colistin-resistant E. coli based on LC-MS/MS analysis. Interestingly, equisetin boosted ROS accumulation in E. coli in the presence of colistin. Moreover, we found that equisetin and colistin lost the synergistic effect in two LPS-deficient A. baumannii strains. These findings suggest that colistin destroys the hydrophobic barrier of Gram-negative bacteria, facilitating equisetin to enter the cell and exert its antibacterial effect. Lastly, equisetin restored the activity of colistin in a G. mellonella larvae infection model. Collectively, these results reveal that equisetin can potentiate colistin activity against MDR Gram-negative bacteria including colistin-resistant strains, providing an alternative approach to address Gram-negative pathogens associated with infections in clinics.


Introduction
The alarming emergence and dissemination of antimicrobial resistance constitute a crisis to One Health. It is predicted that antimicrobial resistance will lead to the death of 10 million people/year by 2050 [1]. The overuse and misuse of antibiotics and the scarcity of new drugs result in the migration of multidrug-resistant (MDR) bacteria from hospitals to the community. In 2017, the World Health Organization (WHO) introduced a list of "critical priority" pathogens-including the "ESKAPE" pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter species)-responsible for a global health problem [2]. Compared to Grampositive bacteria, Gram-negative bacteria (Escherichia coli, K. pneumoniae, P. aeruginosa and A. baumannii) are among the primary infectious agents, owing to their dual-membrane envelope, which prevent many antibiotics from accessing targets. Therefore, future development strategies should pay more attention to antibiotics that are active against MDR Gram-negative bacteria. There was almost no novel compounds discovered that were active against Gram-negative bacteria in the past fifty years [3]. Thus, it is imperative to explore novel antibiotics or other approaches against MDR Gram-negative bacteria-

Equisetin Synergies with Colistin against MDR Gram-Negative Bacteria
To expand the antibacterial spectrum of equisetin and to improve the efficiency of currently available antibiotics, we systematically assessed the synergistic activity between equisetin and other major classes of antibiotics against E. coli B2 (NDM-5 + MCR-1). We found that equisetin had no synergistic activity with other antibiotics except colistin, as was previously reported [15] (Figure 1a, Table S1). Then, we focused on the synergistic effect of equisetin and colistin against colistin-resistant bacteria. We tested the synergistic antibacterial effect of equisetin and colistin on 23 clinical mcr-1 positive resistant isolates at different concentrations and found that equisetin (4 µg/mL) combined with colistin (1 µg/mL) inhibited 100% of the tested bacteria (Table 1 and Table S1). Consistently, equisetin restored colistin sensitivity in 10 species of mcr-1 positive Gram-negative bacteria in the presence of equisetin (4 µg/mL) plus colistin (1 µg/mL) (Figure 1b, FIC index ≤ 0.5), which is consistent with our previous report [15]. Neither equisetin alone (4 µg/mL), nor sublethal levels of colistin (1 µg/mL, 1/8 MIC), affected the growth of E. coli (Figure 1c) but their combination sharply inhibited the growth of E. coli. Furthermore, the time-killing curve showed that equisetin combined with colistin rapidly killed more than 99% of E. coli within 1 h (Figure 1d).

Mechanism of Equisetin Combined with Colistin against Bacteria
Colistin is thought to require the insertion of the fatty-acyl chain into the OM to kill the bacteria [16]. Interestingly, the synergistic antibacterial activity can also be achieved by PBNP (Figure 2a), which lacks the fatty-acyl moiety found on colistin and is therefore incapable of inducing lysis [17]. Deacylated PMBN can sufficiently interact with and disrupt the OM, which is largely irrespective of the antibacterial mechanism [18]. Addition-

Mechanism of Equisetin Combined with Colistin against Bacteria
Colistin is thought to require the insertion of the fatty-acyl chain into the OM to kill the bacteria [16]. Interestingly, the synergistic antibacterial activity can also be achieved by PBNP (Figure 2a), which lacks the fatty-acyl moiety found on colistin and is therefore incapable of inducing lysis [17]. Deacylated PMBN can sufficiently interact with and disrupt the OM, which is largely irrespective of the antibacterial mechanism [18]. Additionally, the OM disruption of bacteria and potentiation of antibiotics by PBNP and colistin are unaffected by the mcr-1 expression [17,18]. Accordingly, we speculated that the role of colistin in drug combination is based on the destruction of the membrane. Indeed, colistin can significantly enhance the amount of other antibiotics entering the cell and exerting antibacterial synergy [19][20][21]. We used an optimized LC-MS/MS detection method to determine the intracellular accumulation of equisetin when combined with colistin or equisetin alone and found that colistin can significantly increase the accumulation of equisetin in the cell (Figure 2b). The result may indicate that equisetin enters the cytoplasm to exert its antibacterial function. Next, we used the hydrophobic fluorescent probe 1-N-phenylnaphthylamine (NPN) to evaluate the damaging effects of equisetin combined with colistin or equisetin and colistin alone on the OM of E. coli B2. As shown in Figure  3c, we found that equisetin combined with colistin showed significant damaging effects to the OM and that equisetin or colistin alone cannot damage the OM. In addition to the bacterial OM, polymyxin also plays an important role in the permeability of the bacterial cytoplasmic membrane (CM) [22]. Therefore, we used propidium iodide (PI) mediated fluorescent dyes to test the effect of the drugs on the permeability of the inner membrane. As expected, equisetin plus colistin exposure resulted in a strong PI signal from E. coli B2 cells (Figure 2d), indicative of the damaged CM causing the PI to fluoresce upon contact with DNA in the bacterial cytoplasm. Taken together, equisetin combined with colistin destroy the OM and CM of the bacteria. Additionally, equisetin also inhibited transcription of mcr-1, but not in a dose-dependent manner ( Figure S2). phenylnaphthylamine (NPN) to evaluate the damaging effects of equisetin combined with colistin or equisetin and colistin alone on the OM of E. coli B2. As shown in Figure 3c, we found that equisetin combined with colistin showed significant damaging effects to the OM and that equisetin or colistin alone cannot damage the OM. In addition to the bacterial OM, polymyxin also plays an important role in the permeability of the bacterial cytoplasmic membrane (CM) [22]. Therefore, we used propidium iodide (PI) mediated fluorescent dyes to test the effect of the drugs on the permeability of the inner membrane. As expected, equisetin plus colistin exposure resulted in a strong PI signal from E. coli B2 cells ( Figure  2d), indicative of the damaged CM causing the PI to fluoresce upon contact with DNA in the bacterial cytoplasm. Taken together, equisetin combined with colistin destroy the OM and CM of the bacteria. Additionally, equisetin also inhibited transcription of mcr-1, but not in a dose-dependent manner ( Figure S2).
Since the bactericidal activity was rapid, the mechanistic basis of equisetin was considered to be associated with bacterial propagation. Thus, we evaluated whether the bacterial metabolism affects the antibacterial activity of equisetin combined with colistin and found that the combination rapidly eliminated metabolically active bacteria in 2 h (37 °C) but lost its antibacterial activity against bacteria in metabolically suppressed states (0 °C, Figure 2e). According to the rapid sterilization rate of the drug combination, we speculated that the bacteria accumulated a large number of toxic substances in a short time. Hence, we detected the content of reactive oxygen species (ROS) in bacterial cells after drug treatments. Indeed, in low levels of colistin, the accumulation of ROS in cells gradually increased with the addition of equisetin ( Figure 2f).

In Vivo Efficacy of Equisetin Combined with Colistin
The encouraging antibacterial activity of equisetin combined with colistin in vitro inspired us to further investigate their therapeutic potential in vivo. We constructed a resistant bacteria infection model of E. coli B2 in G. mellonella larvae, as was previously reported [19]. After 1 h post-infection, equisetin combined with colistin was supplied with different dose gradients (Figure 4a). The experimental results showed that equisetin Since the bactericidal activity was rapid, the mechanistic basis of equisetin was considered to be associated with bacterial propagation. Thus, we evaluated whether the bacterial metabolism affects the antibacterial activity of equisetin combined with colistin and found that the combination rapidly eliminated metabolically active bacteria in 2 h (37 • C) but lost its antibacterial activity against bacteria in metabolically suppressed states (0 • C, Figure 2e). According to the rapid sterilization rate of the drug combination, we speculated that the bacteria accumulated a large number of toxic substances in a short time. Hence, we detected the content of reactive oxygen species (ROS) in bacterial cells after drug treatments. Indeed, in low levels of colistin, the accumulation of ROS in cells gradually increased with the addition of equisetin (Figure 2f).

FIC for Parent and LPS-Deficient A. baumannii Strains
To further explore the synergistic mechanism of equisetin and colistin, we studied two LPS-deficient A. baumannii strains that are inactivated in the lipid A biosynthesis [23,24]. We first determined the MIC of equisetin and colistin on wild-type and LPS-deficient A. baumannii isolates and found that colistin resistance increased more than 100-fold, whereas equisetin resistance decreased more than 100-fold relative to the wild-type parent A. baumannii (Table 2). Our results demonstrated that LPS deficiency of strains fostered resistance to colistin but was sensitive to equisetin, which is exclusively active against Grampositive bacteria. Furthermore, equisetin lacked synergy with colistin on LPS-deficient A. baumannii, contrary to the wild type ( Figure 3). These findings denote that outer-membrane LPS are the major obstacle to equisetin-dependent killing in A. baumannii.

In Vivo Efficacy of Equisetin Combined with Colistin
The encouraging antibacterial activity of equisetin combined with colistin in vitro inspired us to further investigate their therapeutic potential in vivo. We constructed a resistant bacteria infection model of E. coli B2 in G. mellonella larvae, as was previously reported [19]. After 1 h post-infection, equisetin combined with colistin was supplied with different dose gradients (Figure 4a). The experimental results showed that equisetin combined with colistin at 8 mg kg −1 + 4 mg kg −1 fully protected G. mellonella from infection, which suggests that equisetin combined with colistin is a promising solution to treat MDRassociated infections in vivo (Figure 4b).

Discussion
We are in a "post-antibiotic era" and there are few clinical antibacterial candidates, especially for Gram-negative pathogens [25]. The OM of Gram-negative bacteria is an effective barrier in the outer leaflet that protects bacteria from toxic environmental insults, including antibiotics. Given the long development duration of antibacterial drugs and the high probability of failure, we must preserve the existing drugs. Combination therapy with existing drugs is a key to combat MDR infections and extend their shelf lives [26]. Potentiators that restore the activity of current antibiotics can expand the antibacterial spectrum as well. For example, the inhibitors of β-lactamases and sulfonamide synergists overcome a number of pathogens [27]. Notably, colistin disrupts the OM of Gram-negative bacteria to permit other compounds access to their cytoplasmic targets [28]. Thus, combinations of colistin with hydrophobic antibiotics [29] or adjuvants [30] are often used in the treatment of MDR Gram-negative infections. Colistin binding to the negatively charged lipid A phosphate groups of an LPS component causes membrane destabilization, leading to the increase of cell envelope permeability, leakage of cellular contents and ultimately cell death [18]. Despite that the medication of phosphate groups decreases the therapeutic utility of colistin, colistin still can paralyze the integrity of colistin-resistant bacteria [31,32]. For instance, colistin in combination with rifampicin [33], macrolides [34], tigecycline [35], carbapenems and glycopeptides [36] achieves promising therapeutic outcomes. Indeed, although hydrophobic antibiotics are conventionally active against Gram-

Discussion
We are in a "post-antibiotic era" and there are few clinical antibacterial candidates, especially for Gram-negative pathogens [25]. The OM of Gram-negative bacteria is an effective barrier in the outer leaflet that protects bacteria from toxic environmental insults, including antibiotics. Given the long development duration of antibacterial drugs and the high probability of failure, we must preserve the existing drugs. Combination therapy with existing drugs is a key to combat MDR infections and extend their shelf lives [26]. Potentiators that restore the activity of current antibiotics can expand the antibacterial spectrum as well. For example, the inhibitors of β-lactamases and sulfonamide synergists overcome a number of pathogens [27]. Notably, colistin disrupts the OM of Gram-negative bacteria to permit other compounds access to their cytoplasmic targets [28]. Thus, combinations of colistin with hydrophobic antibiotics [29] or adjuvants [30] are often used in the treatment of MDR Gram-negative infections. Colistin binding to the negatively charged lipid A phosphate groups of an LPS component causes membrane destabilization, leading to the increase of cell envelope permeability, leakage of cellular contents and ultimately cell death [18]. Despite that the medication of phosphate groups decreases the therapeutic utility of colistin, colistin still can paralyze the integrity of colistin-resistant bacteria [31,32]. For instance, colistin in combination with rifampicin [33], macrolides [34], tigecycline [35], carbapenems and glycopeptides [36] achieves promising therapeutic outcomes. Indeed, although hydrophobic antibiotics are conventionally active against Gram-positive bacteria such as rifampicin and macrolides, they have shown favorable prospects against Gramnegative pathogens in the presence of colistin or other Gram-negative OM disrupting compounds [37].
Compared to natural antibiotics of terrestrial origins, marine organisms that survive in high salinity, high pressure, low temperature and oligotrophic environments become appealing for the discovery of new compounds. Marine fungi derived from sponges are outstanding sources of natural products with various biological activities [38]. Equisetin isolated from a marine sponge-associated fungal strain Fusarium can kill diverse MDR Gram-positive bacteria without cross-resistance and detectable resistance [15,39]. In addition to effective antimicrobial effects, equisetin also has activity against HIV-1 integrasel [40] and mitochondria [41]. In combination with colistin, equisetin displays antibacterial activity against Gram-negative pathogens including colistin-resistant bacteria, enhancing colistin activity substantially and reducing its cytotoxicity accordingly.
Interestingly, the rapid bactericidal ability of equisetin combined with colistin against E. coli (Figure 1d) is similar to the equisetin-mediated killing curve of S. aureus [15]. Moreover, the result that LPS-deficient Gram-negative bacteria are sensitive to equisetin indicates that equisetin may target the same component in both Gram-positive and Gram-negative bacteria. Previous studies suggest that the formation of ROS is a common mechanism of bacterial death induced by bactericidal antibiotics [42][43][44]. Accumulated ROS break DNA, peroxidate lipids, carbonylate proteins and so on [45]. Correspondingly, ROSmediated damages accelerate ROS accumulation, leading to metabolic perturbation and cell death [46]. Equisetin shows an ROS-independent bactericidal mechanism (data unpublished). Thus, we speculate that colistin facilitates the entry of equisetin to the MDR Gram-negative bacteria, while colistin promotes intracellular ROS accumulation that contributes to rapid bacterial killing. Additionally, combination antibiotic therapy is frequently used to treat severe Gram-negative infections in clinics [47]. Equisetin combined with colistin is highly effective in eradicating MDR Gram-negative bacteria in G. mellonella infection models. Altogether, these findings suggest that equisetin exhibits a distinct mode of action against bacteria and possesses promising pharmacokinetic properties.
In summary, our study shows that equisetin acts synergistically with colistin to kill various MDR Gram-negative bacteria (including colistin-resistant strains) and restores the efficacy of colistin in vivo. We find drug combinations alter the permeability of the OM and CM of Gram-negative bacteria, permitting the increased accumulation of intracellular equisetin to exert antibacterial activity. Collectively, we demonstrate that equisetin combined with colistin represents an attractive strategy for the treatment of infections caused by MDR Gram-negative pathogens.

Bacterial Isolates
The standard strains in this study were purchased from China General Microbial Collection and Management Center (CGMCC) and the other clinically resistant strains were isolated by our laboratory, including E. coli B2 and 23 mcr-1 positive bacteria [19].

Antibacterial Test
The minimum inhibitory concentrations (MICs) of equisetin and colistin were determined by the standard broth microdilution method, according to the CLSI 2021 guideline. Briefly, equisetin or colistin were mixed with equal volumes of bacterial suspensions in

FIC Index Determination
FIC index (FICI) was determined using chequerboard assays. Colistin was diluted along the ordinate, while equisetin was diluted along the abscissa by MHB in a 96-well microtiter plate in a final volume of 100 µL. Then, bacterial suspensions were diluted by approximately 1.5 × 10 6 CFUs/mL in MHB and 100 µL were added to 96-well microtiter plates containing antibiotics. After 16-20 h of incubation at 37 • C, the FICI was calculated according to the following formula:

Growth Curves of Bacteria
The overnight cultures of E. coli B2 were standardized to match a 0.5 McFarland turbidity standard and followed by 1:100 dilution in MHB. Different concentrations of equisetin, colistin or the combination of equisetin with colistin were added to a 96-well microplate and mixed with an equal volume of culture diluted 1:100 in MHB with the plate lid covered. The growth curves were recorded under the wavelength of 600 nm with an interval of 4 h at 37 • C, using an Infinite M200 Microplate reader (Tecan) in the long-term model. PBS was used as a negative control in the experiment.

Time-Kill Assays
E. coli B2 was cultured to the exponential phase at 37 • C with shaking at 200 revolutions per minute (rpm) for 2 h and then diluted in MHB to a desired concentration at 10 6 to 10 7 CFU mL −1 . Then, bacteria were treated by equisetin, colistin or the combination of equisetin with colistin in culture tubes at 37 • C with shaking at 200 rpm. Subsequently, 10-fold serially diluted suspensions were plated on Tryptose Soya Agar (TSA) plates and incubated at 37 • C overnight for enumeration of bacterial colonies. PBS was used as a negative control in the experiment.

Outer Membrane Permeability Assay
The fluorescent probe NPN was used to evaluate the outer membrane integrity of E. coli B2 when treated by different concentrations of equisetin combined with colistin or with drugs alone. Briefly, E. coli B2 grown overnight at 37 • C were washed and resuspended in 5 mmol/L of HEPES (pH 7.2, +5 mmol/L of glucose) to obtain an OD 600 of 0.5. NPN was added to a final concentration of 10 µmol/L and the mixture was incubated at 37 • C for 30 min. A total of 190 µL of probe-labelled bacterial cells were added to a 96-well plate and then 10 µL of the drug combination or the drug alone was added. After incubating for Antibiotics 2021, 10, 1263 9 of 12 30 min, fluorescence was measured on an Infinite M200 Microplate reader (Tecan) with the excitation wavelength at 350 nm and the emission wavelength at 420 nm.

Cytoplasmic Membrane Permeability Assay
E. coli B2 grown overnight at 37 • C were washed and resuspended in 0.01 mol/L of PBS (pH 7.4) to obtain an OD 600 of 0.5. PI was added to a final concentration of 10 µmol/L and the mixture was incubated at 37 • C for 10 min. Then, after being treated by the drug combination or drug alone, fluorescence was measured with the excitation wavelength at 535 nm and emission wavelength at 615 nm.

ROS Measurement
The levels of ROS in E. coli

Reverse Transcription-(RT)PCR Analysis
E. coli B2 was grown overnight at 37 • C and treated with equisetin (0-64 µg/mL). Total RNA was extracted using the EASYspin Plus kit (Aidlab, catalogue no. RN0801) and quantified by the ratio of absorbance (260 nm:280 nm) using a Nanodrop spectrophotometer (Thermo Fisher Scientific, Waltham, American). Before the complementary DNA synthesis, the RNA from all bacterial cells was adjusted to an identical concentration of 1 µg and a reverse transcription of extracted RNA was performed using the PrimeScript RT reagent Kit (Takara, catalogue no. RR047A), following the manufacturer's protocol.
The levels of mcr-1 in treated E. coli groups were determined by RT-PCR relative to the 16S rRNA mRNA levels of the control gene. According to the previously reported primers for 16S rRNA and mcr-1 [35], RT-PCR was performed with SYBR Green qPCR kit (TaKaRa). Thermal cycling was performed by a standard two-step PCR amplification method, 95 • C for 30 s, 40 cycles of 95 • C for 5 s and 60 • C for 34 s. The ABI quantstudio TM 7 detection system (Applied Biosystem) for RT-PCR detection and a 2 −∆∆Ct method was used to determine the fold change of gene expression.

G. mellonella Infection Model
The synergy between equisetin and colistin was accessed in the G. mellonella infection model. The larvae of G. mellonella (purchased from Tianjin Huiyude Biotech Company) were randomly divided into four groups (n = 10 per group) and infected with 10 µL of E. coli B2 suspension (1.0 × 10 5 CFUs) at the right posterior gastropoda. After 1 h post-infection, G. mellonella was treated with PBS, colistin (8 mg kg −1 ) or the combination of equisetin with colistin (8 mg kg −1 + 4 mg kg −1 and 4 mg kg −1 + 2 mg kg −1 ) at the left posterior gastropoda. Survival rates of G. mellonella were recorded over 48 h. PBS was used as negative control and colistin at 8 mg kg −1 was used as a positive control in the experiment.

Conclusions
In conclusion, these results demonstrate that equisetin combined with colistin shows great efficacy for treating MDR Gram-negative bacteria infections in vitro and in vivo, which suggests that drug combination holds promise for overcoming antibacterial resistance. Therefore, given the fact that development of new antibiotics declines rapidly, drug combination can be a promising approach to extend the life of the already existing antibacterial drug arsenal.