Polymyxin E Induces Rapid Paenibacillus polymyxa Death by Damaging Cell Membrane while Ca2+ Can Protect Cells from Damage

Response of Paenibacillus polymyxa SC2 to the stress of polymyxin B and a key ABC transporter YwjA involved

Polymyxins are cationic peptide antibiotics and regarded as the "final line of defense" against multidrug-resistant bacterial infections. Meanwhile, some polymyxin-resistant strains and the corresponding resistance mechanisms have also been reported. However, the response of the polymyxin-producing strain Paenibacillus polymyxa to polymyxin stress remains unclear. The purpose of this study was to investigate the stress response of gram-positive P. polymyxa SC2 to polymyxin B and to identify functional genes involved in the stress response process. Polymyxin B treatment upregulated the expression of genes related to basal metabolism, transcriptional regulation, transport, and flagella formation and increased intracellular ROS levels, flagellar motility, and biofilm formation in P. polymyxa SC2. Adding magnesium, calcium, and iron alleviated the stress of polymyxin B on P. polymyxa SC2, furthermore, magnesium and calcium could improve the resistance of P. polymyxa SC2 to polymyxin B by promoting biofilm formation. Meanwhile, functional identification of differentially expressed genes indicated that an ABC superfamily transporter YwjA was involved in the stress response to polymyxin B of P. polymyxa SC2. This study provides an important reference for improving the resistance of P. polymyxa to polymyxins and increasing the yield of polymyxins. KEY POINTS: • Phenotypic responses of P. polymyxa to polymyxin B was performed and indicated by RNA-seq • Forming biofilm was a key strategy of P. polymyxa to alleviate polymyxin stress • ABC transporter YwjA was involved in the stress resistance of P. polymyxa to polymyxin B.

From bacteria to biomedicine: Developing therapies exploiting NAD+ metabolism

Nicotinamide adenine dinucleotide (NAD+) serves as a critical cofactor in cellular metabolism and redox reactions. Bacterial pathways rely on NAD+ participation, where its stability and concentration govern essential homeostasis and functions. This review delves into the role and metabolic regulation of NAD+ in bacteria, highlighting its influence on physiology and virulence. Notably, we explore enzymes linked to NAD+ metabolism as antibacterial drug targets and vaccine candidates. Moreover, we scrutinize NAD+'s medical potential, offering insights for its application in biomedicine. This comprehensive assessment informs future research directions in the dynamic realm of NAD+ and its biomedical significance.

Carbonyl Cyanide 3-Chloro Phenyl Hydrazone (CCCP) Restores the Colistin Sensitivity in Brucella intermedia

Brucella intermedia (formerly Ochrobactrum intermedium), a non-fermentative bacterium, has been isolated from animals and human clinical specimens. It is naturally resistant to polymyxins, including colistin (CO), and may cause opportunistic infections in humans. We isolated six Brucella intermedia strains from Senegalese monkey stool. In order to determine whether an efflux pump mechanism was involved in CO resistance in B. intermedia, we evaluated the effects of verapamil (VRP), reserpine (RSP), phe-arg β-naphthylamide dihydrochloride (PAβN) and carbonyl cyanide 3-chloro phenyl hydrazone (CCCP), four efflux pump inhibitors, on these colistin-resistant strains. Using the broth microdilution method, a CO and CCCP combination of 2 µg/mL and 10 µg/mL, respectively, significantly reduced the CO minimal inhibitory concentration (MIC) of B. intermedia, supporting an efflux pump mechanism. In contrast, VRP, PAβN and RSP did not restore CO susceptibility. A time kill assay showed a bactericidal effect of the CO-CCCP combination. Genomic analysis revealed a potential implication in the CO resistance mechanism of some conserved efflux pumps, such as YejABEF, NorM and EmrAB, as previously reported in other bacteria. An inhibitory effect of the CO-CCCP combination was observed on biofilm formation using the crystal violet method. These results suggest that the intrinsic CO resistance in Brucella intermedia is linked to an efflux pump mechanism and that the synergistic effect of CO-CCCP may open a new field to identify new treatments to restore antibiotic efficacy in humans.

Efflux pump inhibitor combined with ofloxacin decreases MRSA biofilm formation by regulating the gene expression of NorA and quorum sensing

Carbonyl cyanide p-nitrophenylhydrazone (2e) displayed a lone or synergistic efficacy against MRSA (RSC Adv., 2020, 10, 17854). In this work, the synergistic mechanism of 2e with ofloxacin was studied. MRSA2858 had potential for biofilm formation, and the value of MBEC of 2e alone was 0.78-1.56 μM, while that of 2e + ofloxacin was 0.39-0.78 μM. 2e combined with ofloxacin showed a synergistic anti-biofilm effect against MRSA. Efflux pump inhibitor 2e can better bind to NorA protein. After MRSA2858 was treated with 2e of 1/2MIC (0.78 μM) and ofloxacin of 1/8MIC (0.097 μM), the transcript levels of efflux genes (norA) and quorum-sensing (QS) regulatory genes (agrA, sarA, icaA, hla) were substantially down-regulated, and alpha-hemolysin (Hla) was inhibited by 99.15%. 2e combined with ofloxacin was more effective than 2e alone in reducing bacterial load in vivo. All in all, efflux pump inhibitor 2e enhanced the bactericidal activities of antibiotics through regulating the gene expression of NorA and QS system.

Synergistic antibacterial activity of baicalin and EDTA in combination with colistin against colistin-resistant Salmonella

The emergence and rapid spread of multidrug resistant (MDR) Gram-negative bacteria have posed a serious threat to global health and security. Because of the time-consuming, high cost and high risk of developing new antibiotics, a significant method is to use antibiotic adjuvants to revitalize the existing antibiotics. The purpose of the study is to research the traditional Chinese medicine baicalin with the function of inhibiting the efflux pump and EDTA whether their single or combination can increase the activity of colistin against colistin-resistant Salmonella in vitro and in vivo, and to explore its molecular mechanisms. In vitro antibacterial experiments, we have observed that baicalin and EDTA alone could enhance the antibacterial activity of colistin. At the same time, the combination of baicalin and EDTA also showed a stronger synergistic effect on colistin, reversing the colistin resistance of all Salmonella strains. Molecular docking and RT-PCR results showed that the combination of baicalin and EDTA not only affected the expression of mcr-1, but also was an effective inhibitor of MCR-1. In-depth synergistic mechanism analysis revealed that baicalin and EDTA enhanced colistin activity through multiple pathways, including accelerating the tricarboxylic acid cycle (TCA cycle), inhibiting the bacterial antioxidant system and lipopolysaccharide (LPS) modification, depriving multidrug efflux pump functions and attenuating bacterial virulence. In addition, the combinational therapy of colistin, baicalin and EDTA displayed an obvious reduction in bacterial loads cfus of liver and spleen compared with monotherapy and 2-drug combination therapy. In conclusion, our study indicates that the combination of baicalin and EDTA as a novel colistin adjuvant can provide a reliable basis for formulating the therapeutic regimen for colistin resistant bacterial infection.

An experimental study to evaluate the effect of polymixin E (Colistin) alone or in combination with gentamicin in McCarey-Kaufman corneal preservation medium on various drug resistant bacterial and fungal isolates

Purpose

To assess the efficacy of the addition of polymyxin E (colistin) in the McCarey-Kaufman (MK) corneal storage solution against multi-drug resistant strains of Enterobacteriaceae, Staphylococcus aureus, and Candida spp.

Methods

A standard micro broth dilution test and a checkerboard assay were performed for five multi-drug resistant (MDR) clinical strains of P. aeruginosa and five clinical strains of methicillin-resistant S. aureus (MRSA) and C. albicans against colistin and gentamicin alone and in combination. The minimum inhibitory concentration (MIC) and the fractional inhibitory concentration index (FICI) were calculated to assess the efficacy of each combination.

Results

The MIC of colistin was in the range of 1-2 μg/mL for P. aeruginosa, whereas it was 256-1024 μg/mL against S. aureus. In comparison, the MIC of gentamicin was found to be 0.5-512 μg/mL and 0.5-8 μg/mL against P. aeruginosa and S. aureus, respectively. All five isolates of C. albicans did not exhibit any susceptibility to either colistin or gentamicin even at a concentration of ≥ 512 μg/mL each. The checkerboard assay was performed to evaluate the nature of the interaction of the combination of colistin and gentamicin. Based on the FICI, it was observed that the colistin and gentamicin combination has a maximum synergistic effect (FIC <0.5) in 80% (4/5) for S. aureus isolates, whereas the maximum additive effect (FIC >0.5-4) was 100% (5/5) for P. aeruginosa and the minimum additive effect was 20% (1/5) for S. aureus isolates. Antagonism (FIC ≥ 4) was not observed in any combination between the strains used in the study. Both colistin and gentamicin alone or in combination were, however, ineffective against Candida spp.

Conclusion

The addition of colistin has an inhibitory effect on bacterial contamination that could be possibly caused by MDR strains and could potentially be considered as an additional additive in corneal storage media.

Polymyxins and Bacterial Membranes: A Review of Antibacterial Activity and Mechanisms of Resistance

Following their initial discovery in the 1940s, polymyxin antibiotics fell into disfavor due to their potential clinical toxicity, especially nephrotoxicity. However, the dry antibiotic development pipeline, together with the rising global prevalence of infections caused by multidrug-resistant (MDR) Gram-negative bacteria have both rejuvenated clinical interest in these polypeptide antibiotics. Parallel to the revival of their use, investigations into the mechanisms of action and resistance to polymyxins have intensified. With an initial known effect on biological membranes, research has uncovered the detailed molecular and chemical interactions that polymyxins have with Gram-negative outer membranes and lipopolysaccharide structure. In addition, genetic and epidemiological studies have revealed the basis of resistance to these agents. Nowadays, resistance to polymyxins in MDR Gram-negative pathogens is well elucidated, with chromosomal as well as plasmid-encoded, transferrable pathways. The aims of the current review are to highlight the important chemical, microbiological, and pharmacological properties of polymyxins, to discuss their mechanistic effects on bacterial membranes, and to revise the current knowledge about Gram-negative acquired resistance to these agents. Finally, recent research, directed towards new perspectives for improving these old agents utilized in the 21st century, to combat drug-resistant pathogens, is summarized.

Novel synthetic polymyxins kill Gram-positive bacteria

Background

Staphylococcus aureus, including 'superbug' MRSA, is a major cause of nosocomial infections. In the European Union, up to 171 200 new nosocomial MRSA infections are acquired annually, and in the USA S. aureus causes more deaths than HIV/AIDS and tuberculosis combined. MRSA is also the first group of pathogens that infect the pulmonary tract in young patients with cystic fibrosis.

Objectives

We describe two newly developed and synthesized colistin (polymyxin E)-inspired molecules.

Methods

A collection of several isolates of S. aureus [including MRSA and vancomycin-resistant S. aureus (VRSA)] was tested. To check the antimicrobial activity, we performed time-kill curves, growth curves, biofilm eradication, toxicity and isothermal titration calorimetry.

Results

Both peptides showed high antimicrobial activities (MIC 4 mg/L) and low relative toxicities (selectivity index close to 23).

Conclusions

Successful production of polymyxin-scaffold molecules active against S. aureus, both MRSA and VRSA, opens up new approaches to the treatment of these complicated infections.

Cannabidiol Is a Novel Modulator of Bacterial Membrane Vesicles

Membrane vesicles (MVs) released from bacteria participate in cell communication and host-pathogen interactions. Roles for MVs in antibiotic resistance are gaining increased attention and in this study we investigated if known anti-bacterial effects of cannabidiol (CBD), a phytocannabinoid from Cannabis sativa, could be in part attributed to effects on bacterial MV profile and MV release. We found that CBD is a strong inhibitor of MV release from Gram-negative bacteria (E. coli VCS257), while inhibitory effect on MV release from Gram-positive bacteria (S. aureus subsp. aureus Rosenbach) was negligible. When used in combination with selected antibiotics, CBD significantly increased the bactericidal action of several antibiotics in the Gram-negative bacteria. In addition, CBD increased antibiotic effects of kanamycin in the Gram-positive bacteria, without affecting MV release. CBD furthermore changed protein profiles of MVs released from E. coli after 1 h CBD treatment. Our findings indicate that CBD may pose as a putative adjuvant agent for tailored co-application with selected antibiotics, depending on bacterial species, to increase antibiotic activity, including via MV inhibition, and help reduce antibiotic resistance.

Non-invasive, ratiometric determination of intracellular pH in Pseudomonas species using a novel genetically encoded indicator

The ability of Pseudomonas species to thrive in all major natural environments (i.e. terrestrial, freshwater and marine) is based on its exceptional capability to adapt to physicochemical changes. Thus, environmental bacteria have to tightly control the maintenance of numerous physiological traits across different conditions. The intracellular pH (pHi ) homoeostasis is a particularly important feature, since the pHi influences a large portion of the biochemical processes in the cell. Despite its importance, relatively few reliable, easy-to-implement tools have been designed for quantifying in vivo pHi changes in Gram-negative bacteria with minimal manipulations. Here we describe a convenient, non-invasive protocol for the quantification of the pHi in bacteria, which is based on the ratiometric fluorescent indicator protein PHP (pH indicator for Pseudomonas). The DNA sequence encoding PHP was thoroughly adapted to guarantee optimal transcription and translation of the indicator in Pseudomonas species. Our PHP-based quantification method demonstrated that pHi is tightly regulated over a narrow range of pH values not only in Pseudomonas, but also in other Gram-negative bacterial species such as Escherichia coli. The maintenance of the cytoplasmic pH homoeostasis in vivo could also be observed upon internal (e.g. redirection of glucose consumption pathways in P. putida) and external (e.g. antibiotic exposure in P. aeruginosa) perturbations, and the PHP indicator was also used to follow dynamic changes in the pHi upon external pH shifts. In summary, our work describes a reliable method for measuring pHi in Pseudomonas, allowing for the detailed investigation of bacterial pHi homoeostasis and its regulation.

Peptidylarginine Deiminase Inhibitors Reduce Bacterial Membrane Vesicle Release and Sensitize Bacteria to Antibiotic Treatment

Outer membrane and membrane vesicles (OMV/MV) are released from bacteria and participate in cell communication, biofilm formation and host-pathogen interactions. Peptidylarginine deiminases (PADs) are phylogenetically conserved enzymes that catalyze post-translational deimination/citrullination of proteins, causing structural and functional changes in target proteins. PADs also play major roles in the regulation of eukaryotic extracellular vesicle release. Here we show phylogenetically conserved pathways of PAD-mediated OMV/MV release in bacteria and describe deiminated/citrullinated proteins in E. coli and their derived OMV/MVs. Furthermore, we show that PAD inhibitors can be used to effectively reduce OMV/MV release, both in Gram-negative and Gram-positive bacteria. Importantly, this resulted in enhanced antibiotic sensitivity of both E. coli and S. aureus to a range of antibiotics tested. Our findings reveal novel strategies for applying pharmacological OMV/MV-inhibition to reduce antibiotic resistance.

Inactivation of Polymyxin by Hydrolytic Mechanism

Polymyxins are nonribosomal peptide antibiotics used as the last-resort drug for treatment of multidrug-resistant Gram-negative bacteria. However, strains that are resistant to polymyxins have emerged in many countries. Although several mechanisms for polymyxin resistance have been well described, there is little knowledge on the hydrolytic mechanism of polymyxin. Here, we identified a polymyxin-inactivating enzyme from Bacillus licheniformis strain DC-1 which was produced and secreted into the medium during entry into stationary phase. After purification, sequencing, and heterologous expression, we found that the alkaline protease Apr is responsible for inactivation of polymyxins. Analysis of inactivation products demonstrated that Apr cleaves polymyxin E at two peptide bonds: one is between the tripeptide side chain and the cyclic heptapeptide ring, the other between l-Thr and l-α-γ-diaminobutyric acid (l-Dab) within the cyclic heptapeptide ring. Apr is highly conserved among several genera of Gram-positive bacteria, including Bacillus and Paenibacillus It is noteworthy that two peptidases S8 from Gram-negative bacteria shared high levels of sequence identity with Apr. Our results indicate that polymyxin resistance may result from inactivation of antibiotics by hydrolysis.

Enhanced NADH Metabolism Involves Colistin-Induced Killing of Bacillus subtilis and Paenibacillus polymyxa

The commonly believed mechanism of colistin against Gram-negative bacteria is to cause cell membrane lysis, whereas the mechanism of colistin against Gram-positive bacteria is extremely fragmented. In this study, we found that colistin treatment on Bacillus subtilis WB800, Paenibacillus polymyxa C12 and Paenibacillus polymyxa ATCC842 enhances not only the activities of α-ketoglutaric dehydrogenase and malate dehydrogenase in tricarboxylic acid (TCA) cycle, but also the relative expression levels of their encoding genes. Additionally, the oxaloacetate concentration also increases. Interestingly, the analysis of the relative expression of genes specific for respiratory chain showed that colistin treatment stimulates the respiratory chain in Gram-positive bacteria. Accordingly, the NAD⁺/NADH ratio increases and the oxidative level is then boosted up. As a result, the intensive oxidative damages are induced in Gram-positive bacteria and cells are killed. Notably, both rotenone and oligomycin, respectively, inhibiting NADH dehydrogenase and phosphorylation on respiratory chain can downgrade oxidative stress formation, thus alleviating the colistin-induced killing of Gram-positive cells. Besides, thiourea-based scavenging for reactive oxygen species also rescues the colistin-subjected cells. These data collectively demonstrate that colistin stimulates both TCA cycle and respiratory chain in Gram-positive bacteria, leading to the enhancement of NADH metabolism and resulting in the generation of oxidative damages in Gram-positive cells. Our studies provide a better understanding of antibacterial mechanism of colistin against Gram-positive bacteria, which is important for knowledge on bacterial resistance to colistin happening via the inhibition of respiratory chain and manipulation of its production.

Enhanced Production of Polymyxin E in Paenibacillus polymyxa by Replacement of Glucose by Starch

Polymyxin E or colistin, produced by Paenibacillus polymyxa, is an important antibiotic against Gram-negative pathogens. The objective of this study is to evaluate the effect of starch in fermentation medium on colistin biosynthesis in P. polymyxa. The results indicated that replacement of glucose by starch stimulated colistin production and biosynthesis rate. Overall, the stimulation extent was starch concentration-dependent. As expected, addition of starch induced the expression of amyE encoding amylase and increased amylase activity in fermentation solution. Additionally, replacement of glucose by starch resulted in residue reducing sugar and pH of fermentation mixture low relative to glucose as the sole sugar source. At the molecular level, it was found that replacement of glucose by starch has enhanced the relative expression level of ccpA encoding catabolite control protein A. Therefore, the repression of starch utilization by glucose could be probably relieved. In addition, use of starch stimulated the expression of regulatory gene spo0A but repressed the expression of another regulatory gene abrB. As a result, the expression of genes directly involved in colistin biosynthesis and secretion increased, indicating that at the transcriptional level spo0A and abrB played opposite roles in regulating colistin biosynthesis in P. polymyxa. Taken together, our data demonstrated that starch instead of glucose can promote colistin production probably by affecting the expression of colistin biosynthesis-related genes, as well as reducing the repression of glucose to a secondary metabolic product.

Reversing Antimicrobial Resistance in Multidrug-Resistant Klebsiella pneumoniae of Clinical Origin Using 1-(1-Naphthylmethyl)-Piperazine

Eleven clinical Klebsiella pneumoniae fluoroquinolone-resistant isolates were tested to access the potential of adjuvant therapies to reduce antimicrobial resistance using fixed concentrations of the chemosensitizers chlorpromazine (CPZ), thioridazine (TZ), phenylalanine-arginine-β-naphthylamide (PAβN), and 1-(1-naphthylmethyl)-piperazine-(NMP) with varying concentrations of antimicrobial agents nalidixic acid (NAL), ciprofloxacin (CIP), moxifloxacin (MXF), tetracycline (TET), and chloramphenicol (CHL). Ethidium bromide dye was used together with the chemosensitizers to investigate permeabilization effects. NMP was assessed for its capacity to reduce the mass of biofilm alone and in combination with CIP and MXF. Of the selected chemosensitizers, NMP exhibited the greatest capacity to reverse resistance and inhibit efflux, based on the concentrations tested. Susceptibility to antimicrobial agents including (fluoro)quinolones, TET, and CHL were found to be increased in the presence of NMP, in a concentration-dependent manner. PAβN also demonstrated similar effects when combined with the chemosensitizers tested. In the case of half of the isolates studied, NMP alone reduced preformed biofilm biomass. Combinations of latter along with CIP or MXF were also found to reduce the mass of preformed biofilm, in the case of only some of the bacterial isolates. The capacity of NMP to reduce antimicrobial resistance could be of relevance as a strategy to limit bacterial colonization on abiotic surfaces.

Fungal silver nanoparticles: synthesis, application and challenges

PURPOSE

This paper aims to summarize recent developments regarding the synthesis, application and challenges of fungal AgNPs. Possible methods to overcome the challenge of synthesis and reduce the toxicity of AgNPs have been discussed.

MATERIALS AND METHODS

This review consults and summary a large number of papers.

RESULTS

Silver nanoparticles (AgNPs) have great potential in many areas, as they possess multiple novel characteristics. Conventional methods for AgNPs biosynthesis involve chemical agents, causing environmental toxicity and high energy consumption. Fungal bioconversion is a simple, low-cost and energy-efficient biological method, which could successfully be used for AgNPs synthesis. Fungi can produce enzymes that act as both reducing and capping agents, to form stable and shape-controlled AgNPs.

CONCLUSIONS

AgNPs have great potential in the medical and food industries, due to their antimicrobial, anticancer, anti-HIV, and catalytic activities. However, the observed in vitro and in vivo toxicity poses considerable challenges in the synthesis and application of AgNPs.

Oxidative Stress Induced by Polymyxin E Is Involved in Rapid Killing of Paenibacillus polymyxa

Historically, the colistin has been thought to kill bacteria through membrane lysis. Here, we present an alternative mechanism that colistin induces rapid Paenibacillus polymyxa death through reactive oxygen species production. This significantly augments our understanding of the mechanism of colistin action, which is critical knowledge toward the yield development of colistin in the future.

Carbonyl Cyanide m-Chlorophenylhydrazine (CCCP) Reverses Resistance to Colistin, but Not to Carbapenems and Tigecycline in Multidrug-Resistant Enterobacteriaceae

Background: Carbapenems (CAR), colistin (CST), and tigecycline (TGC) alone or in combination therapy has become the last-resort antibiotics for treating infections caused by multidrug resistant (MDR) bacteria. However, resistance to these reserve antibiotics are increasingly being reported worldwide. Hence, the quest to find other agents that will synergistically restore the efficacy of these antibiotics have increased. Methods: Sixty-three clinical Enterobacteriaceae isolates comprising of Klebsiella pneumoniae (n = 24), Enterobacter spp. (n = 15), Serratia marcescens (n = 12), Citrobacter freundii (n = 8), Escherichia coli (n = 2), and K. oxytoca/michiganensis (n = 2) with known carbapenem resistance mechanisms and undescribed CST and TGC resistance mechanisms were subjected to broth microdilution and meropenem (MEM) disc synergy test in the presence and absence of carbonyl cyanide m-chlorophenylhydrazine (CCCP), a H⁺ conductor (protonophore). Results and conclusions: Susceptibility to MEM, imipenem (IMP), CST, and TGC was found in only 2, 0, 17, and 9 isolates respectively. Addition of CCCP reversed resistance to CST, TGC, IMP, and MEM in 44, 3, 0, and 0 isolates respectively; CST had the highest mean minimum inhibitory concentration (MIC) fold change (193.12; p < 0.0001) post CCCP compared to that of MEM (1.70), IMP (1.49) and TGC (1.16). Eight isolates tested positive for the MEM-CCCP disc synergy test. We concluded that CCCP reverse CST resistance in CST-resistant Enterobacteriaceae. Although CCCP is an experimental agent with no therapeutic value clinically, further studies are necessary to decipher the mechanisms underlying the CST-CCCP synergy to inform the development of adjuvants that could be therapeutically effective in CST-resistant infections.