Colistin and its role in the Era of antibiotic resistance: an extended review (2000-2019)

Novel nitroxoline derivative combating resistant bacterial infections through outer membrane disruption and competitive NDM-1 inhibition

ABSTRACTNew Delhi metallo-β-lactamase-1 (NDM-1) has rapidly disseminated worldwide, leading to multidrug resistance and worse clinical prognosis. Designing and developing effective NDM-1 inhibitors is a critical and urgent challenge. In this study, we constructed a library of long-lasting nitroxoline derivatives and identified ASN-1733 as a promising dual-functional antibiotic. ASN-1733 can effectively compete for Ca2+ on the bacterial surface, causing the detachment of lipopolysaccharides (LPS), thereby compromising the outer membrane integrity and permeability and exhibiting broad-spectrum bactericidal activity. Moreover, ASN-1733 demonstrated wider therapeutic applications than nitroxoline in mouse sepsis, thigh and mild abdominal infections. Furthermore, ASN-1733 can effectively inhibit the hydrolytic capability of NDM-1 and exhibits synergistic killing effects in combination with meropenem against NDM-1 positive bacteria. Mechanistic studies using enzymatic experiments and computer simulations revealed that ASN-1733 can bind to key residues on Loop10 of NDM-1, hindering substrate entry into the enzyme's active site and achieving potent inhibitory activity (Ki = 0.22 µM), even in the presence of excessive Zn2+. These findings elucidate the antibacterial mechanism of nitroxoline and its derivatives, expand their potential application in the field of antibacterial agents and provide new insights into the development of novel NDM-1 inhibitors.

Expression and characterization of the new antimicrobial peptide AP138L-arg26 anti Staphylococcus aureus

The low activity and yield of antimicrobial peptides (AMPs) are pressing problems. The improvement of activity and yield through modification and heterologous expression, a potential way to solve the problem, is a research hot-pot. In this work, a new plectasin-derived variant L-type AP138 (AP138L-arg26) was constructed for the study of recombination expression and druggablity. As a result, the total protein concentration of AP138L-arg26 was 3.1 mg/mL in Pichia pastoris X-33 supernatant after 5 days of induction expression in a 5-L fermenter. The recombinant peptide AP138L-arg26 has potential antibacterial activity against selected standard and clinical Gram-positive bacteria (G+, minimum inhibitory concentration (MIC) 2-16 µg/mL) and high stability under different conditions (temperature, pH, ion concentration) and 2 × MIC of AP138L-arg26 could rapidly kill Staphylococcus aureus (S. aureus) (> 99.99%) within 1.5 h. It showed a high safety in vivo and in vivo and a long post-antibiotic effect (PAE, 1.91 h) compared with vancomycin (1.2 h). Furthermore, the bactericidal mechanism was revealed from two dimensions related to its disruption of the cell membrane resulting in intracellular potassium leakage (2.5-fold higher than control), and an increase in intracellular adenosine triphosphate (ATP), and reactive oxygen species (ROS), the decrease of lactate dehydrogenase (LDH) and further intervening metabolism in S. aureus. These results indicate that AP138L-arg26 as a new peptide candidate could be used for more in-depth development in the future. KEY POINTS: • The AP138L-arg26 was expressed in the P. pastoris expression system with high yield • The AP138 L-arg26 showed high stability and safety in vitro and in vivo • The AP138L-arg26 killed S. aureus by affecting cell membranes and metabolism.

Global epidemiology and genetic diversity of mcr-positive Klebsiella pneumoniae: A systematic review and genomic analysis

The rapid increase of mcr-positive Klebsiella pneumoniae (K. pneumoniae) has received considerable attention and poses a major public health concern. Here, we systematically analyzed the global distribution of mcr-positive K. pneumoniae isolates based on published articles as well as publicly available genomes. Combining strain information from 78 articles and 673 K. pneumoniae genomes, a total of 1000 mcr-positive K. pneumoniae isolates were identified. We found that mcr-positive K. pneumoniae has disseminated widely worldwide, especially in Asia, with a higher diversity of sequence types (STs). These isolates were disseminated in 57 countries and were associated with 12 different hosts. Most of the isolates were found in China and were isolated from human sources. Moreover, MLST analysis showed that ST15 and ST11 accounted for the majority of mcr-positive K. pneumoniae, which deserve sustained attention in further surveillance programs. mcr-1 and mcr-9 were the dominant mcr variants in mcr-positive K. pneumoniae. Furthermore, a Genome-wide association study (GWAS) demonstrated that mcr-1- and mcr-9-producing genomes exhibited different antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs), thereby indicating a distinct evolutionary path. Notably, the phylogenetic analysis suggested that certain mcr-positive K. pneumoniae genomes from various geographical areas and hosts harbored a high degree of genetic similarities (<20 SNPs), suggesting frequent cross-region and cross-host clonal transmission. Overall, our results emphasize the significance of monitoring and exploring the transmission and evolution of mcr-positive K. pneumoniae in the context of "One health".

The multifaceted roles of phosphoethanolamine-modified lipopolysaccharides: from stress response and virulence to cationic antimicrobial resistance

SUMMARYLipopolysaccharides (LPS) are an integral part of the outer membrane of Gram-negative bacteria and play essential structural and functional roles in maintaining membrane integrity as well as in stress response and virulence. LPS comprises a membrane-anchored lipid A group, a sugar-based core region, and an O-antigen formed by repeating oligosaccharide units. 3-Deoxy-D-manno-octulosonic acid-lipid A (Kdo2-lipid A) is the minimum LPS component required for bacterial survival. While LPS modifications are not essential, they play multifaceted roles in stress response and host-pathogen interactions. Gram-negative bacteria encode several distinct LPS-modifying phosphoethanolamine transferases (PET) that add phosphoethanolamine (pEtN) to lipid A or the core region of LPS. The pet genes differ in their genomic locations, regulation mechanisms, and modification targets of the encoded enzyme, consistent with their various roles in different growth niches and under varied stress conditions. The discovery of mobile colistin resistance genes, which represent lipid A-modifying pet genes that are encoded on mobile elements and associated with resistance to the last-resort antibiotic colistin, has led to substantial interest in PETs and pEtN-modified LPS over the last decade. Here, we will review the current knowledge of the functional diversity of pEtN-based LPS modifications, including possible roles in niche-specific fitness advantages and resistance to host-produced antimicrobial peptides, and discuss how the genetic and structural diversities of PETs may impact their function. An improved understanding of the PET group will further enhance our comprehension of the stress response and virulence of Gram-negative bacteria and help contextualize host-pathogen interactions.

Intravenous combined with aerosolised polymyxins versus intravenous polymyxins monotherapy for ventilator-associated pneumonia: a systematic review and meta-analysis

Polymyxins was applied to treat ventilator associated pneumonia (VAP) caused by carbapenem-resistant Gram-negative bacteria (CR-GNB) via different administration routes. The potential benefits of aerosolised polymyxins as adjunctive treatment for patients still were contradictory. This review assessed the safety and efficacy of intravenous combined with aerosolised polymyxins versus intravenous polymyxins monotherapy in patients with VAP caused by CR-GNB. Two reviewers independently evaluated and extracted date from Pubmed, Embase, Cochrane library and Web of science. The primary outcome was all-cause mortality and secondary outcomes included clinical cure rate, clinical improvement rate, microbiological eradication and nephrotoxicity. Differences for dichotomous outcomes were expressed as odds ratios (OR) with 95% confidence intervals (CI). Eleven eligible studies were included. The results showed that compared with intravenous polymyxins monotherapy, intravenous plus aerosolised polymyxins therapy significantly reduced all-cause mortality rate (OR = 0.75, 95% CI 0.57 - 0.99, P = 0.045) and improved clinical improvement rate (OR = 1.62, 95% CI 1.02 - 2.60, P = 0.043) and microbial eradication rate (OR = 2.07, 95% CI 1.40 - 3.05, P = 0.000). However, there were no significant difference in terms of clinical cure rate (OR = 1.59, 95% CI 0.96 - 2.63, P = 0.072) and nephrotoxicity (OR = 1.14, 95% CI 0.80 - 1.63, P = 0.467) for intravenous plus aerosolised polymyxins therapy. Subgroup analysis revealed that the clinical improvement rate was improved significantly in case-control studies. Aerosolised polymyxins maybe a useful adjunct to intravenous polymyxins for CR-GNB VAP patients.

Plumbagin enhances antimicrobial and anti-biofilm capacities of chlorhexidine against clinical Klebsiella pneumoniae while reducing resistance mutations

With the widespread misuse of disinfectants, the clinical susceptibility of Klebsiella pneumoniae (K. pneumoniae) to chlorhexidine (CHX) has gradually diminished, posing significant challenges to clinical disinfection and infection control. K. pneumoniae employs overexpression of efflux pumps and the formation of thick biofilms to evade the lethal effects of CHX. Plumbagin (PLU) is a natural plant extract that enhances membrane permeability and reduces proton motive force. In this study, we elucidated the synergistic antimicrobial activity of PLU in combination with CHX, effectively reducing the MIC of CHX against K. pneumoniae to 1 µg/mL and below. Crucially, through crystal violet staining and confocal laser scanning microscopy live/dead staining, we discovered that PLU significantly enhances the anti-biofilm capability of CHX. Mechanistically, experiments involving membrane permeability, alkaline phosphatase leakage, reactive oxygen species, and RT-qPCR suggest that the combination of PLU and CHX improves the permeability of bacterial inner and outer membranes, promotes bacterial oxidative stress, and inhibits oqxA/B efflux pump expression. Furthermore, we conducted surface disinfection experiments on medical instruments to simulate clinical environments, demonstrating that the combination effectively reduces bacterial loads by more than 3 log10 CFU/mL. Additionally, results from resistance mutation frequency experiments indicate that combined treatment reduces the generation of resistant mutants within the bacterial population. In summary, PLU can serve as an adjuvant, enhancing the anti-biofilm capability of CHX and reducing the occurrence of resistance mutations, thereby extending the lifespan of CHX.IMPORTANCEAs disinfectants are extensively and excessively utilized worldwide, clinical pathogens are progressively acquiring resistance against these substances. However, high concentrations of disinfectants can lead to cross-resistance to antibiotics, and concurrent use of different disinfectants can promote bacterial resistance mutations and facilitate the horizontal transfer of resistance genes, which poses significant challenges for clinical treatment. Compared with the lengthy process of developing new disinfectants, enhancing the effectiveness of existing disinfectants with natural plant extracts is important and meaningful. CHX is particularly common and widely used compared with other disinfectants. Meanwhile, Klebsiella pneumoniae, as a clinically significant pathogen, exhibits high rates of resistance and pathogenicity. Previous studies and our data indicate a significant decrease in the sensitivity of clinical K. pneumoniae to CHX, highlighting the urgent need for novel strategies to address this issue. In light of this, our research is meaningful.

Cinnamaldehyde and baicalin reverse colistin resistance in Enterobacterales and Acinetobacter baumannii strains

PURPOSE

Colistin is used as a last resort antibiotic against infections caused by multidrug-resistant gram-negative bacteria, especially carbapenem-resistant bacteria. However, colistin-resistance in clinical isolates is becoming more prevalent. Cinnamaldehyde and baicalin, which are the major active constituents of Cinnamomum and Scutellaria, have been reported to exhibit antibacterial properties. The aim of this study was to evaluate the capacity of cinnamaldehyde and baicalin to enhance the antibiotic activity of colistin in Enterobacterales and Acinetobacter baumannii strains.

METHODS

The MICs of colistin were determined with and without fixed concentrations of cinnamaldehyde and baicalin by the broth microdilution method. The FIC indices were also calculated. In addition, time-kill assays were performed with colistin alone and in combination with cinnamaldehyde and baicalin to determine the bactericidal action of the combinations. Similarly, the effects of L-arginine, L-glutamic acid and sucrose on the MICs of colistin combined with cinnamaldehyde and baicalin were studied to evaluate the possible effects of these compounds on the charge of the bacterial cell- wall.

RESULTS

At nontoxic concentrations, cinnamaldehyde and baicalin partially or fully reversed resistance to colistin in Enterobacterales and A. baumannii. The combinations of the two compounds with colistin had bactericidal or synergistic effects on the most resistant strains. The ability of these agents to reverse colistin resistance could be associated with bacterial cell wall damage and increased permeability.

CONCLUSION

Cinnamaldehyde and baicalin are good adjuvants for the antibiotic colistin against Enterobacterales- and A. baumannii-resistant strains.

Advances in the clinical treatment of multidrug-resistant pathogens using polymyxins

OBJECTIVES

Polymyxins are a vital class of antibiotics used to combat multidrug-resistant Gram-negative bacteria. However, their use is limited due to potential nephrotoxicity and the availability of alternative antibiotics. This review aims to examine the properties of polymyxins and the clinical advances in their use for treating infections caused by carbapenem-resistant Gram-negative bacteria (CR-GNB).

METHODS

This review analyses literature on polymyxin properties and various clinical approaches, including intravenous drip infusion, nebulized or dry powder inhalation, and ointment application. Treatment efficacy in terms of bacterial eradication, cure rate and mortality rate are reviewed and evaluated.

RESULTS

Polymyxins have been reintroduced to treat critical infections due to the increasing prevalence of CR-GNB. Clinical trials and studies have confirmed that polymyxins can effectively treat CR-GNB infections when the formulation and administration are appropriate, with acceptable levels of nephrotoxicity.

CONCLUSIONS

In the future, the development of polymyxin formulations will aim to improve their clinical effectiveness while reducing toxicity and side effects and preventing the emergence of polymyxin-resistant strains. Enhanced efficacy and minimized potential side effects can be achieved by developing new polymyxin-delivery systems that provide a smart and controlled release or customized patient administration.

Immunomodulator AS101 restores colistin susceptibility of clinical colistin-resistant Escherichia coli and Klebsiella pneumoniae in vitro and in vivo

OBJECTIVES

Colistin (COL) was once considered to be the last line of defence against multidrug-resistant bacteria belonging to the family Enterobacteriaceae. Due to the misuse of COL, COL-resistant (COL-R) Enterobacteriaceae have emerged. To address this clinical issue and combat COL resistance, novel approaches are urgently needed.

METHODS

In this study, the in vitro and in vivo antimicrobial and antibiofilm effects of the immunomodulator AS101 were investigated in combination with COL against COL-R Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae).

RESULTS

Checkerboard assay, time-kill assay, and scanning electron microscopy confirmed the in vitro antimicrobial phenotype, whereas, crystal violet staining and multidimensional confocal laser scanning microscopy with live/dead staining confirmed the antibiofilm capability of the combination therapy. Moreover, the Galleria mellonella infection model and the mouse infection model indicated the high in vivo efficacy of the combination therapy. Additionally, cytotoxicity experiments performed using human kidney-derived HK-2 cells and haemolysis assays performed using human erythrocytes collectively demonstrated safety at effective combination concentrations. Furthermore, quantification of the expression of inflammatory cytokines via enzyme-linked immunosorbent assay confirmed the anti-inflammatory advantage of combination therapy. At the mechanistic level, changes in outer and inner membrane permeability and accumulation of ROS levels, which might be potential mechanisms for synergistic antimicrobial effects.

CONCLUSIONS

This study found that AS101 can restore COL susceptibility in clinical COL-R E. coli and K. pneumoniae and also has synergistic antibiofilm and anti-inflammatory capabilities. This study provided a novel strategy to combat clinical infections caused by COL-R E. coli and K. pneumoniae.

Cyclic peptides: A powerful instrument for advancing biomedical nanotechnologies and drug development

Cyclic peptides have emerged as an essential tool in the advancement of biomedical nanotechnologies, offering unique structural and functional advantages over linear peptides. This review article aims to highlight the roles of cyclic peptides in the development of biomedical fields, with a particular focus on their application in drug discovery and delivery. Cyclic peptides exhibit exceptional stability, bioavailability, and binding specificity, making them ideal candidates for therapeutic and diagnostic applications. We explore the synthesis and design strategies that enable the precise control of cyclic peptide structures, leading to enhanced performance in targeting specific cellular pathways. The article also highlights recent breakthroughs in the use of cyclic peptides for creating innovative drug delivery systems, including nanoparticle conjugates and peptide-drug conjugates, which have shown promise in improving the efficacy and safety profiles of existing traditional treatments. The integration of cyclic peptides into nanotechnological frameworks holds significant promise for addressing unmet medical needs, providing a foundation for future advancements in personalized medicine and targeted drug delivery.

Mechanism of synergistic action of colistin with resveratrol and baicalin against mcr-1-positive Escherichia coli

The rising incidence of colistin (COL) resistance poses a significant challenge, undermining the therapeutic efficacy of COL against life-threatening bacterial infections. Therefore, the urgent identification and development of new therapeutics are imperative. It has been proven that combinations of antibiotics and promising non-antibiotic agents could be a potential strategy to combat infections caused by MDR pathogens. Due to various antimicrobial properties, medicinal plants have attracted significant attention, which could be promising adjuvant. In this study, we investigated the synergistic effects of combining COL with resveratrol (RST) and baicalin (BAI) against mcr-1-positive Escherichia coli through antibiotic susceptibility testing, checkerboard method and time-killing assays. The mechanisms of combination treatment were analyzed using SEM, fluorometric assays and transcriptome analysis. The molecular docking assay was conducted to elucidate potential interactions between RST, BAI and the MCR-1 protein. Finally, we assessed the in vivo efficacy of combination against mcr-1-positive Escherichia coli. The results demonstrated that the combination of RST, BAI and COL showed significant synergistic activity both in vitro and in vivo. Further mechanistic study revealed that the combination could increase the membrane-damaging ability of COL, disrupt the homeostasis of proton motive force (PMF), inhibit the activity of efflux pumps and impair ATP supply. The molecular docking revealed that RST and BAI could bind to MCR-1 stably, indicating the combination of RST and BAI may be an effective MCR-1 inhibitor. Our findings demonstrated that the combination of RST and BAI might be potential COL adjuvant, providing an alternative approach to address mcr-1-positive Escherichia coli infections.

Exploring Structure-Activity Relationships and Modes of Action of Laterocidine

A significant increase in life-threatening infections caused by Gram-negative "superbugs" is a serious threat to global health. With a dearth of new antibiotics in the developmental pipeline, antibiotics with novel mechanisms of action are urgently required to prevent a return to the preantibiotic era. A key strategy to develop novel anti-infective treatments is to discover new natural scaffolds with distinct mechanisms of action. Laterocidine is a unique cyclic lipodepsipeptide with activity against multiple problematic multidrug-resistant Gram-negative pathogens, including Pseudomonas aeruginosa, Acinetobacter baumannii, and Enterobacterales. Here, we developed a total chemical synthesis methodology for laterocidine and undertook systematic structure-activity relationship studies with chemical biology and NMR. We discovered important structural features that drive the antimicrobial activity of laterocidine, leading to the discovery of an engineered peptide surpassing the efficacy of the original peptide. This engineered peptide demonstrated complete inhibition of the growth of a polymyxin-resistant strain of Pseudomonas aeruginosa in static time-kill experiments.

Antimicrobial peptide-based strategies to overcome antimicrobial resistance

Antibiotic resistance has emerged as a global threat, rendering the existing conventional treatment strategies ineffective. In view of this, antimicrobial peptides (AMPs) have proven to be potent alternative therapeutic interventions with a wide range of applications in clinical health. AMPs are small peptides produced naturally as a part of the innate immune responses against a broad range of bacterial, fungal and viral pathogens. AMPs present a myriad of advantages over traditional antibiotics, including their ability to target multiple sites, reduced susceptibility to resistance development, and high efficacy at low doses. These peptides have demonstrated notable potential in inhibiting microbes resistant to traditional antibiotics, including the notorious ESKAPE pathogens, recognized as the primary culprits behind nosocomial infections. AMPs, with their multifaceted benefits, emerge as promising candidates in the ongoing efforts to combat the escalating challenges posed by antibiotic resistance. This in-depth review provides a detailed discussion on AMPs, encompassing their classification, mechanism of action, and diverse clinical applications. Focus has been laid on combating newly emerging drug-resistant organisms, emphasizing the significance of AMPs in mitigating this pressing challenge. The review also illuminates potential future strategies that may be implemented to improve AMP efficacy, such as structural modifications and using AMPs in combination with antibiotics and matrix-inhibiting compounds.

Screening a Compound Library to Identify Additives That Boost Cytochrome P450 Enzyme Function in Vascularised Liver Spheres

To accurately study human organ function and disease 'in the dish', it is necessary to develop reliable cell-based models that closely track human physiology. Our interest lay with the liver, which is the largest solid organ in the body. The liver is a multifunctional and highly regenerative organ; however, severe liver damage can have dire consequences for human health. A common cause of liver damage is adverse reactions to prescription drugs. Therefore, the development of predictive liver models that capture human drug metabolism patterns is required to optimise the drug development process. In our study, we aimed to identify compounds that could improve the metabolic function of stem cell-derived liver tissue. Therefore, we screened a compound library to identify additives that improved the maturity of in vitro-engineered human tissue, with the rationale that by taking such an approach, we would be able to fine-tune neonatal and adult cytochrome P450 metabolic function in stem cell-derived liver tissue.

Carbapenem-Resistant Enterobacterales in the Western Balkans: Addressing Gaps in European AMR Surveillance Map

In the context of global efforts to combat antimicrobial resistance (AMR), the importance of comprehensive AMR data is more crucial than ever. AMR surveillance networks, such as the European Antimicrobial Resistance Surveillance Network (EARS-Net) and the Central Asian and European Surveillance of Antimicrobial Resistance (CAESAR), support member states in obtaining high-quality AMR data. Nevertheless, data gaps persist in some countries, including those in the Western Balkans (WBs), a region with high AMR rates. This review analyzed existing research on carbapenem-resistant Enterobacterales (CRE) to better understand the AMR landscape in the WB countries. The most prevalent CRE was Klebsiella pneumoniae, followed by Escherichia coli, Enterobacter cloacae, and Proteus mirabilis, with sporadic cases of Morganella morganii, Providencia spp., Klebsiella oxytoca, and Citrobacter sedlakii. Carbapenemase production was identified as the most common mechanism of carbapenem resistance, but other resistance mechanisms were not investigated. An increasing trend in carbapenem resistance has been observed over the last decade, alongside a shift in carbapenemase epidemiology from the NDM type in 2013-2014 to the OXA-48 type in recent years. Few studies have applied whole-genome sequencing for CRE analysis, which has demonstrated the spread of resistance determinants across different niches and over time, emphasizing the importance of molecular-based research. The overall low number of studies in the WB countries can be attributed to limited resources, highlighting the need for enhanced support in education, training, technology, and equipment to improve data collection and evaluation.

Antibiotic-Polyphosphate Nanocomplexes: A Promising System for Effective Biofilm Eradication

Purpose

The eradication of bacterial biofilms poses an enormous challenge owing to the inherently low antibiotic susceptibility of the resident microbiota. The complexation of antibiotics with polyphosphate can substantially improve antimicrobial performance.

Methods

Nanoparticular complexes of the model drug colistin and polyphosphate (CP-NPs) were developed and characterized in terms of their particle size and morphology, polydispersity index (PDI), zeta potential, and cytotoxicity. Enzyme-triggered monophosphate and colistin release from the CP-NPs was evaluated in the presence of alkaline phosphatase (AP). Subsequently, antimicrobial efficacy was assessed by inhibition experiments on planktonic cultures, as well as time-kill assays on biofilms formed by the model organism Micrococcus luteus.

Results

The CP-NPs exhibited a spherical morphology with particle sizes <200 nm, PDI <0.25, and negative zeta potential. They showed reduced cytotoxicity toward two human cell lines and significantly decreased hemotoxicity compared with native colistin. Release experiments with AP verified the enzymatic cleavage of polyphosphate and subsequent release of monophosphate and colistin from CP-NPs. Although CP-NPs were ineffective against planktonic M. luteus cultures, they showed major activity against bacterial biofilms, outperforming native colistin treatment. Strongly elevated AP levels in the biofilm state were identified as a potential key factor for the observed findings.

Conclusion

Accordingly, polyphosphate-based nanocomplexes represent a promising tool to tackle bacterial biofilm.

Valnemulin restores colistin sensitivity against multidrug-resistant gram-negative pathogens

Colistin is one of the last-resort antibiotics in treating infections caused by multidrug-resistant (MDR) pathogens. Unfortunately, the emergence of colistin-resistant gram-negative strains limit its clinical application. Here, we identify an FDA-approved drug, valnemulin (Val), exhibit a synergistic effect with colistin in eradicating both colistin-resistant and colistin-susceptible gram-negative pathogens both in vitro and in the mouse infection model. Furthermore, Val acts synergistically with colistin in eliminating intracellular bacteria in vitro. Functional studies and transcriptional analysis confirm that the combinational use of Val and colistin could cause membrane permeabilization, proton motive force dissipation, reduction in intracellular ATP level, and suppression in bacterial motility, which result in bacterial membrane disruption and finally cell death. Our findings reveal the potential of Val as a colistin adjuvant to combat MDR bacterial pathogens and treat recalcitrant infections.

Pharmacokinetics and pharmacodynamics of bacteriophage therapy: a review with a focus on multidrug-resistant Gram-negative bacterial infections

SUMMARYDespite the early recognition of their therapeutic potential and the current escalation of multidrug-resistant (MDR) pathogens, the adoption of bacteriophages into mainstream clinical practice is hindered by unfamiliarity with their basic pharmacokinetic (PK) and pharmacodynamic (PD) properties, among others. Given the self-replicative nature of bacteriophages in the presence of host bacteria, the adsorption rate, and the clearance by the host's immunity, their PK/PD characteristics cannot be estimated by conventional approaches, and thus, the introduction of new considerations is required. Furthermore, the multitude of different bacteriophage types, preparations, and treatment schedules impedes drawing general conclusions on their in vivo PK/PD features. Additionally, the drawback of acquired bacteriophage resistance of MDR pathogens with clinical and environmental implications should be taken into consideration. Here, we provide an overview of the current state of the field of PK and PD of bacteriophage therapy with a focus on its application against MDR Gram-negative infections, highlighting the potential knowledge gaps and the challenges in translation from the bench to the bedside. After reviewing the in vitro PKs and PDs of bacteriophages against the four major MDR Gram-negative pathogens, Klebsiella pneumoniae, Acinetobacter baumannii complex, Pseudomonas aeruginosa, and Escherichia coli, specific data on in vivo PKs (tissue distribution, route of administration, and basic PK parameters in animals and humans) and PDs (survival and reduction of bacterial burden in relation to the route of administration, timing of therapy, dosing regimens, and resistance) are summarized. Currently available data merit close scrutiny, and optimization of bacteriophage therapy in the context of a better understanding of the underlying PK/PD principles is urgent to improve its therapeutic effect and to minimize the occurrence of bacteriophage resistance.

Convergence of mcr-1 and broad-spectrum β-lactamase genes in Escherichia coli strains from the environmental sector

The mcr-type gene encodes the main plasmid-mediated mechanism of colistin resistance and has been reported in several bacterial species obtained from different sources. Anthropogenic activities in the environment favor the evolution of antimicrobial resistance. Indeed, mcr-1-positive Escherichia coli strains were susceptible to non-polymyxins antimicrobials, but now emerging as multidrug-resistant (MDR) lineages. In this regard, hundreds of surface water and agricultural soil samples were screened for the presence of E. coli carrying the mcr-type genes and mcr-1-positive strains were subjected to in-depth genomic analysis. Almost all colistin-resistant strains were classified as MDR, highlighting those obtained from soils that showed resistance to extended-spectrum cephalosporins and carbapenems. International and high-risk clones of E. coli were identified, with ST10 and ST1720 shared between water and soil samples. Resistome analysis showed a broad resistome (AMR, metal tolerance, and biocide resistance). The mcr-1.1 and mcr-1.26 allelic variants were detected on IncX4 and IncI2 plasmids. Curiously, mcr-1-positive E. coli strains from agricultural soils harbored plasmid-mediated blaCTX-M-1, blaCTX-M-8, or blaKPC-2 genes. Virulome analysis demonstrated traits of a high putative virulence potential, with the presence of extraintestinal pathogenic E. coli. Comparative analysis revealed the persistence and dissemination of plasmid-mediated antimicrobial resistance genes in genetically diversity E. coli strains at the human-animal-environmental interface. These findings demonstrate a possible emerging AMR trend with the convergence of resistance to colistin and broad-spectrum β-lactams in environmental-derived E. coli strains.

Topical adjunctive treatment with flagellin augments pulmonary neutrophil responses and reduces bacterial dissemination in multidrug-resistant K. pneumoniae infection

Objective

Antimicrobial resistance is an emerging problem and multi-drug resistant (MDR) Klebsiella pneumoniae (K. pneumoniae) represents an enormous risk of failing therapy in hospital-acquired pneumonia. The current study aimed to determine the immunomodulatory effect of topical flagellin in addition to antibiotic treatment during respiratory infection evoked by hypervirulent antibiotic-susceptible and antibiotic-resistant K. pneumoniae in mice.

Methods

C57BL6 mice were inoculated intranasally with hypervirulent K. pneumoniae (K2:O1) which was either antibiotic-susceptible or multi-drug resistant. Six hours after infection, mice were treated with antibiotics intraperitoneally and flagellin or vehicle intranasally. Mice were sacrificed 24 hours after infection. Samples were analyzed for bacterial loads and for inflammatory and coagulation markers.

Results

Flagellin therapy induced neutrophil influx in the lung during antibiotic-treated pneumonia evoked by either antibiotic-susceptible or -resistant K. pneumoniae. The pulmonary neutrophil response was matched by elevated levels of neutrophil-attracting chemokines, neutrophil degranulation products, and local coagulation activation. The combined therapy of effective antibiotics and flagellin did not impact K. pneumoniae outgrowth in the lung, but decreased bacterial counts in distant organs. Neutrophil depletion abrogated the flagellin-mediated effect on bacterial dissemination and local coagulation responses.

Conclusion

Topical flagellin administration as an adjunctive to antibiotic treatment augments neutrophil responses during pneumonia evoked by MDR-K. pneumoniae, thereby reducing bacterial dissemination to distant organs.

In treacherous waters: detection of colistin-resistant bacteria in water and plastic litter from a recreational estuary

Colistin resistance poses a major therapeutic challenge and resistant strains have now been reported worldwide. However, the occurrence of such bacteria in aquatic environments is considerably less understood. This study aimed to isolate and characterize colistin-resistant strains from water and plastic litter collected in an urban recreational estuary. Altogether, 64 strains with acquired colistin resistance were identified, mainly Acinetobacter spp. and Enterobacter spp. From these, 40.6% were positive for at least one mcr variant (1-9), 26.5% harbored, extended-spectrum beta-lactamases, 23.4% harbored, sulfonamide resistance genes, and 9.3% harbored, quinolone resistance genes. merA, encoding mercury resistance, was detected in 10.5% of these strains, most of which were also strong biofilm producers. The minimum inhibitory concentration toward colistin was determined for the mcr-positive strains and ranged from 2 to ≥512 µg ml-1. Our findings suggest that Gram-negative bacteria highly resistant to a last-resort antimicrobial can be found in recreational waters and plastic litter, thereby evidencing the urgency of the One Health approach to mitigate the antimicrobial resistance crisis.

Mutations in the pmrB gene constitute the major mechanism underlying chromosomally encoded colistin resistance in clinical Escherichia coli

OBJECTIVES

The mechanisms underlying chromosomally encoded colistin resistance in Escherichia coli remain insufficiently investigated. In this study, we investigated the contribution of various pmrB mutations from E. coli clinical isolates to colistin resistance.

METHODS

The resistance mechanisms in eight mcr-negative colistin-resistant E. coli isolates obtained from a nationwide surveillance program in Taiwan using recombinant DNA techniques and complementary experiments were investigated. The minimal inhibitory concentrations (MICs) of colistin in the recombinant strains were compared with those in the parental strains. The expression levels of pmrA and pmrK (which are part of the pmrCAB and pmrHFIJKLM operons associated with colistin resistance) were measured using reverse transcription-quantitative real-time polymerase chain reaction.

RESULTS

In the complementation experiments, various mutated pmrB alleles from the eight mcr-negative colistin-resistant E. coli strains were introduced into an ATCC25922 mutant with a PmrB deletion, which resulted in colistin resistance. The MIC levels of colistin in the most complemented strains were comparable to those of the parental colistin-resistant strains. Increased expression levels of pmrA and pmrK were consistently detected in most complemented strains. The impact for colistin resistance was confirmed for various novel amino acid substitutions, P94L, G19E, L194P, L98R and R27L in PmrB from the parental clinical strains. The detected amino acid substitutions are distributed in the different functional domains of PmrB.

CONCLUSIONS

Colistin resistance mediated by amino acid substitutions in PmrB is a major chromosomally encoded mechanism in E. coli of clinical origin.

Evaluating the accuracy of the MBT Lipid Xtract Kit for assessing colistin resistance in comparison to broth microdilution

Colistin resistance testing methods such as broth microdilution (BMD) are time-consuming and labour intensive for clinical laboratories. MBT Lipid Xtract Kit on MALDI Biotyper Sirius System (Bruker, Billerica, MA, USA) utilizes lipidomic analysis to identify specific cell wall modifications associated with colistin resistance. We compared MBT to BMD (ComASP Colistin, Liofilchem) across 36 Gram-negative isolates (non-resistant MIC ≤2 µg ml-1, resistant MIC ≥4 µg ml-1). All samples were tested twice on MBT with discrepant results repeated before assessing categorical agreement between MBT and BMD. 44.4% (16/36) of isolates were colistin resistant via BMD. MBT Lipid Xtract had 80.6% agreement (29/36) with BMD, with 5/7 discrepancies corrected to match upon repeat testing. There was 100% agreement for Escherichia coli isolates (n=16). The whole-genome sequencing was completed on the two discrepant Klebsiella pneumoniae isolates, with variants within colistin resistance-associated loci identified (MIC 0.5 µg ml-1: arnC S30T, pmrB T246A, lapB N212T, lpxM S253G, crrB Q287K and MIC >16 µg ml-1: arnC S30T, pmrB R90insRN, pmrB T246A, pmrA E57G, lpxM S253G). Further evaluation, particularly for non-E. coli, of MBT is required prior to implementation in clinical laboratories.

Insertion with long target duplication in polymyxin B-induced resistant mutant of Salmonella

OBJECTIVES

A Salmonella enterica subsp. diarizonae (hereafter S. diarizonae) clinical strain S499 demonstrated unique genomic features. The strain S499 was treated with polymyxin B in vitro to investigate the mechanism of resistance.

METHODS

S499 was treated with polymyxin B by increasing concentration gradually to obtain a resistant mutant S499V. Whole genomes of the two strains were sequenced using Illumina HiSeq X-10 and PacBio RS II platforms. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was performed to compare the gene expression.

RESULTS

The chromosome of strain S499 contained a 40-kb DNA region that was replicated after treatment with polymyxin B and generated a triple tandem DNA repeat region in the chromosome of mutant strain S499V. This repeat region in S499V was flanked by IS1 and contained pmrD, pmrG, and arnBCADTEF operon. In comparison to the homologous 40-kb DNA region of strain S499, a few genes in the repeat DNA region of strain S499V contained truncating mutations that generate two open reading frames (ORFs). The expression of pmrD, pmrG, and arnT was significantly upregulated in S499V.

CONCLUSION

The duplication and overexpression of pmrD, pmrG, and arnT operon may be responsible for the polymyxin B resistance of mutant strain S499V.

Genome analysis of extensively drug-resistant Pseudomonas aeruginosa ST1971 from a patient in China hospitalized for severe pneumonia

OBJECTIVES

The emergence and outbreak of carbapenem-resistant Pseudomonas aeruginosa are a major global public threat. In this study we aimed to characterize the genome of drug-resistant and virulent genes in an extremely drug-resistant (XDR) P. aeruginosa strain to understand its antimicrobial resistance trends and pathogenicity.

METHODS

An XDR P. aeruginosa strain was isolated in China from a patient with severe pneumonia. Antimicrobial susceptibility testing, genome sequencing, and phylogenetic analysis were performed. Predictions were fulfilled using curated bioinformatics tools.

RESULTS

Assembly of the strain (CRPA190) comprised 76 contigs with a total length of 7 009 318 bp. CRPA190 belongs to sequence type 1971 (ST1971) and the O11 serogroup. Nine prophage regions, three CRISPR arrays, and two Cas clusters were identified. However, no plasmids were predicted. Antibiotic susceptibility tests showed that CRPA190 was resistant to all the tested antibiotics, including carbapenem, polymyxin B, and ceftazidime-avibactam. Forty antimicrobial resistance genes were predicted in CRPA190, including several carbapenemase genes such as blaPDC-142, blaPME-1, blaNDM-1, and blaOXA-902. The isolate was predicted to be pathogenic and possess strong biofilm-forming ability. It harbours virulence genes that are associated with an arsenal of virulence determinants involved in adherence, motility, exotoxins, exoenzymes, immune modulation, biofilms, nutritional/metabolic factors, and effector delivery systems.

CONCLUSIONS

These findings enhance our understanding of the resistance and pathogenicity of the ST1971 P. aeruginosa strain that is unique in China and provide a broader perspective on the global epidemiological landscape, suggesting the emergence of P. aeruginosa ST1971, which requires control measures to limit its dissemination.

Coordinated elimination of bacterial taxa optimally attenuates alloimmunity and prolongs allograft survival

This study aimed to dissect the relationship between specific gut commensal bacterial subgroups, their functional metabolic pathways, and their impact on skin allograft outcome and alloimmunity. We previously showed that oral broad-spectrum antibiotic (Abx) pretreatment in mice delayed skin, heart, and lung allograft rejection and dampened alloimmune responses. Here, rationally designed Abx combinations targeting major bacterial groups were used to elucidate their individual contribution to modulating alloimmune responses. Abx cocktails targeting intestinal gram-negative, gram-positive, or anaerobic/gram-positive bacteria by oral gavage, all delayed skin allograft rejection, and reduced alloreactive T cell priming to different extents. Notably, the most pronounced extension of skin allograft survival and attenuation of alloimmunity were achieved when all gut bacterial groups were simultaneously targeted. These results suggest a model in which the strength of the alloimmune response is additively tuned up by gut microbial diversity. Shotgun metagenomic sequencing enabled strain-level resolution and identified a shared commensal, Parabacteroides distasonis, as the most enriched following all Abx treatments. Oral administration of P.distasonis to mice harboring a diverse microbiota significantly prolonged skin allograft survival, identifying a probiotic with therapeutic benefit in transplantation.

Antimicrobial Resistance in Sepsis Cases Due to Escherichia coli and Klebsiella pneumoniae: Pre-Pandemic Insights from a Single Center in Southwestern Romania

Sepsis is an uncontrolled reaction of the body to an infection, and if not effectively treated, it can progress to septic shock, multiple organ failure, and ultimately, death.

OBJECTIVE

To determine the resistance profile of Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae) strains isolated in sepsis cases diagnosed at the Infectious Diseases Clinic in Craiova, Romania.

METHODS

The bacteria responsible for sepsis cases were identified using the Vitek 2 Systems version 06.01, which was then employed to assess their antimicrobial susceptibility (Global CLSI and Phenotypic 2017).

RESULTS

We have identified 989 patients diagnosed with bacterial sepsis. Among these, 953 cases were caused by Gram-negative rods, with 415 attributed to E. coli and 278 to K. pneumoniae. High levels of resistance to ampicillin were recorded for E. coli strains isolated in sepsis cases (64.6%); adding sulbactam lowers the level of resistance to 41.8%. Resistance to 3rd generation cephalosporins varied between 7.47 and 14.6% and another 3.41 to 11.1% are dose-dependent susceptibility strains. Resistance to carbapenems (i.e., ertapenem, meropenem) is low-2.18-2.42%. More than 95% of the tested K. pneumoniae strains were resistant to ampicillin and adding sulbactam as a β-lactamase inhibitor only halves that level. Resistance to 3rd generation cephalosporins varied between 20.7% and 22.5%; resistance levels for K. pneumoniae were notably higher than those for E. coli. Over 95% of K. pneumoniae strains showed resistance to ampicillin, and resistance to 3rd generation cephalosporins varied between 20.7% and 22.5%. Additionally, K. pneumoniae exhibited higher resistance to carbapenems (13.7-19.5%) compared to E. coli (2.18-2.42%).

CONCLUSIONS

Antimicrobial resistance levels are generally lower than continental and national data, except for ampicillin and carbapenems (meropenem and ertapenem). K. pneumoniae strains are significantly more resistant than E. coli strains.

Genomic Insights into Vietnamese Extended-Spectrum β-Lactamase-9-Producing Extensively Drug-Resistant Pseudomonas aeruginosa Isolates Belonging to the High-Risk Clone ST357 Obtained from Bulgarian Intensive Care Unit Patients

Extensively drug-resistant P. aeruginosa (XDR-PA) has been highlighted as a serious public health threat. The present study aimed to explore the genomic characteristics of two Vietnamese extended-spectrum β-lactamase-9 (VEB-9)-producing XDR-PA isolates from Bulgaria in comparison to all blaVEB-9-positive strains with available genomes. The isolates designated Pae51 and Pae52 were obtained from tracheobronchial aspirates of intensive care unit (ICU) patients. Antimicrobial susceptibility testing, whole-genome sequencing, RT-qPCR, and phylogenomic analysis were performed. Pae51 and Pae52 were resistant to most antipseudomonal β-lactams including carbapenems, aminoglycosides, and fluoroquinolones but remained susceptible to colistin and cefiderocol. Numerous resistance determinants were detected: blaVEB-9, blaPDC-3, blaOXA-10, blaOXA-50, aac(6')-II, ant(2″)-Ia, ant(3″)-IIa, aph(3')-IIb, cprP, catB7, dfrB2, sul1, fosA, and tet(A). Both isolates carried complex integrons with blaVEB-9 and tet(A) embedded next to the conservative 3' end sequences. A variety of virulence factors were also identified, including the type III secretion system exotoxin U. Pae51 and Pae52 differed by only four SNPs and belonged to the high-risk clone ST357. To our knowledge, this is the first report of blaVEB-9-positive XDR-PA isolates in Bulgaria presenting a detailed genomic analysis. The development of novel antimicrobial strategies for such pathogens should be an essential part of infection control stewardship practices in ICU wards.

Antibiotic Susceptibility of Aerobic and Facultative Anaerobic Gram-Negative Rods in Hong Kong and Implications on Usefulness of Ceftazidime-Avibactam and Ceftolozane-Tazobactam

Due to the increasing resistance of aerobic and facultative anaerobic Gram-negative rods, ceftazidime-avibactam and ceftolozane-tazobactam have been launched in the market in the last few years. In this study, we analyzed the susceptibility pattern of the major aerobic and facultative anaerobic Gram-negative rods in Hong Kong for ceftazidime-avibactam, ceftolozane-tazobactam, four other broad-spectrum antibiotics commonly used in Hong Kong and colistin. For 300 isolates collected from January to December 2021, non-ESBL-producing Enterobacterales, ESBL-producing Enterobacterales and Pseudomonas aeruginosa were highly susceptible to ceftazidime-avibactam (all 100%) and ceftolozane-tazobactam (98.7%, 99.7% and 94.3%). For 32 archived ESBL-producing Klebsiella pneumoniae isolates collected between January 2014 and March 2023, all were susceptible to ceftazidime-avibactam and ceftolozane-tazobactam. For 101 archived carbapenemase-producing Enterobacterales, their susceptibilities to ceftazidime-avibactam and ceftolozane-tazobactam varied depending on the type of carbapenemase produced. Both had high activities against OXA-producing strains (97.1% and 76.5%, respectively) but were 100% resistant for NDM-producing and NDM+OXA-producing strains. All KPC-producing strains were susceptible to ceftazidime-avibactam but resistant to ceftolozane-tazobactam. Ceftazidime-avibactam and ceftolozane-tazobactam are good alternatives for the management of infections caused by ESBL-producing Enterobacterales and selective strains of carbapenemase-producing Enterobacterales in Hong Kong.

Synthesis, Characterization, and Biological Evaluation of Chitosan Nanoparticles Cross-Linked with Phytic Acid and Loaded with Colistin against Extensively Drug-Resistant Bacteria

The natural evolution of microorganisms, as well as the inappropriate use of medicines, have accelerated the problem of drug resistance to many of the antibiotics employed today. Colistin, a lipopeptide antibiotic used as a last resort against multi-resistant strains, has also begun to present these challenges. Therefore, this study was focused on establishing whether colistin associated with chitosan nanoparticles could improve its antibiotic activity on an extremely resistant clinical isolate of Pseudomonas aeruginosa, which is a clinically relevant Gram-negative bacterium. For this aim, nanoparticulate systems based on phytic acid cross-linked chitosan and loaded with colistin were prepared by the ionic gelation method. The characterization included particle size, polydispersity index-PDI, and zeta potential measurements, as well as thermal (DSC) and spectrophotometric (FTIR) analysis. Encapsulation efficiency was assessed by the bicinchoninic acid (BCA) method, while the antimicrobial evaluation was made following the CLSI guidelines. The results showed that colistin-loaded nanoparticles were monodispersed (PDI = 0.196) with a particle size of around 266 nm and a positive zeta potential (+33.5 mV), and were able to associate with around 65.8% of colistin and decrease the minimum inhibitory concentration from 16 μg/mL to 4 μg/mL. These results suggest that the association of antibiotics with nanostructured systems could be an interesting alternative to recover the antimicrobial activity on resistant strains.

Limited impact of colistin resistance on mortality of intensive care patients with carbapenem-resistant bacteraemia

BACKGROUND

Increasing incidence of carbapenem-resistant Gram-negative bacteraemia (CR-GNB) has triggered increased use of polymyxins, likely fuelling the emergence and spread of colistin resistance.

AIM

To estimate the excess clinical burden of colistin resistance in intensive care patients with CR-GNB.

METHODS

A cohort of patients with CR-GNB during their stay in the intensive care unit (ICU) of a university hospital in Greece over a 4-year period (2020-2023) was constructed. Competing risks survival analysis was performed to estimate the burden associated with colistin resistance.

FINDINGS

Of the 177 ICU patients with CR-GNB, 134 (76%) had colistin-resistant isolates, predominantly Acinetobacter baumannii (79%), identified by broth microdilution. Patients with colistin-resistant infection were similar to those with colistin-susceptible infection with respect to age, sex, APACHE II score, Charlson comorbidity index score, Pitt bacteraemia score, prior surgery and the occurrence of polymicrobial cultures. However, patients in the colistin-resistant group had lower risk of mortality compared with those in the colistin-susceptible group (31% vs 44%, P = 0.004 at 14 days, respectively; 46% vs 56% at 28 days, respectively; P = 0.173). Multi-variable regression analysis confirmed that colistin-resistant CR-GNB was associated with significantly lower risk of inpatient death compared with colistin-susceptible CR-GNB within 14 days [cause-specific hazard ratio (csHR) 0.53, 95% CI 0.28-1.01) and 28 days (csHR 0.55, 95% CI 0.31-0.95) of infection onset.

CONCLUSION

Limited impact of colistin resistance on mortality was demonstrated in a large contemporary cohort of ICU patients with CR-GNB, possibly reflecting the recent shift away from colistin-based treatment regimens.

Diclofenac Sodium Restores the Sensitivity of Colistin-Resistant Gram-Negative Bacteria to Colistin

The continuous rise of multidrug-resistant (MDR) Gram-negative bacteria poses a severe threat to public health worldwide. Colistin(COL), employed as the last-line antibiotic against MDR pathogens, is now at risk due to the emergence of colistin-resistant (COL-R) bacteria, potentially leading to adverse patient outcomes. In this study, synergistic activity was observed when colistin and diclofenac sodium (DS) were combined and used against clinical COL-R strains of Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae), Acinetobacter baumannii (A. baumannii), and Pseudomonas aeruginosa (P. aeruginosa) both in vitro and in vivo. The checkerboard method and time-killing assay showed that DS, when combined with COL, exhibited enhanced antibacterial activity compared to DS and COL monotherapies. Crystal violet staining and scanning electron microscopy showed that COL-DS inhibited biofilm formation compared with monotherapy. The in vivo experiment showed that the combination of DS and COL reduced bacterial loads in infected mouse thighs. Synergistic activity was observed when COL and DS were use in combination against clinical COL-R strains of E. coli, K. pneumoniae, A. baumannii and P. aeruginosa both in vitro and in vivo. The synergistic antibacterial effect of the COL-DS combination has been confirmed by performing various in vitro and in vivo experiments, which provides a new treatment strategy for infections caused by MDR bacteria.

Phenotypic and genotypic antimicrobial resistance patterns in honey bee (Apis mellifera L.) bacterial symbionts

Antimicrobial resistance (AMR) is a major global public health problem. Nevertheless, the knowledge of the factors driving the spread of resistance among environmental microorganisms is limited, and few studies have been performed worldwide. Honey bees (Apis mellifera L.) have long been considered bioindicators of environmental pollution and more recently also of AMR. In this study, 53 bacterial strains isolated from the body surface of honey bees at three ontogenetic stages, collected from ten different geographic locations, were tested for their phenotypic and genotypic resistance to eight classes of the most widely used antimicrobials in human and veterinary medicine. Results showed that 83% of the strains were resistant to at least one antimicrobial and 62% were multidrug-resistant bacteria, with a prevalence of resistance to nalidixic acid, cefotaxime, and aztreonam. A high percentage of isolates harbouring at least one antimicrobial gene was also observed (85%). The gene encoding resistance to colistin mcr-1 was the most abundant, followed by those for tetracycline tetM and tetC. Geographical features influenced the distribution of these traits more than bacterial species or bee stage, supporting the use of honey bee colonies and their associated bacteria as indicators to monitor environmental resistance. This approach can improve the scientific understanding of this global threat by increasing data collection capacity.

An allosteric inhibitor of the PhoQ histidine kinase with therapeutic potential against Salmonella infection

BACKGROUND

The upsurge of antimicrobial resistance demands innovative strategies to fight bacterial infections. With traditional antibiotics becoming less effective, anti-virulence agents or pathoblockers, arise as an alternative approach that seeks to disarm pathogens without affecting their viability, thereby reducing selective pressure for the emergence of resistance mechanisms.

OBJECTIVES

To elucidate the mechanism of action of compound N'-(thiophen-2-ylmethylene)benzohydrazide (A16B1), a potent synthetic hydrazone inhibitor against the Salmonella PhoP/PhoQ system, essential for virulence.

MATERIALS AND METHODS

The measurement of the activity of PhoP/PhoQ-dependent and -independent reporter genes was used to evaluate the specificity of A16B1 to the PhoP regulon. Autokinase activity assays with either the native or truncated versions of PhoQ were used to dissect the A16B1 mechanism of action. The effect of A16B1 on Salmonella intramacrophage replication was assessed using the gentamicin protection assay. The checkerboard assay approach was used to analyse potentiation effects of colistin with the hydrazone. The Galleria mellonella infection model was chosen to evaluate A16B1 as an in vivo therapy against Salmonella.

RESULTS

A16B1 repressed the Salmonella PhoP/PhoQ system activity, specifically targeting PhoQ within the second transmembrane region. A16B1 demonstrates synergy with the antimicrobial peptide colistin, reduces the intramacrophage proliferation of Salmonella without being cytotoxic and enhances the survival of G. mellonella larvae systemically infected with Salmonella.

CONCLUSIONS

A16B1 selectively inhibits the activity of the Salmonella PhoP/PhoQ system through a novel inhibitory mechanism, representing a promising synthetic hydrazone compound with the potential to function as a Salmonella pathoblocker. This offers innovative prospects for combating Salmonella infections while mitigating the risk of antimicrobial resistance emergence.

Using honey bee colonies to monitor phenotypic and genotypic resistance to colistin

Colistin is a polymyxin antimicrobic mainly used to treat infection caused by multi-drug resistant Gram-negative bacteria. Mechanisms of colistin resistance are linked to the mobile colistin resistance (mcr) genes, which are transferable within mobile plasmids. Currently, there is limited research on the environmental dissemination of these genes. The behavioural and morphological characteristics of Apis mellifera L. make honey bees effective environmental bioindicators for assessing the prevalence of antimicrobial-resistant bacteria. This study aims to evaluate the colistin phenotypic and genotypic resistance in environmental Gram-negative bacteria isolated from foraging honey bees, across a network of 33 colonies distributed across the Emilia-Romagna region in Italy. Phenotypic resistances were determined through a microdilution assay using the minimum inhibitory concentration (MIC) with dilutions ranging from 0.5 μg/ml to 256 μg/ml. Strains with MIC values gather than 2 μg/ml were classified as resistant. Also, the identification of the nine mcr genes was carried out using two separate multiplex PCR assays. The study found that 68.5% of isolates were resistant and the genus with the higher resistance rates observed in Enterobacter spp. (84.5%). At least one mcr gene was found in 137 strains (53.3%). The most detected gene was mcr5 (35.3%), which was the most frequently detected gene in the seven provinces, while the least observed was mcr4 (4.8%), detected only in two provinces. These results suggested the feasibility of detecting specific colistin resistance genes in environmentally spread bacteria and understanding their distribution at the environmental level, despite their restricted clinical use. In a One-Health approach, this capability enables valuable environmental monitoring, considering the significant role of colistin in the context of public health.

Genomic characterization of carbapenemase-producing Enterobacterales from Dhaka food markets unveils the spread of high-risk antimicrobial-resistant clones and plasmids co-carrying bla NDM and mcr-1.1

Background

The transmission of carbapenemase-producing Enterobacterales (CPE) in the external environment, especially through food, presents a significant public health risk.

Objectives

To investigate the prevalence and genetic characteristics of CPE in food markets of Dhaka, Bangladesh, using WGS.

Methods

CPE isolates were obtained from different food and water samples collected from food markets in the southern part of Dhaka, Bangladesh. The isolates subsequently underwent molecular typing, WGS employing both short- and long-read sequencers, and plasmid analysis.

Results

This study unveiled an extensive spread of CPE, with no significant difference in contamination rates observed in samples (N = 136), including meat (n = 8), fish (n = 5), vegetables (n = 36) or various food-washed water (n = 65) from markets near hospitals or residential areas. Thirty-eight Enterobacterales from 33 samples carried carbapenemase genes (bla NDM-1, -4, -7, bla KPC-2, bla OXA-181 or bla IMI-1). Among these, the high-risk Escherichia coli ST410 clone was the most prevalent and distributed across various locations. Furthermore, the identification of IncHI2 plasmids co-harbouring resistance genes like bla NDM-5 and mcr-1.1, without discernible epidemiological connections, is a unique finding, suggesting their widespread dissemination.

Conclusions

The analysis unveils a dynamic landscape of CPE dissemination in food markets, underscored by the proliferation of novel IncHI2 hybrid plasmids carrying both colistin- and carbapenem-resistance genes. This illuminates the ever-evolving landscape of antimicrobial resistance in Dhaka, urging us to confront its emergent challenges.

IS1-mediated chromosomal amplification of the arn operon leads to polymyxin B resistance in Escherichia coli B strains

Polymyxins [colistin and polymyxin B (PMB)] comprise an important class of natural product lipopeptide antibiotics used to treat multidrug-resistant Gram-negative bacterial infections. These positively charged lipopeptides interact with lipopolysaccharide (LPS) located in the outer membrane and disrupt the permeability barrier, leading to increased uptake and bacterial cell death. Many bacteria counter polymyxins by upregulating genes involved in the biosynthesis and transfer of amine-containing moieties to increase positively charged residues on LPS. Although 4-deoxy-l-aminoarabinose (Ara4N) and phosphoethanolamine (PEtN) are highly conserved LPS modifications in Escherichia coli, different lineages exhibit variable PMB susceptibilities and frequencies of resistance for reasons that are poorly understood. Herein, we describe a mechanism prevalent in E. coli B strains that depends on specific insertion sequence 1 (IS1) elements that flank genes involved in the biosynthesis and transfer of Ara4N to LPS. Spontaneous and transient chromosomal amplifications mediated by IS1 raise the frequency of PMB resistance by 10- to 100-fold in comparison to strains where a single IS1 element located 90 kb away from the end of the arn operon has been deleted. Amplification involving IS1 becomes the dominant resistance mechanism in the absence of PEtN modification. Isolates with amplified arn operons gradually lose their PMB-resistant phenotype with passaging, consistent with classical PMB heteroresistance behavior. Analysis of the whole genome transcriptome profile showed altered expression of genes residing both within and outside of the duplicated chromosomal segment, suggesting complex phenotypes including PMB resistance can result from tandem amplification events.IMPORTANCEPhenotypic variation in susceptibility and the emergence of resistant subpopulations are major challenges to the clinical use of polymyxins. While a large database of genes and alleles that can confer polymyxin resistance has been compiled, this report demonstrates that the chromosomal insertion sequence (IS) content and distribution warrant consideration as well. Amplification of large chromosomal segments containing the arn operon by IS1 increases the Ara4N content of the lipopolysaccharide layer in Escherichia coli B lineages using a mechanism that is orthogonal to transcriptional upregulation through two-component regulatory systems. Altogether, our work highlights the importance of IS elements in modulating gene expression and generating diverse subpopulations that can contribute to phenotypic polymyxin B heteroresistance.

Antimicrobial Resistance Surveillance: Data Harmonisation and Data Selection within Secondary Data Use

Resistance to last-resort antibiotics is a global threat to public health. Therefore, surveillance and monitoring systems for antimicrobial resistance should be established on a national and international scale. For the development of a One Health surveillance system, we collected exemplary data on carbapenem and colistin-resistant bacterial isolates from human, animal, food, and environmental sources. We pooled secondary data from routine screenings, hospital outbreak investigations, and studies on antimicrobial resistance. For a joint One Health evaluation, this study incorporates epidemiological metadata with phenotypic resistance information and molecular data on the isolate level. To harmonise the heterogeneous original information for the intended use, we developed a generic strategy. By defining and categorising variables, followed by plausibility checks, we created a catalogue for prospective data collections and applied it to our dataset, enabling us to perform preliminary descriptive statistical analyses. This study shows the complexity of data management using heterogeneous secondary data pools and gives an insight into the early stages of the development of an AMR surveillance programme using secondary data.

Efficacy and safety of different polymyxin-containing regimens for the treatment of pneumonia caused by multidrug-resistant gram-negative bacteria: a systematic review and network meta-analysis

BACKGROUND

The optimal administration of polymyxins for treating multidrug-resistant gram-negative bacterial (MDR-GNB) pneumonia remains unclear. This study aimed to systematically assess the efficacy and safety of three polymyxin-containing regimens by conducting a comprehensive network meta-analysis.

METHODS

We comprehensively searched nine databases. Overall mortality was the primary outcome, whereas the secondary outcomes encompassed microbial eradication rate, clinical success, acute kidney injury, and incidence of bronchospasm. Extracted study data were analyzed by pairwise and network meta-analyses. Version 2 of the Cochrane risk-of-bias tool and the Risk of Bias in Nonrandomized Studies of Interventions (ROBINS-I) assessment tool were used to assess the risk of bias in randomized trials and cohort studies, respectively.

RESULTS

This study included 19 observational studies and 3 randomized controlled trials (RCTs), encompassing 3318 patients. Six studies with high risk of bias were excluded from the primary analysis. In the pairwise meta-analysis, compared to the intravenous (IV) polymyxin-containing regimen, the intravenous plus inhaled (IV + IH) polymyxin-containing regimen showed a significant decrease in overall mortality, while no statistically significant difference was found in the inhaled (IH) polymyxin-containing regimen. The network meta-analysis indicated that the IV + IH polymyxin-containing regimen had significantly lower overall mortality (OR 0.67; 95% confidence interval [CI] 0.50-0.88), higher clinical success rate (OR 1.90; 95% CI 1.20-3.00), better microbial eradication rate (OR 2.70; 95% CI 1.90-3.90) than the IV polymyxin-containing regimen, and significantly better microbial eradication rate when compared with the IH polymyxin-containing regimen (OR 2.30; 95% CI 1.30-4.20). Furthermore, compared with IV + IH and IV polymyxin-containing regimens, the IH polymyxin-containing regimen showed a significant reduction in acute kidney injury.

CONCLUSIONS

Our study indicates that among the three administration regimens, the IV + IH polymyxin-containing regimen may be the most effective for treating MDR-GNB pneumonia, with a significantly lower overall mortality compared to the IV regimen and a considerably higher microbial eradication rate compared to the IH regimen. The IH regimen may be considered superior to the IV regimen due to its substantially lower incidence of acute kidney injury, even though the reduction in overall mortality was not significant.

Adequacy of the Dosing and Infusion Time of Ceftazidime/Avibactam for the Treatment of Gram-Negative Bacterial Infections: A PK/PD Simulation Study

Introduction

Recent studies suggested the potential benefits of extended infusion times to optimize the treatment efficacy of ceftazidime/avibactam, which indicated that the current pharmacokinetic/pharmacodynamic (PK/PD) target may not be sufficient, especially for severe infections. The purpose of this study is to assess the adequacy of dosing strategies and infusion durations of ceftazidime/avibactam when applying higher PK/PD targets.

Methods

This study utilized published PK parameters to conduct Monte Carlo simulations. Different dosages including the recommended regimen based on renal function were simulated and evaluated by the probability of target attainment (PTA) and cumulative fraction of response (CFR). Different PK/PD targets were set for ceftazidime and avibactam. MIC distributions from various sources were used to calculate the CFR.

Results

Multiple PK/PD targets have been set in this study, All recommended dosage could easily achieve the target of 50%fT ≥ MIC (ceftazidime) and 50%fT ≥ CT=1.0 mg/L (avibactam). However, for severe infection patients with normal renal function and augmented renal clearance at the recommended dosage (2000 mg/500 mg, every 8 hours), the infusion duration needs to be extended to 3 hours and 4 hours to achieve the targets of 100%fT ≥ MIC and 100%fT ≥ CT=1.0 mg/L. Only continuous infusion at higher dosages achieved 100%fT ≥ 4×MIC and 100%fT ≥ CT=4.0 mg/L targets to all currently recommended regimens. According to the varying MIC distributions, higher concentrations are needed for Pseudomonas aeruginosa, with the attainment rates vary across different regions.

Conclusion

The current recommended dosing regimen of ceftazidime/avibactam is insufficient for severe infection patients, and continuous infusion is suggested.

In Vitro Evaluation of Colistin Conjugated with Chitosan-Capped Gold Nanoparticles as a Possible Formulation Applied in a Metered-Dose Inhaler

Inhaled colistin is used to treat pneumonia and respiratory infections through nebulization or dry powder inhalers. Nevertheless, the development of a metered-dose inhaler (MDI) for colistin, which could enhance patient convenience and treatment efficacy, has not yet been developed. Colistin is known for its ability to induce cellular toxicity. Gold nanoparticles (AuNPs) can potentially mitigate colistin toxicity. Therefore, this study aimed to evaluate the antimicrobial effectiveness of colistin conjugated with chitosan-capped gold nanoparticles (Col-CS-AuNPs) and their potential formulation for use with MDIs to deliver the aerosol directly to the deep lung. Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and elemental analysis were used to characterize the synthesized Col-CS-AuNPs. Drug release profiles fitted with the most suitable release kinetic model were evaluated. An MDI formulation containing 100 µg of colistin per puff was prepared. The aerosol properties used to determine the MDI performance included the fine particle fraction, mass median aerodynamic diameter, and geometric standard deviation, which were evaluated using the Andersen Cascade Impactor. The delivered dose uniformity was also determined. The antimicrobial efficacy of the Col-CS-AuNP formulation in the MDI was assessed. The chitosan-capped gold nanoparticles (CS-AuNPs) and Col-CS-AuNPs had particle sizes of 44.34 ± 1.02 and 174.50 ± 4.46 nm, respectively. CS-AuNPs effectively entrapped 76.4% of colistin. Col-CS-AuNPs exhibited an initial burst release of up to 60% colistin within the first 6 h. The release mechanism was accurately described by the Korsmeyer-Peppas model, with an R2 > 0.95. The aerosol properties of the Col-CS-AuNP formulation in the MDI revealed a high fine particle fraction of 61.08%, mass median aerodynamic diameter of 2.34 µm, and geometric standard deviation of 0.21, with a delivered dose uniformity within 75-125% of the labeled claim. The Col-CS-AuNP MDI formulation completely killed Escherichia coli at 5× and 10× minimum inhibitory concentrations after 6 and 12 h of incubation, respectively. The toxicity of CS-AuNP and Col-CS-AuNP MDI formulations in upper and lower respiratory tract cell lines was lower than that of free colistin. The stability of the Col-CS-AuNP MDI formulation was maintained for at least 3 months. The Col-CS-AuNP MDI formulation effectively eradicated bacteria over a 12-h period, showing promise for advancing lung infection treatments.

Tigecycline-Based Regimens for Complicated Urinary Tract Infections Caused by Carbapenem-Resistant Gram-Negative Bacteria: Case Series

There is existing controversy regarding the efficacy of tigecycline (TG) in treating complicated urinary tract infections (cUTIs) because of its pharmacokinetic concerns. We present three patients with cUTIs caused by carbapenem-resistant gram-negative (GN) pathogens successfully treated with high-dose tigecycline (HDT)-based regimens, as cefiderocol and aztreonam were not available in our country. The first case describes a 67-year-old patient with diabetes, prostate cancer, and double J ureteral stenting who was hospitalized with a febrile, complicated urinary tract infection (cUTI). Urine and blood cultures were positive for metallo-beta-lactamases (MBL)-producing extensively drug-resistant (XDR) Klebsiella pneumoniae (cefiderocol-susceptible). The synergy between TG and colistin using the in vitro E-test was demonstrated, and the patient was started on this regimen using HDT. Clinical and microbiological cures were achieved, and the patient was discharged home. The second case presents a 70-year-old patient with urethral pathology who was hospitalized with the diagnosis of a lower cUTI caused by an MBL-producing pan-drug-resistant (PDR) Klebsiella pneumoniae. The in vitro E-test showed synergy between TG and colistin, and our patient was successfully treated with this HDT-based combination. The third case emphasizes a 63-year-old patient with insulin-dependent diabetes, Child B cirrhosis, and a right double J ureteral stent who was hospitalized with a febrile cUTI. Urine and blood cultures were positive for carbapenem-resistant XDR Acinetobacter baumannii (susceptible to colistin and TG). Colistin was administered for only 96 hours because of stage II acute kidney injury, and we continued the treatment with HDT in monotherapy. The patient was discharged home, and no urinary tract infection relapse was seen for six months. Favorable clinical and microbiological outcomes were achieved with TG-based regimens in our cUTI cases. We highlight the role of antibiotic synergy determined by the in vitro E-test in two cases of MBL-producing XDR/PDR Klebsiella pneumoniae.

Current and novel therapies for management of Acinetobacter baumannii-associated pneumonia

Acinetobacter baumannii is a common pathogen associated with hospital-acquired pneumonia showing increased resistance to carbapenem and colistin antibiotics nowadays. Infections with A. baumannii cause high patient fatalities due to their capability to evade current antimicrobial therapies, emphasizing the urgency of developing viable therapeutics to treat A. baumannii-associated pneumonia. In this review, we explore current and novel therapeutic options for overcoming therapeutic failure when dealing with A. baumannii-associated pneumonia. Among them, antibiotic combination therapy administering several drugs simultaneously or alternately, is one promising approach for optimizing therapeutic success. However, it has been associated with inconsistent and inconclusive therapeutic outcomes across different studies. Therefore, it is critical to undertake additional clinical trials to ascertain the clinical effectiveness of different antibiotic combinations. We also discuss the prospective roles of novel antimicrobial therapies including antimicrobial peptides, bacteriophage-based therapy, repurposed drugs, naturally-occurring compounds, nanoparticle-based therapy, anti-virulence strategies, immunotherapy, photodynamic and sonodynamic therapy, for utilizing them as additional alternative therapy while tackling A. baumannii-associated pneumonia. Importantly, these innovative therapies further require pharmacokinetic and pharmacodynamic evaluation for safety, stability, immunogenicity, toxicity, and tolerability before they can be clinically approved as an alternative rescue therapy for A. baumannii-associated pulmonary infections.

Mobile Colistin Resistance and Plasmid-Mediated Quinolone Resistance Genes in Escherichia coli from China, 1993-2019

Plasmid-mediated quinolone resistance (PMQR) genes and mobile colistin resistance (MCR) genes in Escherichia coli (E. coli) have been widely identified, which is considered a global threat to public health. In the present study, we conducted an analysis of MCR genes (mcr-1, mcr-2, mcr-3, mcr-4, and mcr-5) and PMQR genes [qnrA, qnrB, qnrC, qnrD, qnrE1, qnrVC, qnrS, aac(6')-Ib-cr, qepA, and oqxAB] in E. coli from China, 1993-2019. From the 3,663 E. coli isolates examined, 1,613 (44.0%) tested positive for PMQR genes, either individually or in combination. Meanwhile, 262 isolates (7.0%) carried the MCR genes. Minimum inhibitory concentration (MIC) analyses of 17 antibiotics for the MCR gene-carrying strains revealed universal multidrug resistance. Resistance to polymyxin varied between 4 μg/mL and 64 μg/mL, with MIC50 and MIC90 at 8 μg/mL and 16 μg/mL, respectively. In addition, fluctuations in the detection rates of these resistant genes correlated with the introduction of antibiotic policies, host origin, temporal trends, and geographical distribution. Continuous surveillance of PMQR and MCR variants in bacteria is required to implement control and prevention strategies.

Characterization of NMCR-3, NMCR-4 and NMCR-5, three novel non-mobile colistin resistance determinants: Implications for MCR-3, MCR-7, and MCR-5 progenitors, respectively

In this study, the progenitors of MCR-3, MCR-7 and MCR-5, namely NMCR-3, NMCR-4 and NMCR-5, were firstly discovered and indicating Aeromonas was a natural reservoir for MCR-3 and MCR-7. Furthermore, different evolutionary models for MCR-3, MCR-7 and MCR-5 were proposed.

Restoring colistin sensitivity in colistin-resistant Salmonella and Escherichia coli: combinatorial use of berberine and EDTA with colistin

The appearance and prevalence of multidrug-resistance (MDR) Gram-negative bacteria (GNB) have limited our antibiotic capacity to control bacterial infections. The clinical efficacy of colistin (COL), considered as the "last resort" for treating GNB infections, has been severely hindered by its increased use as well as the emergence and prevalence of mobile colistin resistance (MCR)-mediated acquired drug resistance. Identifying promising compounds to restore antibiotic activity is becoming an effective strategy to alleviate the crisis of increasing MDR. We first demonstrated that the combination of berberine (BBR) and EDTA substantially restored COL sensitivity against COL-resistant Salmonella and Escherichia coli. Molecular docking indicated that BBR can interact with MCR-1 and the efflux pump system AcrAB-TolC, and BBR combined with EDTA downregulated the expression level of mcr-1 and tolC. Mechanically, BBR combined with EDTA could increase bacterial membrane damage, inhibit the function of multidrug efflux pump, and promote oxidative damage, thereby boosting the action of COL. In addition, transcriptome analysis found that the combination of BBR and EDTA can accelerate the tricarboxylic acid cycle, inhibit cationic antimicrobial peptide (CAMP) resistance, and attenuate Salmonella virulence. Notably, the combination of BBR and EDTA with COL significantly reduced the bacterial load in the liver and spleen of a mice model infected with Salmonella. Our findings revealed that BBR and EDTA can be used as adjuvants collectively with COL to synergistically reverse the COL resistance of bacteria.

IMPORTANCE

Colistin is last-resort antibiotic used to treat serious clinical infections caused by MDR bacterial pathogens. The recent emergence of transferable plasmid-mediated COL resistance gene mcr-1 has raised the specter of a rapid worldwide spread of COL resistance. Coupled with the fact of barren antibiotic development pipeline nowadays, a critical approach is to revitalize existing antibiotics using antibiotic adjuvants. Our research showed that berberine combined with EDTA effectively reversed COL resistance both in vivo and in vitro through multiple modes of action. The discovery of berberine in combination with EDTA as a new and safe COL adjuvant provides a therapeutic regimen for combating Gram-negative bacteria infections. Our findings provide a potential therapeutic option using existing antibiotics in combination with antibiotic adjuvants and address the prevalent infections caused by MDR Gram-negative pathogens worldwide.

Colistin: Lights and Shadows of an Older Antibiotic

The emergence of antimicrobial resistance represents a serious threat to public health and for infections due to multidrug-resistant (MDR) microorganisms, representing one of the most important causes of death worldwide. The renewal of old antimicrobials, such as colistin, has been proposed as a valuable therapeutic alternative to the emergence of the MDR microorganisms. Although colistin is well known to present several adverse toxic effects, its usage in clinical practice has been reconsidered due to its broad spectrum of activity against Gram-negative (GN) bacteria and its important role of "last resort" agent against MDR-GN. Despite the revolutionary perspective of treatment with this old antimicrobial molecule, many questions remain open regarding the emergence of novel phenotypic traits of resistance and the optimal usage of the colistin in clinical practice. In last years, several forward steps have been made in the understanding of the resistance determinants, clinical usage, and pharmacological dosage of this molecule; however, different points regarding the role of colistin in clinical practice and the optimal pharmacokinetic/pharmacodynamic targets are not yet well defined. In this review, we summarize the mode of action, the emerging resistance determinants, and its optimal administration in the treatment of infections that are difficult to treat due to MDR Gram-negative bacteria.

Molecular Mechanisms of Bacterial Resistance to Antimicrobial Peptides in the Modern Era: An Updated Review

Antimicrobial resistance (AMR) poses a serious global health concern, resulting in a significant number of deaths annually due to infections that are resistant to treatment. Amidst this crisis, antimicrobial peptides (AMPs) have emerged as promising alternatives to conventional antibiotics (ATBs). These cationic peptides, naturally produced by all kingdoms of life, play a crucial role in the innate immune system of multicellular organisms and in bacterial interspecies competition by exhibiting broad-spectrum activity against bacteria, fungi, viruses, and parasites. AMPs target bacterial pathogens through multiple mechanisms, most importantly by disrupting their membranes, leading to cell lysis. However, bacterial resistance to host AMPs has emerged due to a slow co-evolutionary process between microorganisms and their hosts. Alarmingly, the development of resistance to last-resort AMPs in the treatment of MDR infections, such as colistin, is attributed to the misuse of this peptide and the high rate of horizontal genetic transfer of the corresponding resistance genes. AMP-resistant bacteria employ diverse mechanisms, including but not limited to proteolytic degradation, extracellular trapping and inactivation, active efflux, as well as complex modifications in bacterial cell wall and membrane structures. This review comprehensively examines all constitutive and inducible molecular resistance mechanisms to AMPs supported by experimental evidence described to date in bacterial pathogens. We also explore the specificity of these mechanisms toward structurally diverse AMPs to broaden and enhance their potential in developing and applying them as therapeutics for MDR bacteria. Additionally, we provide insights into the significance of AMP resistance within the context of host-pathogen interactions.