Chromosomally encoded and plasmid-mediated polymyxins resistance in Acinetobacter baumannii: a huge public health threat

Polymyxins retain in vitro activity and in vivo efficacy against "resistant" Acinetobacter baumannii strains when tested in physiological conditions

The emergence of plasmid-mediated resistance threatens the efficacy of polymyxins as the last line of defense against pan-drug-resistant infections. However, we have found that using Mueller-Hinton II (MHII), the standard minimum inhibitory concentration (MIC) medium, results in MIC data that are disconnected from in vivo treatment outcomes. We found that culturing putative colistin-resistant Acinetobacter baumannii clinical isolates, as defined by MICs of >2 mg/L in standard MHII testing conditions, in bicarbonate-containing media reduced MICs to the susceptible range by preventing colistin resistance-conferring lipopolysaccharide modifications from occurring. Furthermore, the lower MICs in bicarbonate-containing media accurately predicted in vivo efficacy of a human-simulated dosing strategy of colistin and polymyxin B in a lethal murine infection model for some polymyxin-resistant A. baumannii strains. Thus, current polymyxin susceptibility testing methods overestimate the contribution of polymyxin resistance-conferring mutations and incorrectly predict antibiotic activity in vivo. Polymyxins may remain a viable therapeutic option against Acinetobacter baumannii strains heretofore determined to be "pan-resistant."

Polymyxins: recent advances and challenges

Antibiotic resistance is a pressing global health challenge, and polymyxins have emerged as the last line of defense against multidrug-resistant Gram-negative (MDR-GRN) bacterial infections. Despite the longstanding utility of colistin, the complexities surrounding polymyxins in terms of resistance mechanisms and pharmacological properties warrant critical attention. This review consolidates current literature, focusing on polymyxins antibacterial mechanisms, resistance pathways, and innovative strategies to mitigate resistance. We are also investigating the pharmacokinetics of polymyxins to elucidate factors that influence their in vivo behavior. A comprehensive understanding of these aspects is pivotal for developing next-generation antimicrobials and optimizing therapeutic regimens. We underscore the urgent need for advancing research on polymyxins to ensure their continued efficacy against formidable bacterial challenges.

A molecular overview of the polymyxin-LPS interaction in the context of its mode of action and resistance development

With the rising incidences of antimicrobial resistance (AMR) and the diminishing options of novel antimicrobial agents, it is paramount to decipher the molecular mechanisms of action and the emergence of resistance to the existing drugs. Polymyxin, a cationic antimicrobial lipopeptide, is used to treat infections by Gram-negative bacterial pathogens as a last option. Though polymyxins were identified almost seventy years back, their use has been restricted owing to toxicity issues in humans. However, their clinical use has been increasing in recent times resulting in the rise of polymyxin resistance. Moreover, the detection of "mobile colistin resistance (mcr)" genes in the environment and their spread across the globe have complicated the scenario. The mechanism of polymyxin action and the development of resistance is not thoroughly understood. Specifically, the polymyxin-bacterial lipopolysaccharide (LPS) interaction is a challenging area of investigation. The use of advanced biophysical techniques and improvement in molecular dynamics simulation approaches have furthered our understanding of this interaction, which will help develop polymyxin analogs with better bactericidal effects and lesser toxicity in the future. In this review, we have delved deeper into the mechanisms of polymyxin-LPS interactions, highlighting several models proposed, and the mechanisms of polymyxin resistance development in some of the most critical Gram-negative pathogens.

Multi-drug Resistant Acinetobacter baumannii: Phenotypic and Genotypic Resistance Profiles and the Associated Risk Factors in Teaching Hospital in Jordan

BACKGROUND

This study aimed to determine the prevalence of Antimicrobial Resistance Genes (ARGs), with a focus on colistin resistance in clinical A. baumannii, as well as the risk factors associated with A. baumannii infection in Jordanian patients.

METHODS

In total, 150 A. baumannii isolates were obtained from patients at a teaching hospital. The isolates were tested for antimicrobial susceptibility using disc diffusion and microdilution methods. PCR amplification was used to detect ARGs, and statistical analysis was conducted to evaluate the influence of identified risk factors on the ARGs acquisition.

RESULTS

More than 90% of A. baumannii isolates were resistant to monobactam, carbapenem, cephalosporins, Fluoroquinolones, penicillin, and β-lactam agents. Moreover, 20.6% of the isolates (n = 31) were colistin-resistant. Several ARGs were also detected in A. baumannii isolates. Univariate analysis indicated that risk factors and the carriage of ARGs were significantly associated P ≤ (0.05) with gender, invasive devices, immunodeficiency, systemic diseases, tumors, and covid-19. Logistic regression analysis indicated seven risk factors, and three protective factors were associated with the ARGs (armA, strA, and strB) P ≤ (0.05). In contrast, tetB and TEM were associated with 2 risk factors each P ≤ (0.05).

CONCLUSION

Our study indicates a high prevalence of MDR A. baumannii infections in ICU patients, as well as describing the case of colistin-resistant A. baumannii for the first time in Jordan. Additionally, the risk factors associated with ARGs-producing A. baumannii infections among ICU patients suggest a rapid emergence and spread of MDR A. baumannii without adequate surveillance and control measures.

Prevalence of colistin resistance in clinical isolates of Acinetobacter baumannii: a systematic review and meta-analysis

INTRODUCTION

The development of colistin resistance in Acinetobacter baumannii during treatment has been identified in certain patients, often leading to prolonged or recurrent infections. As colistin, is the last line of therapy for A. baumannii infections that are resistant to almost all other antibiotics, colistin-resistant A. baumannii strains currently represent a significant public health threat, particularly in healthcare settings where there is significant selective pressure.

AIM

The aim of this study was to comprehensively determine the prevalence of colistin resistance in A. baumannii from clinical samples. Regional differences in these rates were also investigated using subgroup analyses.

METHOD

The comprehensive search was conducted using "Acinetobacter baumannii", "Colistin resistant" and all relevant keywords. A systematic literature search was performed after searching in PubMed, Embase, Web of Science, and Scopus databases up to April 25, 2023. Statistical analysis was performed using Stata software version 17 and sources of heterogeneity were evaluated using I2. The potential for publication bias was explored using Egger's tests. A total of 30,307 articles were retrieved. After a thorough evaluation, 734 studies were finally eligible for inclusion in the present systematic review and meta-analysis.

RESULT

According to the results, the prevalence of resistance to colistin among A. baumannii isolates was 4% (95% CI 3-5%), which has increased significantly from 2% before 2011 to 5% after 2012. South America had the highest resistance rate to this antibiotic. The broth microdilution method had the highest level of resistance, while the agar dilution showed the lowest level.

CONCLUSIONS

This meta-analysis found a low prevalence of colistin resistance among A. baumannii isolates responsible for infections worldwide from 2000 to 2023. However, there is a high prevalence of colistin-resistant isolates in certain countries. This implies an urgent public health threat, as colistin is one of the last antibiotics available for the treatment of infections caused by XDR strains of A. baumannii.

Mechanistic and biophysical characterization of polymyxin resistance response regulator PmrA in Acinetobacter baumannii

Introduction

Acinetobacter baumannii PmrAB is a crucial two-component regulatory system (TCS) that plays a vital role in conferring resistance to polymyxin. PmrA, a response regulator belonging to the OmpR/PhoB family, is composed of a C-terminal DNA-binding effector domain and an N-terminal receiver domain. The receiver domain can be phosphorylated by PmrB, a transmembrane sensor histidine kinase that interacts with PmrA. Once phosphorylated, PmrA undergoes a conformational change, resulting in the formation of a symmetric dimer in the receiver domain. This conformational change facilitates the recognition of promoter DNA by the DNA-binding domain of PmrA, leading to the activation of adaptive responses.

Methods

X-ray crystallography was carried out to solve the structure of PmrA receiver domain. Electrophoretic mobility shift assay and Isothermal titration calorimetry were recruited to validate the interaction between the recombinant PmrA protein and target DNA. Field-emission scanning electron microscopy (FE-SEM) was employed to characterize the surface morphology of A. baumannii in both the PmrA knockout and mutation strains.

Results

The receiver domain of PmrA follows the canonical α5β5 response regulator assembly, which undergoes dimerization upon phosphorylation and activation. Beryllium trifluoride is utilized as an aspartate phosphorylation mimic in this process. Mutations involved in phosphorylation and dimerization significantly affected the expression of downstream pmrC and naxD genes. This impact resulted in an enhanced cell surface smoothness with fewer modifications, ultimately contributing to a decrease in colistin (polymyxin E) and polymyxin B resistance. Additionally, a conservative direct-repeat DNA PmrA binding sequence TTTAAGNNNNNTTTAAG was identified at the promoter region of the pmrC and naxD gene. These findings provide structural insights into the PmrA receiver domain and reveal the mechanism of polymyxin resistance, suggesting that PmrA could be a potential drug target to reverse polymyxin resistance in Acinetobacter baumannii.

Portrait of a killer: Uncovering resistance mechanisms and global spread of Acinetobacter baumannii

Antibiotic resistance is a growing global concern in the field of medicine as it renders bacterial infections difficult to treat and often more severe. Acinetobacter baumannii is a gram-negative bacterial pathogen causing a wide range of infections, including pneumonia, sepsis, urinary tract infections, and wound infections. A. baumannii has emerged as a significant healthcare-associated pathogen due to its high level of antibiotic resistance. The global spread of antibiotic-resistant strains of A. baumannii has resulted in limited treatment options, leading to increased morbidity and mortality rates, especially in vulnerable populations such as the elderly and immunocompromised individuals, as well as longer hospital stays and higher healthcare costs. Further complicating the situation, multi- and pan-drug-resistant strains of A. baumannii are becoming increasingly common, and these deadly strains are resistant to all or almost all available antibiotics. A. baumannii employs various clever strategies to develop antibiotic resistance, including horizontal transfer of resistance genes, overexpression of inherent efflux pumps that remove drugs from the cell, intrinsic mutations, combined with natural selection under antibiotic selective pressure leading to emergence of successful resistance clones. The typical multidrug resistance phenotype of A. baumannii is, therefore, an orchestrated collimation of all these mechanisms combined with the worldwide spread of "global clones," rendering infections caused by this pathogen challenging to control and treat. To address the escalating problem of antibiotic resistance in A. baumannii, there is a need for increased surveillance, strict infection control measures, and the development of new treatment strategies, requiring a concerted effort by healthcare professionals, researchers, and policymakers.

CRISPR-Based Gene Editing in Acinetobacter baumannii to Combat Antimicrobial Resistance

Antimicrobial resistance (AMR) poses a significant threat to the health, social, environment, and economic sectors on a global scale and requires serious attention to addressing this issue. Acinetobacter baumannii was given top priority among infectious bacteria because of its extensive resistance to nearly all antibiotic classes and treatment options. Carbapenem-resistant A. baumannii is classified as one of the critical-priority pathogens on the World Health Organization (WHO) priority list of antibiotic-resistant bacteria for effective drug development. Although available genetic manipulation approaches are successful in A. baumannii laboratory strains, they are limited when employed on newly acquired clinical strains since such strains have higher levels of AMR than those used to select them for genetic manipulation. Recently, the CRISPR-Cas (Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein) system has emerged as one of the most effective, efficient, and precise methods of genome editing and offers target-specific gene editing of AMR genes in a specific bacterial strain. CRISPR-based genome editing has been successfully applied in various bacterial strains to combat AMR; however, this strategy has not yet been extensively explored in A. baumannii. This review provides detailed insight into the progress, current scenario, and future potential of CRISPR-Cas usage for AMR-related gene manipulation in A. baumannii.

Drug‑resistant Acinetobacter baumannii: From molecular mechanisms to potential therapeutics (Review)

Bacterial drug resistance is increasingly becoming an important problem that needs to be solved urgently in modern clinical practices. Infection caused by Acinetobacter baumannii is a serious threat to the life and health of patients. The drug resistance rate of Acinetobacter baumannii strains is increasing, thus research on the drug resistance of Acinetobacter baumannii has also seen an increase. When patients are infected with drug-resistant Acinetobacter baumannii, the availability of suitable antibiotics commonly used in clinical practices is becoming increasingly limited and the prognosis of patients is worsening. Studying the molecular mechanism of the drug resistance of Acinetobacter baumannii is fundamental to solving the problem of drug-resistant Acinetobacter baumannii and potentially other 'super bacteria'. Drug resistance mechanisms primarily include enzymes, membrane proteins, efflux pumps and beneficial mutations. Research on the underlying mechanisms provides a theoretical basis for the use and development of antibiotics and the development of novel treatment methods.

Swine farm groundwater is a hidden hotspot for antibiotic-resistant pathogenic Acinetobacter

Acinetobacter is present in the livestock environment, but little is known about their antibiotic resistance and pathogenic species in the farm groundwater. Here we investigated antibiotic resistance of Acinetobacter in the swine farm groundwater (JZPG) and residential groundwater (JZG) of a swine farming village, in comparison to a nearby (3.5 km) non-farming village (WTG) using metagenomic and culture-based approaches. Results showed that the abundance of antibiotic resistome in some JZG and all JZPG (~3.4 copies/16S rRNA gene) was higher than that in WTG (~0.7 copies/16S rRNA gene), indicating the influence of farming activities on both groundwater types. Acinetobacter accounted for ~95.7% of the bacteria in JZG and JZPG, but only ~8.0% in WTG. They were potential hosts of ~95.6% of the resistome in farm affected groundwater, which includes 99 ARG subtypes against 23 antibiotic classes. These ARGs were associated with diverse intrinsic and acquired resistance mechanisms, and the predominant ARGs were tetracyclines and fluoroquinolones resistance genes. Metagenomic binning analysis elucidated that non-baumannii Acinetobacter including A. oleivorans, A. beijerinckii, A. seifertii, A. bereziniae and A. modestus might pose environmental risks because of multidrug resistance, pathogenicity and massive existence in the groundwater. Antibiotic susceptibility tests showed that the isolated strains were resistant to multiple antibiotics including sulfamethoxazole (resistance ratio: 96.2%), levofloxacin (42.5%), gatifloxacin (39.0%), ciprofloxacin (32.6%), tetracycline (32.0%), doxycycline (29.0%) and ampicillin (12.0%) as well as last-resort polymyxin B (31.7%), colistin (24.1%) and tigecycline (4.1%). The findings highlight potential prevalence of groundwater-borne antibiotic-resistant pathogenic Acinetobacter in the livestock environment.

Therapeutic Prospection of Animal Venoms-Derived Antimicrobial Peptides against Infections by Multidrug-Resistant Acinetobacter baumannii: A Systematic Review of Pre-Clinical Studies

Infections caused by multidrug-resistant Acinetobacter baumannii (MDR-Ab) have become a public health emergency. Due to the small therapeutic arsenal available to treat these infections, health agencies have highlighted the importance of developing new antimicrobials against MDR-Ab. In this context, antimicrobial peptides (AMPs) stand out, and animal venoms are a rich source of these compounds. Here, we aimed to summarize the current knowledge on the use of animal venom-derived AMPs in the treatment of MDR-Ab infections in vivo. A systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The eight studies included in this review identified the antibacterial activity of eleven different AMPs against MDR-Ab. Most of the studied AMPs originated from arthropod venoms. In addition, all AMPs are positively charged and rich in lysine residues. In vivo assays showed that the use of these compounds reduces MDR-Ab-induced lethality and bacterial load in invasive (bacteremia and pneumonia) and superficial (wounds) infection models. Moreover, animal venom-derived AMPs have pleiotropic effects, such as pro-healing, anti-inflammatory, and antioxidant activities, that help treat infections. Animal venom-derived AMPs are a potential source of prototype molecules for the development of new therapeutic agents against MDR-Ab.

Gram-negative bacilli carrying mcr gene in Brazil: a pathogen on the rise

The incidence of infections caused by resistant Gram-negative pathogens has become a critical factor in public health due to the limitation of therapeutic options for the control of infections caused, especially, by Enterobacteriaceae (Escherichia coli and Klebsiella pneumoniae), Pseudomonas spp., and Acinetobacter spp. Thus, given the increase in resistant pathogens and the reduction of therapeutic options, polymyxins were reintroduced into the clinic. As the last treatment option, polymyxins were regarded as the therapeutic key, since they were one of the few classes of antimicrobials that had activity against multidrug-resistant Gram-negative bacilli. Nonetheless, over the years, the frequent use of this antimicrobial has led to reports of resistance cases. In 2015, mcr (mobile colistin resistance), a colistin resistance gene, was described in China. Due to its location on carrier plasmids, this gene is characterized by rapid spread through conjugation. It has thus been classified as a rising threat to public health worldwide. In conclusion, based on several reports that show the emergence of mcr in different regional and climatic contexts and species of isolates, this work aims to review the literature on the incidence of the mcr gene in Brazil in different regions, types of samples identified, species of isolates, and type of carrier plasmid.

Adverse Events and Drug Resistance in Critically Ill Patients Treated with Colistimethate Sodium: A Review of the Literature

The adverse events related to sodium colistimethate have had variability regarding the prevalence of nephrotoxicity, neurotoxicity, and less frequent respiratory depression. In recent years, its use has been relevant due to the increase of multidrug-resistant bacteria since it is considered the last-line drug, being its main adverse event and reason for discrepancies between authors' nephrotoxicity. The indiscriminate use of antibiotic therapy has generated multiple mechanisms of resistance, the most common being related to Colistin, the bactericidal escape effect. Based on the search criteria, no randomized clinical trials were identified showing safety and efficacy with the use of Colistin, inferring that the application of the appropriate dose is governed by expert opinion and retrospective and prospective observational studies, which confounding factors such as the severity of the patient and the predisposition to develop acute renal failure are constant. In this review, we focus on identifying the mechanism of nephrotoxicity and bacterial resistance, where much remains to be known.

Colistin Resistance in Acinetobacter baumannii: Molecular Mechanisms and Epidemiology

Acinetobacter baumannii is recognized as a clinically significant pathogen causing a wide spectrum of nosocomial infections. Colistin was considered a last-resort antibiotic for the treatment of infections caused by multidrug-resistant A. baumannii. Since the reintroduction of colistin, a number of mechanisms of colistin resistance in A. baumannii have been reported, including complete loss of LPS by inactivation of the biosynthetic pathway, modifications of target LPS driven by the addition of phosphoethanolamine (PEtN) moieties to lipid A mediated by the chromosomal pmrCAB operon and eptA gene-encoded enzymes or plasmid-encoded mcr genes and efflux of colistin from the cell. In addition to resistance to colistin, widespread heteroresistance is another feature of A. baumannii that leads to colistin treatment failure. This review aims to present a critical assessment of relevant published (>50 experimental papers) up-to-date knowledge on the molecular mechanisms of colistin resistance in A. baumannii with a detailed review of implicated mutations and the global distribution of colistin-resistant strains.

Molecular characterisation of colistin and carbapenem-resistant clinical isolates of Acinetobacter baumannii from Southeast Europe

OBJECTIVES

To characterise 11 colistin- and carbapenem-resistant Acinetobacter baumannii isolates recently emerging in hospital settings.

METHODS

A. baumannii isolates were collected from hospitalised patients under colistin treatment in three countries of Southeast Europe: Turkey, Croatia, and Bosnia and Herzegovina. Isolates were identified using molecular methods.

RESULTS

Isolates from Turkey and Croatia belong to the sequence types ST195 or ST281 of the clone lineage 2, while the single isolate from Bosnia and Herzegovina belongs to the ST231 of clone lineage 1. All isolates turned out to be highly resistant to colistin (MIC ≥ 16 mg/L) and have point mutations in pmrCAB operon genes. The colistin-resistant isolate from Bosnia and Herzegovina had a unique P170L point mutation in the pmrB gene and the R125H point mutation in the pmrC gene. The L20S mutation in the pmrA gene was detected only in isolates from Croatia and has never been reported before in isolates from this country.

CONCLUSION

Colistin resistance in A. baumannii in hospitalised patients receiving colistin treatment is a result of chromosomal mutations. The pattern of point mutations in pmrCAB genes suggests a spread of specific colistin-resistant isolates within the hospital.

Genomic Analysis of Group B Streptococcus from Neonatal Sepsis Reveals Clonal CC17 Expansion and Virulence- and Resistance-Associated Traits After Intrapartum Antibiotic Prophylaxis

BACKGROUND

Group B Streptococcus (GBS) is a leading cause of invasive neonatal infections. This study aimed to investigate the trend of GBS serotype and genotype change and their correlation with antimicrobial resistance before and after implementation of intrapartum antibiotic prophylaxis (IAP).

METHODS

We performed serotyping, whole-genome sequencing, antimicrobial susceptibility testing, and single-nucleotide polymorphism (SNP)-based phylogenetic analysis on 238 invasive GBS isolates collected from October 1998 to February 2020 in Taiwan.

RESULTS

There were 7 serotypes and 6 clonal complexes (CCs) among the 238 GBS isolates, and more than half of the isolates carried multiple antimicrobial resistance genes. The expansion of CC17 strains and the increase in late-onset disease occurred synchronously after the implementation of IAP. Analysis of the carriage isolates from pregnant women showed diverse serotype distribution in the IAP era. The antimicrobial susceptibility testing showed that all 238 strains were susceptible to ampicillin and penicillin, while the number of various resistance genes in GBS genomes was found increased with the expansion of CC17. Compared with reference genomes, 697 nonsynonymous SNPs in 443 protein-coding genes were CC17 specific.

CONCLUSIONS

The study revealed the expansion of GBS CC17 and the increase of late-onset disease that occurred simultaneously with the implementation of IAP. Although the susceptibility of CC17 to antimicrobial agents is not different from that of other sequence types at present, GBS with phenotypic resistance to antimicrobials may emerge in the future, given the environmental selection pressure and the continued accumulation of SNP mutations.

Preferential Selection of Low-Frequency, Lipopolysaccharide-Modified, Colistin-Resistant Mutants with a Combination of Antimicrobials in Acinetobacter baumannii

Colistin, which targets lipopolysaccharide (LPS), is used as a last-resort drug against severe infections caused by drug-resistant Acinetobacter baumannii. However, A. baumannii possesses two colistin-resistance mechanisms. LPS modification caused by mutations in pmrAB genes is often observed in clinical isolates of multidrug-resistant Gram-negative pathogens. In addition to LPS modification, A. baumannii has a unique colistin resistance mechanism, a complete loss of LPS due to mutations in the lpxACD genes, which are involved in LPS biosynthesis. This study aimed to elucidate the detailed mechanism of the emergence of colistin-resistant A. baumannii using strains with the same genetic background. Various colistin-resistant strains were generated experimentally using colistin alone and in combination with other antimicrobials, such as meropenem and ciprofloxacin, and the mutation spectrum was analyzed. In vitro selection of A. baumannii in the presence of colistin led to the emergence of strains harboring mutations in lpxACD genes, resulting in LPS-deficient colistin-resistant strains. However, combination of colistin with other antimicrobials led to the selection of pmrAB mutant strains, resulting in strains with modified LPS (LPS-modified strains). Further, the LPS-deficient strains showed decreased fitness and increased susceptibility to many antibiotics and disinfectants. As LPS-deficient strains have a higher biological cost than LPS-modified strains, our findings suggested that pmrAB mutants are more likely to be isolated in clinical settings. We provide novel insights into the mechanisms of resistance to colistin and provide substantial solutions along with precautions for facilitating current research and treatment of colistin-resistant A. baumannii infections. IMPORTANCE Acinetobacter baumannii has developed resistance to various antimicrobial drugs, and its drug-resistant strains cause nosocomial infections. Controlling these infections has become a global clinical challenge. Carbapenem antibiotics are the frontline treatment drugs for infectious diseases caused by A. baumannii. For patients with infections caused by carbapenem-resistant A. baumannii, colistin-based therapy is often the only treatment option. However, A. baumannii readily acquires resistance to colistin. Many patients infected with colistin-resistant A. baumannii undergo colistin treatment before isolation of the colistin-resistant strain, and it is hypothesized that colistin resistance predominantly emerges under selective pressure during colistin therapy. Although the concomitant use of colistin and carbapenems has been reported to have a synergistic effect in vitro against carbapenem-resistant A. baumannii strains, our observations strongly suggest the need for attention to the emergence of strains with a modified lipopolysaccharide during treatment.

A Systematic Review of Antibiotic Resistance Trends and Treatment Options for Hospital-Acquired Multidrug-Resistant Infections

Antimicrobial resistance is a major public health challenge described by the World Health Organization as one of the top 10 public health challenges worldwide. Drug-resistant microbes contribute significantly to morbidity and mortality in the hospital, especially in the critical care unit. The primary etiology of increasing antibiotic resistance is inappropriate and excessive use of antibiotics. The alarming rise of drug-resistant microbes worldwide threatens to erode our ability to treat infections with our current armamentarium of antibiotics. Unfortunately, the pace of development of new antibiotics by the pharmaceutical industry has not kept up with rising resistance to expand our options to treat microbial infections. The costs of antibiotic resistance include death and disability, extended hospital stays due to prolonged sickness, need for expensive therapies, rising healthcare expenditure, reduced productivity from time out of the workforce, and rising penury. This review sums up the common mechanisms, trends, and treatment options for hospital-acquired multidrug-resistant microbes.

Genotypic and phenotypic comparison of clinical and environmental Acinetobacter baumannii strains

The genotypic and phenotypic characteristics and antibiotic resistance (antibiogram) profiles of clinical (n = 13) and environmental (n = 7) Acinetobacter baumannii isolates were compared. Based on the Repetitive Extragenic Palindromic Sequence-based PCR (REP-PCR) analysis, the clinical and environmental A. baumannii isolates shared low genetic relatedness (∼60%). Multilocus sequence typing (MLST, Oxford scheme) indicated that the clinical A. baumannii were assigned to three sequence types (ST231, ST945 and ST848), while the environmental A. baumannii (excluding AB 14) were categorised into the novel ST2520. The majority of the clinical (excluding AB 5, CAB 11, CAC 37) and environmental (excluding AB 14 and AB 16) A. baumannii strains were then capable of phase variation with both the translucent (71.4%; 15/21) and opaque (95.2%; 20/21) colony phenotypes detected. The clinical isolates however, exhibited significantly (p < 0.05) higher biofilm formation capabilities (OD₅₇₀: 2.094 ± 0.497). Moreover, the clinical isolates exhibited significantly (p < 0.05) higher resistance to first line antibiotics, with 92.3% (12/13) characterised as extensively drug resistant (XDR), whereas environmental A. baumannii exhibited increased antibiotic susceptibility with only 57.1% (4/7) characterised as multidrug resistant (MDR). The environmental isolate AB 14 was however, characterised as XDR. In addition, only five clinical A. baumannii isolates exhibited colistin resistance (38.5%; 5/13). The current study highlighted the differences in the genotypic, phenotypic, and antibiotic resistance profiles of clinical and environmental A. baumannii. Moreover, the environmental strains were assigned to the novel ST2520, which substantiates the existence of this opportunistic pathogen in extra-hospital reservoirs.

Apolipoprotein E mimetic peptide COG1410 combats pandrug-resistant Acinetobacter baumannii

The emergence of pandrug-resistant bacteria breaks through the last line of defense and raises fear among people of incurable infections. In the post-antibiotic era, the pharmaceutical field turns to seek non-conventional anti-infective agents. Antimicrobial peptides are considered a prospective solution to the crisis of antimicrobial resistance. In this study, we evaluated the antimicrobial efficiency of an ApoE mimetic peptide, COG1410, which has been confirmed to exhibit strong neural protective activity and immunomodulatory function. COG1410 showed potent antimicrobial activity against pandrug-resistant Acinetobacter baumannii, even eliminating large inocula (108 CFU/ml) within 30 min. LC99.9 in PBS and 50% pooled human plasma was 2 μg/ml (1.4 μM) and 8 μg/ml (5.6 μM), respectively. Moreover, COG1410 exhibited biofilm inhibition and eradication activity, excellent stability in human plasma, and a low propensity to induce resistance. Although COG1410 easily entered bacterial cytoplasm and bound to DNA nonspecifically, the major mechanism of COG1410 killing was to disrupt the integrity of cell membrane and lead to leakage of cytoplasmic contents, without causing obvious pores on the cell surface or cell lysis. Additionally, transcriptome analysis showed that treatment with COG1410-enriched genes involved a series of oxidation–reduction processes. DCFH-DA probe detected an increased ROS level in the presence of COG1410, indicating ROS was another hit of this AMP. Furthermore, the action of COG1410 did not depend on the electronic interaction with the LPS layer, in contrast to polymyxin B. The strong synergistic interaction between COG1410 and polymyxin B dramatically reduced the working concentration of COG1410, expanding the safety window of the application. C. elegans infection model showed that combined therapy of COG1410 and polymyxin B was capable of significantly rescuing the infected nematodes. Taken together, our study demonstrates that COG1410 is a promising drug candidate in the battle against pandrug-resistant A. baumannii.

Evasion of Antimicrobial Activity in Acinetobacter baumannii by Target Site Modifications: An Effective Resistance Mechanism

Acinetobacter baumannii is a Gram-negative bacillus that causes multiple infections that can become severe, mainly in hospitalized patients. Its high ability to persist on abiotic surfaces and to resist stressors, together with its high genomic plasticity, make it a remarkable pathogen. Currently, the isolation of strains with high antimicrobial resistance profiles has gained relevance, which complicates patient treatment and prognosis. This resistance capacity is generated by various mechanisms, including the modification of the target site where antimicrobial action is directed. This mechanism is mainly generated by genetic mutations and contributes to resistance against a wide variety of antimicrobials, such as β-lactams, macrolides, fluoroquinolones, aminoglycosides, among others, including polymyxin resistance, which includes colistin, a rescue antimicrobial used in the treatment of multidrug-resistant strains of A. baumannii and other Gram-negative bacteria. Therefore, the aim of this review is to provide a detailed and up-to-date description of antimicrobial resistance mediated by the target site modification in A. baumannii, as well as to detail the therapeutic options available to fight infections caused by this bacterium.

Biological Control of Acinetobacter baumannii: In Vitro and In Vivo Activity, Limitations, and Combination Therapies

The survival, proliferation, and epidemic spread of Acinetobacter baumannii (A. baumannii) in hospital settings is associated with several characteristics, including resistance to many commercially available antibiotics as well as the expression of multiple virulence mechanisms. This severely limits therapeutic options, with increased mortality and morbidity rates recorded worldwide. The World Health Organisation, thus, recognises A. baumannii as one of the critical pathogens that need to be prioritised for the development of new antibiotics or treatment. The current review will thus provide a brief overview of the antibiotic resistance and virulence mechanisms associated with A. baumannii’s “persist and resist strategy”. Thereafter, the potential of biological control agents including secondary metabolites such as biosurfactants [lipopeptides (surfactin and serrawettin) and glycolipids (rhamnolipid)] as well as predatory bacteria (Bdellovibrio bacteriovorus) and bacteriophages to directly target A. baumannii, will be discussed in terms of their in vitro and in vivo activity. In addition, limitations and corresponding mitigations strategies will be outlined, including curtailing resistance development using combination therapies, product stabilisation, and large-scale (up-scaling) production.

Whole-Genome Sequencing of a Colistin-Resistant Acinetobacter baumannii Strain Isolated at a Tertiary Health Facility in Pretoria, South Africa

BACKGROUND

Acinetobacter baumannii's (A. baumannii) growing resistance to all available antibiotics is of concern. The study describes a colistin-resistant A. baumannii isolated at a clinical facility from a tracheal aspirate sample. Furthermore, it determines the isolates' niche establishment ability within the tertiary health facility.

METHODS

An antimicrobial susceptibility test, conventional PCR, quantitative real-time PCR, phenotypic evaluation of the efflux pump, and whole-genome sequencing and analysis were performed on the isolate.

RESULTS

The antimicrobial susceptibility pattern revealed a resistance to piperacillin/tazobactam, ceftazidime, cefepime, cefotaxime/ceftriaxone, imipenem, meropenem, gentamycin, ciprofloxacin, trimethoprim/sulfamethoxazole, tigecycline, and colistin. A broth microdilution test confirmed the colistin resistance. Conventional PCR and quantitative real-time PCR investigations revealed the presence of adeB, adeR, and adeS, while mcr-1 was not detected. A MIC of 0.38 µg/mL and 0.25 µg/mL was recorded before and after exposure to an AdeABC efflux pump inhibitor. The whole-genome sequence analysis of antimicrobial resistance-associated genes detected beta-lactam: blaOXA-66; blaOXA-23; blaADC-25; blaADC-73; blaA1; blaA2, and blaMBL; aminoglycoside: aph(6)-Id; aph(3″)-Ib; ant(3″)-IIa and armA) and a colistin resistance-associated gene lpsB. The whole-genome sequence virulence analysis revealed a biofilm formation system and cell-cell adhesion-associated genes: bap, bfmR, bfmS, csuA, csuA/B, csuB, csuC, csuD, csuE, pgaA, pgaB, pgaC, and pgaD; and quorum sensing-associated genes: abaI and abaR and iron acquisition system associated genes: barA, barB, basA, basB, basC, basD, basF, basG, basH, basI, basJ, bauA, bauB, bauC, bauD, bauE, bauF, and entE. A sequence type classification based on the Pasteur scheme revealed that the isolate belongs to sequence type ST2.

CONCLUSIONS

The mosaic of the virulence factors coupled with the resistance-associated genes and the phenotypic resistance profile highlights the risk that this strain is at this South African tertiary health facility.

Synergistic Inhibitory Effect of Polymyxin B in Combination with Ceftazidime against Robust Biofilm Formed by Acinetobacter baumannii with Genetic Deficiency in AbaI/AbaR Quorum Sensing

Carbapenem resistance of Acinetobacter baumannii poses challenges to public health. Biofilm contributes to the persistence of A. baumannii cells. This study was designed to investigate the genetic relationships among carbapenem resistance, polymyxin resistance, multidrug resistance, biofilm formation, and surface-associated motility and evaluate the antibiofilm effect of polymyxin in combination with other antibiotics. A total of 103 clinical A. baumannii strains were used to determine antibiotic susceptibility, biofilm formation capacity, and motility. Enterobacterial repetitive intergenic consensus (ERIC)-PCR fingerprinting was used to determine the genetic variation among strains. The distribution of 17 genes related to the resistance-nodulation-cell division (RND)-type efflux, autoinducer-receptor (AbaI/AbaR) quorum sensing, oxacillinases (OXA)-23, and insertion sequence of ISAba1 element was investigated. The representative strains were chosen to evaluate the gene transcription and the antibiofilm activity by polymyxin B (PB) in combination with merapenem, levofloxacin, and ceftazidime, respectively. ERIC-PCR-dependent fingerprints were found to be associated with carbapenem resistance and multidrug resistance. The presence of bla_OXA-23 was found to correlate with genes involved in ISAba1 insertion, AbaI/AbaR quorum sensing, and AdeABC efflux. Carbapenem resistance was observed to be negatively correlated with biofilm formation and positively correlated with motility. PB in combination with ceftazidime displayed a synergistic antibiofilm effect against robust biofilm formed by an A. baumannii strain with deficiency in AbaI/AbaR quorum sensing. Our results not only clarify the genetic correlation among carbapenem resistance, biofilm formation, and pathogenicity in a certain level but also provide a theoretical basis for clinical applications of polymyxin-based combination of antibiotics in antibiofilm therapy. IMPORTANCE Deeper explorations of molecular correlation among antibiotic resistance, biofilm formation, and pathogenicity could provide novel insights that would facilitate the development of therapeutics and prevention against A. baumannii biofilm-related infections. The major finding that polymyxin B in combination with ceftazidime displayed a synergistic antibiofilm effect against robust biofilm formed by an A. baumannii strain with genetic deficiency in AbaI/AbaR quorum sensing further provides a theoretical basis for clinical applications of antibiotics in combination with quorum quenching in antibiofilm therapy.

Netzahualcoyonol from Salacia multiflora (Lam.) DC. (Celastraceae) roots as a bioactive compound against gram-positive pathogens

The expression of virulence factors, such as biofilm formation, in association with the acquisition of resistance to multiple drugs, has evidenced the need for new and effective antimicrobial agents against Staphylococcus aureus. The evaluation of the pharmacological properties of plant-derived compounds is a promising alternative to the development of new antimicrobials. In this study, we aimed to evaluate the antibacterial, antibiofilm, and the synergistic and cytotoxic effects of netzahualcoyonol isolated from Salacia multiflora (Lam.) DC. roots. Netzahualcoyonol presented bacteriostatic (1.56-25.0 µg/mL) and bactericidal (25.0-400.0 µg/mL) effects against Gram-positive bacteria, disrupted the biofilm of S. aureus, and presented a synergistic effect after its combination with β-lactams and aminoglycosides. The low cytotoxicity of netzahualcoyonol (Selectivity Index (SI) for S. aureus (2.56), S. saprophyticus (20.56), and Bacillus subtilis (1.28)) suggests a good security profile. Taken together, these results show that netzahualcoyonol is promising for the development of a new effective antibacterial agent.

Pegylated LyeTx I-b peptide is effective against carbapenem-resistant Acinetobacter baumannii in an in vivo model of pneumonia and shows reduced toxicity

The World Health Organization (WHO) has been warning about the importance of developing new drugs against superbugs. Antimicrobial peptides are an alternative in this context, most of them being involved in innate immunity, acting in various ways, and some even showing synergism with commercial antimicrobial agents. LyeTx I-b is a synthetic peptide derived from native LyeTx I, originally isolated from Lycosa erythrognatha spider venom. Although LyeTx I-b is active against several multidrug-resistant bacteria, it shows some hemolytic and cytotoxic effects. To overcome this hindrance, in the present study we PEGylated LyeTx I-b and evaluated its toxicity and in vitro and in vivo activities on pneumonia caused by multi-resistant Acinetobacter baumannii. PEGylated LyeTx I-b (LyeTx I-bPEG) maintained the same MIC value as the non- PEGylated peptide, showed anti-biofilm activity, synergistic effect with commercial antimicrobial agents, and did not induce resistance. Moreover, in vivo experiments showed its activity against pneumonia. Additionally, LyeTx I-bPEG reduced hemolysis up to 10 times, was approximately 2 times less cytotoxic to HEK-293 cells and 4 times less toxic to mice in acute toxicity models, compared to LyeTx I-b. Our results show LyeTx I-bPEG as a promising antimicrobial candidate, significantly active against pneumonia caused by multidrug-resistant A. baumannii.

Colistin Resistant mcr Genes Prevalence in Livestock Animals (Swine, Bovine, Poultry) from a Multinational Perspective. A Systematic Review

The objective of this review is to collect and present the results of relevant studies on an international level, on the subject of colistin resistance due to mcr genes prevalence in livestock animals. After a literature search, and using PRISMA guidelines principles, a total of 40 swine, 16 bovine and 31 poultry studies were collected concerning mcr-1 gene; five swine, three bovine and three poultry studies referred to mcr-2 gene; eight swine, one bovine, two poultry studies were about mcr-3 gene; six swine, one bovine and one poultry manuscript studied mcr-4 gene; five swine manuscripts studied mcr-5 gene; one swine manuscript was about mcr-6, mcr-7, mcr-8, mcr-9 genes and one poultry study about mcr-10 gene was found. Information about colistin resistance in bacteria derived from animals and animal product foods is still considered limited and that should be continually enhanced; most of the information about clinical isolates are relative to enteropathogens Escherichia coli and Salmonella spp. This review demonstrates the widespread dispersion of mcr genes to livestock animals, indicating the need to further increase measures to control this important threat for public health issue.

Current Update on Intrinsic and Acquired Colistin Resistance Mechanisms in Bacteria

Colistin regained global interest as a consequence of the rising prevalence of multidrug-resistant Gram-negative Enterobacteriaceae. In parallel, colistin-resistant bacteria emerged in response to the unregulated use of this antibiotic. However, some Gram-negative species are intrinsically resistant to colistin activity, such as Neisseria meningitides, Burkholderia species, and Proteus mirabilis. Most identified colistin resistance usually involves modulation of lipid A that decreases or removes early charge-based interaction with colistin through up-regulation of multistep capsular polysaccharide expression. The membrane modifications occur by the addition of cationic phosphoethanolamine (pEtN) or 4-amino-l-arabinose on lipid A that results in decrease in the negative charge on the bacterial surface. Therefore, electrostatic interaction between polycationic colistin and lipopolysaccharide (LPS) is halted. It has been reported that these modifications on the bacterial surface occur due to overexpression of chromosomally mediated two-component system genes (PmrAB and PhoPQ) and mutation in lipid A biosynthesis genes that result in loss of the ability to produce lipid A and consequently LPS chain, thereafter recently identified variants of plasmid-borne genes (mcr-1 to mcr-10). It was hypothesized that mcr genes derived from intrinsically resistant environmental bacteria that carried chromosomal pmrC gene, a part of the pmrCAB operon, code three proteins viz. pEtN response regulator PmrA, sensor kinase protein PmrAB, and phosphotransferase PmrC. These plasmid-borne mcr genes become a serious concern as they assist in the dissemination of colistin resistance to other pathogenic bacteria. This review presents the progress of multiple strategies of colistin resistance mechanisms in bacteria, mainly focusing on surface changes of the outer membrane LPS structure and other resistance genetic determinants. New handier and versatile methods have been discussed for rapid detection of colistin resistance determinants and the latest approaches to revert colistin resistance that include the use of new drugs, drug combinations and inhibitors. Indeed, more investigations are required to identify the exact role of different colistin resistance determinants that will aid in developing new less toxic and potent drugs to treat bacterial infections. Therefore, colistin resistance should be considered a severe medical issue requiring multisectoral research with proper surveillance and suitable monitoring systems to report the dissemination rate of these resistant genes.

Characterisation of Colistin -Resistant Enterobacterales and Acinetobacter Strains Carrying mcr Genes from Asian Aquaculture Products

Aquaculture systems are widely recognised as hotspots for horizontal gene transfer, and the need for screening for bacteria carrying antimicrobial resistance genes in aquaculture systems is becoming more important. In this study, we characterised seventeen bacterial strains (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, and A. nosocomialis) resistant to colistin originating from retailed aquaculture products imported from Vietnam to the Czech Republic. The mcr-1.1 gene was found located on plasmid types IncHI2, IncI2, and IncX4, as well as on the rarely described plasmid types IncFIB-FIC and IncFIB(K), phage-like plasmid p0111, and on the chromosome of E. coli. One E. coli strain carried the mcr-3.5 gene on IncFII(pCoo) plasmid in addition to the mcr-1.1 gene located on IncHI2 plasmid. K. pneumoniae was found to carry the mcr-1.1 and mcr-8.2 genes on IncFIA(HI1) plasmid. The mcr-4.3 gene was found on similar untypeable plasmids of A. baumannii and A. nosocomialis strains, pointing to the possible interspecies transfer of plasmids carrying the mcr-4 gene. Our results highlight that some aquaculture products of Asian origin can represent an important source of variable plasmids carrying mcr genes. The results showed an involvement of phages in the incorporation of the mcr-1 gene into plasmids or the chromosome in E. coli strains from aquaculture. The detection of E. coli with the mcr-1 gene in the chromosome points to the risks associated with the stabilisation of the mcr genes in the bacterial chromosome.

A short synthetic peptide, based on LyeTx I from Lycosa erythrognatha venom, shows potential to treat pneumonia caused by carbapenem-resistant Acinetobacter baumannii without detectable resistance

The emergence of antibiotic-resistant bacteria, especially carbapenem-resistant Acinetobacter baumannii (CRAB), together with relative stagnation in the development of effective antibiotics, has led to enormous health and economic problems. In this study, we aimed to describe the antibacterial spectrum of LyeTx I mnΔK, a short synthetic peptide based on LyeTx I from Lycosa erythrognatha venom, against CRAB. LyeTx I mnΔK showed considerable antibacterial activity against extensively resistant A. baumannii, with minimum inhibitory and bactericidal concentrations ranging from 1 to 16 µM and 2 to 32 µM, respectively. This peptide significantly increased the release of 260 nm-absorbing intracellular material from CRAB, suggesting bacteriolysis. LyeTx I mnΔK was shown to act synergistically with meropenem and colistin against CRAB. The cytotoxic concentration of LyeTx I mnΔK against Vero cells (CC50 = 55.31 ± 5.00 µM) and its hemolytic activity (HC50 = 77.07 ± 4.00 µM) were considerably low; however, its antibacterial activity was significantly reduced in the presence of human and animal serum and trypsin. Nevertheless, the inhalation of this peptide was effective in reducing pulmonary bacterial load in a mouse model of CRAB infection. Altogether, these results demonstrate that the peptide LyeTx I mnΔK is a potential prototype for the development of new effective and safe antibacterial agents against CRAB.

A novel plasmid-encoded mcr-4.3 gene in a colistin-resistant Acinetobacter baumannii clinical strain

OBJECTIVES

To identify the molecular mechanism of colistin resistance in an MDR Acinetobacter baumannii clinical strain isolated in 2008 from a meningitis case in Brazil.

METHODS

Long- and short-read WGS was used to identify colistin resistance genes in A. baumannii strain 597A with a colistin MIC of 64 mg/L. MS was used to analyse lipid A content. mcr was cloned into pET-26b (+) and transformed into Escherichia coli BL21(λDE3)pLysS for analysis.

RESULTS

A novel plasmid (pAb-MCR4.3) harbouring mcr-4.3 within a Tn3-like transposon was identified. The A. baumannii 597A lipid A MS spectra showed a main molecular ion peak at m/z=2034, which indicated the addition of phosphoethanolamine to the lipid A structure. E. coli BL21 transformed with pET-26b-mcr-4.3 gained colistin resistance with a colistin MIC of 8 mg/L.

CONCLUSIONS

Colistin resistance in A. baumannii 597A was correlated with the presence of a novel plasmid-encoded mcr-4.3 gene.

Is it time to reconsider prophylactic antimicrobial use for hematopoietic stem cell transplantation? a narrative review of antimicrobials in stem cell transplantation

INTRODUCTION

Hematopoietic Stem Cell Transplantation (HSCT) is a life-saving procedure for multiple types of hematological cancer, autoimmune diseases, and genetic-linked metabolic diseases in humans. Recipients of HSCT transplant are at high risk of microbial infections that significantly correlate with the presence of graft-versus-host disease (GVHD) and the degree of immunosuppression. Infection in HSCT patients is a leading cause of life-threatening complications and mortality.

AREAS COVERED

This review covers issues pertinent to infection in the HSCT patient, including bacterial and viral infection; strategies to reduce GVHD; infection patterns; resistance and treatment options; adverse drug reactions to antimicrobials, problems of antimicrobial resistance; perturbation of the microbiome; the role of prebiotics, probiotics, and antimicrobial peptides. We highlight potential strategies to minimize the use of antimicrobials.

EXPERT OPINION

Measures to control infection and its transmission remain significant HSCT management policy and planning issues. Transplant centers need to consider carefully prophylactic use of antimicrobials for neutropenic patients. The judicious use of appropriate antimicrobials remains a crucial part of the treatment protocol. However, antimicrobials' adverse effects cause microbiome diversity and dysbiosis and have been shown to increase morbidity and mortality.

Pristimerin isolated from Salacia crassifolia (Mart. Ex. Schult.) G. Don. (Celastraceae) roots as a potential antibacterial agent against Staphylococcus aureus

ETHNOPHARMACOLOGICAL RELEVANCE

Pristimerin is a triterpenoid considered the main component of Salacia crassifolia extracts. This terpene has shown promising antitumor, anti-inflammatory, and antimicrobial effects. Likewise, S. crassifolia has been used in traditional medicine to treat cancer and as an antimicrobial and anti-inflammatory agent.

AIM OF THE STUDY

This study aimed to evaluate the antibacterial activity of the hexane extract of Salacia crassifolia roots (HER) and its isolate, pristimerin, against pathogenic bacteria.

MATERIALS AND METHODS

First, we evaluated the spectrum of action of HER and pristimerin by the determination of the minimum inhibitory concentration (MIC) and the minimal bactericidal concentration (MBC). Subsequently, we analyzed the time-kill curve of these plant-derived compounds against Staphylococcus aureus. Then, we examined their mode of action by three different assays: the crystal violet methodology, the release of intracellular material, and transmission electron microscopy methods (TEM). Finally, we evaluated the effect of HER and pristimerin on the pre-formed biofilm of S. aureus by the crystal violet assay, the synergistic effect by the checkerboard method, the cytotoxicity against Vero cells, and the in silico activity using the online software PASS.

RESULTS

HER and pristimerin presented a narrow spectrum of action against Gram-positive bacteria (MIC 0.195-25 μg/mL), and their primary mode of action is the alteration of membrane permeability of S. aureus. Our results show that the compounds disrupted the pre-formed biofilm of S. aureus in a dose-dependent manner. Furthermore, HER and pristimerin presented a significant synergic effect after the combination with well-known antibiotics, which was associated with the ability of these phytomedicines to change membrane permeability. Regarding the cytotoxic effect, the selective index (SI) of HER ranged from 0.37 to 11.86, and the SI of pristimerin varied from 0.24 to 30.87, according to the bacteria tested.

CONCLUSIONS

Overall, HER and pristimerin showed a promising antibacterial effect in vitro through the alteration of membrane permeability of S. aureus.

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

Increasing antibiotic resistance in multidrug-resistant (MDR) Gram-negative bacteria (MDR-GNB) presents significant health problems worldwide, since the vital available and effective antibiotics, including; broad-spectrum penicillins, fluoroquinolones, aminoglycosides, and β-lactams, such as; carbapenems, monobactam, and cephalosporins; often fail to fight MDR Gram-negative pathogens as well as the absence of new antibiotics that can defeat these "superbugs". All of these has prompted the reconsideration of old drugs such as polymyxins that were reckoned too toxic for clinical use. Only two polymyxins, polymyxin E (colistin) and polymyxin B, are currently commercially available. Colistin has re-emerged as a last-hope treatment in the mid-1990s against MDR Gram-negative pathogens due to the development of extensively drug-resistant GNB. Unfortunately, rapid global resistance towards colistin has emerged following its resurgence. Different mechanisms of colistin resistance have been characterized, including intrinsic, mutational, and transferable mechanisms.In this review, we intend to discuss the progress over the last two decades in understanding the alternative colistin mechanisms of action and different strategies used by bacteria to develop resistance against colistin, besides providing an update about what is previously recognized and what is novel concerning colistin resistance.

In-depth characterization of antibacterial activity of melittin against Staphylococcus aureus and use in a model of non-surgical MRSA-infected skin wounds

Skin infections caused by methicillin-resistant Staphylococcus aureus (MRSA) require the development of new and effective topical antibiotics. In this context, melittin, the main component of apitoxin, has a potent antibacterial effect. However, little is known regarding the anti-inflammatory potential this peptide in infection models, or its ability to induce clinically important resistance. Here, we aimed to conduct an in-depth characterization of the antibacterial potential of melittin in vitro and evaluate the pharmaceutical potential of an ointment containing melittin for the treatment of non-surgical infections induced by MRSA. The minimum inhibitory concentration of melittin varied from 0.12 to 4 μM. The antibacterial effect was mainly bactericidal and fast (approximately 0.5 h after incubation) and was maintained even in stationary cells and mature MRSA biofilms. Melittin interacts synergistically with beta-lactams and aminoglycosides, and its ability to form pores in the membrane reverses the resistance of vancomycin-intermediate Staphylococcus aureus (VISA) to amoxicillin, and vancomycin. Its ability to induce resistance in vitro was absent, and melittin was stable in several conditions often associated with infected wounds. In vivo, aointment containing melittin reduced bacterial load and the content of pro-inflammatory cytokines, such as tumor necrosis factor-α, interleukin-6 (IL-6), and IL-1 beta. Collectively, these data point to melittin as a potential candidate for topical formulations aimed at the treatment of non-surgical infections caused by MRSA.

ISAba1-dependent overexpression of eptA in clinical strains of Acinetobacter baumannii resistant to colistin

BACKGROUND

Colistin resistance in Acinetobacter baumannii often results from mutational activation of the two-component system PmrAB and subsequent addition of phospho-ethanolamine (pEtN) to lipooligosaccharide by up-regulated pEtN transferase PmrC.

OBJECTIVES

To characterize mechanisms of colistin resistance independent of PmrCAB in A. baumannii.

METHODS

Twenty-seven colistin-resistant A. baumannii were collected from 2012 to 2018. Analysis of operon pmrCAB was performed by PCR and sequencing. Seven strains were investigated further by WGS and whole-genome MLST (wgMLST).

RESULTS

Seven out of the 27 selected isolates were found to overexpress eptA, a gene homologous to pmrC, likely as a consequence of upstream insertion of an ISAba1 element. Insertion sites of ISAba1 were mapped 13, 18 and 156 bp ahead of the start codon of eptA in five strains, one strain and one strain, respectively. The finding that the isolates did not cluster together when compared by wgMLST analysis supports the notion that distinct insertion events occurred in close, but different, genetic backgrounds.

CONCLUSIONS

Activation of eptA and subsequent addition of pEtN to the cell surface represents a novel mechanism of resistance to colistin in A. baumannii.

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.

Rate of polymyxin resistance among Acinetobacter baumannii recovered from hospitalized patients: a systematic review and meta-analysis

We conducted a systematic review and meta-analysis to determine the rate of polymyxin resistance among Acinetobacter baumannii isolates causing infection in hospitalized patients around the world during the period of 2010-2019. The systematic review was performed on September 1, 2019, using PubMed/MEDLINE, Scopus, and Web of Science; studies published after January 1, 2010, were selected. The data were summarized in tables, critically analyzed, and treated statistically using the RStudio® Software with Meta package and Metaprop Command. After applying exclusion factors, 41 relevant studies were selected from 969 articles identified on literature search. The overall rate of polymyxin-resistant A. baumannii (PRAB) related to hospitalized patients was estimated to be 13% (95% CI, 0.06-0.27), where a higher rate was observed in America (29%; 95% CI, 0.12-0.55), followed by Europe (13%; 95% CI, 0.02-0.52), and Asia (10%; 95% CI, 0.02-0.32). The extensive use of polymyxins on veterinary to control bacterial infection and growth promotion, as well as the resurgence in prescription and use of polymyxins in the clinics against carbapenem-resistant gram-negative bacteria, may have contributed to the increased incidence of PRAB. The findings of this meta-analysis revealed that the rate of PRAB recovered from hospitalized patients is distinctively high. Thus, action needs to be taken to develop strategies to combat the clinical incidence of PRAB-induced hospital infections.

An Invertebrate Burn Wound Model That Recapitulates the Hallmarks of Burn Trauma and Infection Seen in Mammalian Models

The primary reason for skin graft failure and the mortality of burn wound patients, particularly those in burn intensive care centers, is bacterial infection. Several animal models exist to study burn wound pathogens. The most commonly used model is the mouse, which can be used to study virulence determinants and pathogenicity of a wide range of clinically relevant burn wound pathogens. However, animal models of burn wound pathogenicity are governed by strict ethical guidelines and hindered by high levels of animal suffering and the high level of training that is required to achieve consistent reproducible results. In this study, we describe for the first time an invertebrate model of burn trauma and concomitant wound infection. We demonstrate that this model recapitulates many of the hallmarks of burn trauma and wound infection seen in mammalian models and in human patients. We outline how this model can be used to discriminate between high and low pathogenicity strains of two of the most common burn wound colonizers Pseudomonas aeruginosa and Staphylococcus aureus, and multi-drug resistant Acinetobacter baumannii. This model is less ethically challenging than traditional vertebrate burn wound models and has the capacity to enable experiments such as high throughput screening of both anti-infective compounds and genetic mutant libraries.

Results from the Canadian Nosocomial Infection Surveillance Program for detection of carbapenemase-producing Acinetobacter spp. in Canadian hospitals, 2010-16

Objectives

Globally there is an increased prevalence of carbapenem-resistant Acinetobacter spp. (CRAs) and carbapenemase-producing Acinetobacter spp. (CPAs) in the hospital setting. This increase prompted the Canadian Nosocomial Infection Surveillance Program (CNISP) to conduct surveillance of CRA colonizations and infections identified from patients in CNISP-participating hospitals between 2010 and 2016.

Methods

Participating acute care facilities across Canada submitted CRAs from 1 January 2010 to 31 December 2016. Patient data were collected from medical records using a standardized questionnaire. WGS was conducted on all CRAs and data underwent single nucleotide variant analysis, resistance gene detection and MLST.

Results

The 7 year incidence rate of CRA was 0.02 per 10 000 patient days and 0.015 per 1000 admissions, with no significant increase observed over the surveillance period (P > 0.73). Ninety-four CRA isolates were collected from 58 hospitals, of which 93 (98.9%) were CPA. Carbapenemase OXA-235 group (48.4%) was the most common due to two separate clusters, followed by the OXA-23 group (41.9%). Patients with a travel history were associated with 38.8% of CRA cases. The all-cause 30 day mortality rate for infected cases was 24.4 per 100 CRA cases. Colistin was the most active antimicrobial agent (95.8% susceptibility).

Conclusions

CRA remains uncommon in Canadian hospitals and the incidence did not increase from 2010 to 2016. Almost half of the cases were from two clusters harbouring OXA-235-group enzymes. Previous medical treatment during travel outside of Canada was common.

Insights into Acinetobacter baumannii: A Review of Microbiological, Virulence, and Resistance Traits in a Threatening Nosocomial Pathogen

Being a multidrug-resistant and an invasive pathogen, Acinetobacter baumannii is one of the major causes of nosocomial infections in the current healthcare system. It has been recognized as an agent of pneumonia, septicemia, meningitis, urinary tract and wound infections, and is associated with high mortality. Pathogenesis in A. baumannii infections is an outcome of multiple virulence factors, including porins, capsules, and cell wall lipopolysaccharide, enzymes, biofilm production, motility, and iron-acquisition systems, among others. Such virulence factors help the organism to resist stressful environmental conditions and enable development of severe infections. Parallel to increased prevalence of infections caused by A. baumannii, challenging and diverse resistance mechanisms in this pathogen are well recognized, with major classes of antibiotics becoming minimally effective. Through a wide array of antibiotic-hydrolyzing enzymes, efflux pump changes, impermeability, and antibiotic target mutations, A. baumannii models a unique ability to maintain a multidrug-resistant phenotype, further complicating treatment. Understanding mechanisms behind diseases, virulence, and resistance acquisition are central to infectious disease knowledge about A. baumannii. The aims of this review are to highlight infections and disease-producing factors in A. baumannii and to touch base on mechanisms of resistance to various antibiotic classes.

Identification of pmrB mutations as putative mechanism for colistin resistance in A. baumannii strains isolated after in vivo colistin exposure

Colistin resistance among extensively-resistant Acinetobacter baumannii isolates is a serious health-care problem. Alterations in PmrA-PmrB two-component system have been associated with resistance to colistin. We investigated three pairs of colistin-susceptible and colistin-resistant A. baumannii, sequentially isolated from three patients before and after colistin treatment, respectively. The pmrA and pmrB genes were sequenced by Sanger method. Amino acidic positions and their effect on protein were predicted by InterPro and PROVEAN tools. Expression of pmrA, pmrB and pmrC genes was assessed by semi-quantitative reverse transcription-PCR (qRT-PCR). We found three different nonsynonymous substitutions P233T, E301G and L168K in pmrB coding region, each one in a different colistin resistance strain. The E301G and L168K substitutions represent novel mutations in pmrB, not previously described. Relative expression of pmrA, pmrB and pmrC mRNA increased in all colistin resistant strains. In our study, pmrB substitutions were associated with pmrC over-expression and colistin resistance. Further studies are necessary to understand their impact on modification of lipid A components.

Chemical composition, antioxidant and antimicrobial activities of Himalayan Fraxinus micrantha Lingelsh leaf extract

The present investigation determines the phytochemical analysis, antioxidant and antibacterial potential of methanol extract of Himalayan Fraxinus micrantha (Lingelsh) leaves. Chemical analysis of extract revealed the presence of fifty five compounds. The extract was strongly characterised by tyrosol, esculetin, β-sitosterin, l-(+)-ascorbic acid 2,6-dihexadecanoate, (Z,Z)-6,9-cis-3,4-epoxy nonadecadiene as main components (62.6%). The F. micrantha extract presents IC₅₀, 43.93 µg/ml and 68.90 µg/ml using DPPH and H₂O₂ scavenging assay, respectively. The antibacterial activity of F. micrantha was evaluated for five microorganisms showed that the extract had a remarkable inhibitory potential with a mean zone diameter of inhibition ranging from 11 to 22 mm. The MIC of the extracts ranged from 7.8 to 250 µg/ml and MBC value range 31.25 to 125 µg/ml. F. micrantha showed significant antibacterial activity against B. subtilis (22 mm, ZOI) with MIC and MBC values 7.8 and 31.25 µg/ml respectively.

Pharmacologic potential of new nitro-compounds as antimicrobial agents against nosocomial pathogens: design, synthesis, and in vitro effectiveness

Nosocomial infections are an important cause of morbi-mortality worldwide. The increase in the rate of resistance to conventional drugs in these microorganisms has stimulated the search for new therapeutic options. The nitro moiety (NO₂) is an important pharmacophore of molecules with high anti-infective activity. We aimed to synthesize new nitro-derivates and to evaluate their antibacterial and anti-Candida potential in vitro. Five compounds [3-nitro-2-phenylchroman-4-ol (3); 3-nitro-2-phenyl-2H-chromene (4a); 3-nitro-2-(4-chlorophenyl)-2H-chromene (4b); 3-nitro-2-(4-fluorophenyl)-2H-chromene (4c), and 3-Nitro-2-(2,3-dichlorophenyl)-2H-chromene (4d)] were efficiently synthesized by Michael-aldol reaction of 2-hydroxybenzaldehyde with nitrostyrene, resulting in one β-nitro-alcohol (3) and four nitro-olefins (4a-4d). The antibacterial and anti-Candida potentials were evaluated by assaying minimal inhibitory concentration (MIC), minimum fungicidal concentration (MFC), and minimum bactericidal concentration (MBC). Mono-halogenated nitro-compounds (4b and 4c) showed anti-staphylococcal activity with MIC values of 15.6-62.5 μg/mL and MBC of 62.5 μg/mL. However, the activity against Gram-negative strains was showed to be considerably lower and our data suggests that this effect was associated with the outer membrane. Furthermore, nitro-compounds 4c and 4d presented activity against Candida spp. with MIC values ranging from 7.8-31.25 μg/mL and MFC of 15.6-500 μg/mL. In addition, these compounds were able to induce damage in fungal cells increasing the release of intracellular material, which was associated with actions on the cell wall independent of quantitative changes in chitin and β-glucan. Together, these findings show that nitro-compounds can be exploited as anti-staphylococcal and anti-Candida prototypes.