The Effect of Colistin Resistance-Associated Mutations on the Fitness of Acinetobacter baumannii

Treatment Strategies of Colistin Resistance Acinetobacter baumannii Infections

Acinetobacter baumannii has emerged as a pressing challenge in clinical practice, mainly due to the development of resistance to multiple antibiotics, including colistin, one of the last-resort treatments. This review highlights all the possible mechanisms of colistin resistance and the genetic basis contributing to this resistance, such as modifications to lipopolysaccharide or lipid A structures, alterations in outer membrane permeability via porins and heteroresistance. In light of this escalating threat, the review also evaluates available treatment options. The development of new antibiotics (cefiderocol, sulbactam/durlobactam) although not available everywhere, and the use of various combinations and synergistic drug combinations (including two or more of the following: a polymyxin, ampicillin/sulbactam, carbapenems, fosfomycin, tigecycline/minocycline, a rifamycin, and aminoglycosides) are discussed in the context of overcoming colistin resistance of A. baumannii infections. Although most studied combinations are polymyxin-based combinations, non-polymyxin-based combinations have been emerging as promising options. However, clinical data remain limited and continued investigation is essential to determine optimal therapeutic strategies against colistin-resistant A. baumannii.

Molecular Characteristics of Colistin Resistance in Acinetobacter baumannii and the Activity of Antimicrobial Combination Therapy in a Tertiary Care Medical Center in Lebanon

(1) Background: Infections with pan-drug-resistant (PDR) bacteria, such as A. baumannii, are becoming increasingly common, especially in healthcare facilities. In this study, we selected 15 colistin-resistant clinical A. baumannii isolates from a hospital in Beirut, Lebanon, to test combination therapies and determine their sequence types (STs) and the mechanism of colistin resistance using whole-genome sequencing (WGS). (2) Methods: Antimicrobial susceptibility testing via broth microdilution against 12 antimicrobials from different classes and growth rate assays were performed. A checkerboard assay was conducted on PDR isolates using six different antimicrobials, each in combination with colistin. Genomic DNA was extracted from all isolates and subjected to WGS. (3) Results: All isolates were resistant to all tested antimicrobials with the one exception that was susceptible to gentamicin. Combining colistin with either meropenem, ceftolozane-tazobactam, or teicoplanin showed synergistic activity. Sequencing data revealed that 67% of the isolates belonged to Pasteur ST2 and 33% to ST187. Furthermore, these isolates harbored a number of resistance genes, including blaOXA-23. Mutations in the pmrC gene were behind colistin resistance. (4) Conclusions: With the rise in antimicrobial resistance and the absence of novel antimicrobial production, alternative treatments must be found. The combination therapy results from this study suggest treatment options for PDR ST2 A. baumannii-infected patients.

Characteristics of a Carbapenem-Resistant Acinetobacter baumannii Strain Causing Community-Acquired Pneumonia in a Young Healthy Women

Background

Multidrug-resistant Acinetobacter baumannii rarely causes community-acquired pneumonia. Here, we report the clinical and genomic characteristics of a multidrug-resistant A. baumannii strain responsible for community-acquired pneumonia in a 31-year-old healthy young women.

Methods

A. baumannii strain W2LL was recovered from the alveolar lavage fluid sample of a hospitalized patient with pulmonary infection. Growth rate studies were conducted under various conditions, and virulence assessments were performed using Galleria Mellonella larvae. Whole Genome Sequencing (WGS) was carried out using Oxford Nanopore MinIon and Illumina HiSeq. In silico multilocus sequence typing (MLST), plasmid replicons, antimicrobial resistance genes, and virulence genes were determined using the BacWGSTdb webserver. Phylogenetic analysis between strain W2LL and other closely related A. baumannii genomes retrieved from NCBI database was performed.

Results

WGS identified strain W2LL as a rare sporadic lineage sequence type (ST) 1431. In addition to the detection of the β-lactamase gene (blaOXA-98) on the chromosome, blaOXA-58 was found on a 92,034 bp plasmid. Antimicrobial susceptibility testing revealed this strain was resistant to cephalosporins and carbapenems, with initial treatment using cefoxitin proving ineffective. Subsequent treatment with piperacillin-sulbactam combined with levofloxacin led to gradual improvement. Compared to A. baumannii ATCC 17978, W2LL exhibited similar growth rates at 37°C and 42°C, as well as in the presence of zinc. However, strain W2LL exhibited higher virulence phenotype compared to ATCC 17978 in G. mellonella model. The closest relative of A. baumannii W2LL was CAM180_1, another isolate recovered from Cambodia, which differed by 191 SNPs.

Conclusion

W2LL is a rare ST1431 carbapenem-resistant A. baumannii strain recovered from a patient with no prior hospitalization or typical risk factors. This underscores the growing menace posed by carbapenem-resistant A. baumannii, no longer limited to hospitalized patients, potentially impacting the broader, younger population.

The Gram-negative permeability barrier: tipping the balance of the in and the out

Gram-negative bacteria are intrinsically resistant to many antibiotics, due in large part to the permeability barrier formed by their cell envelope. The complex and synergistic interplay of the two Gram-negative membranes and active efflux prevents the accumulation of a diverse range of compounds that are effective against Gram-positive bacteria. A lack of detailed information on how components of the cell envelope contribute to this has been identified as a key barrier to the rational development of new antibiotics with efficacy against Gram-negative species. This review describes the current understanding of the role of the different components of the Gram-negative cell envelope in preventing compound accumulation and the state of efforts to describe properties that allow compounds to overcome this barrier and apply them to the development of new broad-spectrum antibiotics.

Disrupting iron homeostasis can potentiate colistin activity and overcome colistin resistance mechanisms in Gram-Negative Bacteria

Acinetobacter baumannii is a Gram-negative priority pathogen that can readily overcome antibiotic treatment through a range of intrinsic and acquired resistance mechanisms. Treatment of carbapenem-resistant A. baumannii largely relies on the use of colistin in cases where other treatment options have been exhausted. However, the emergence of resistance against this last-line drug has significantly increased amongst clinical strains. In this study, we identify the phytochemical kaempferol as a potentiator of colistin activity. When administered singularly, kaempferol has no effect on growth but does impact biofilm formation. Nonetheless, co-administration of kaempferol with sub-inhibitory concentrations of colistin exposes bacteria to a metabolic Achilles heel, whereby kaempferol-induced dysregulation of iron homeostasis leads to bacterial killing. We demonstrate that this effect is due to the disruption of Fenton's reaction, and therefore to a lethal build-up of toxic reactive oxygen species in the cell. Furthermore, we show that this vulnerability can be exploited to overcome both intrinsic and acquired colistin resistance in clinical strains of A. baumannii and E. coli in vitro and in the Galleria mellonella model of infection. Overall, our findings provide a proof-of-principle demonstration that targeting iron homeostasis is a promising strategy for enhancing the efficacy of colistin and overcoming colistin-resistant infections.

Molecular characterization and differential effects of levofloxacin and ciprofloxacin on the potential for developing quinolone resistance among clinical Pseudomonas aeruginosa isolates

Background

Fluoroquinolones are some of the most used antimicrobial agents for the treatment of Pseudomonas aeruginosa. This study aimed at exploring the differential activity of ciprofloxacin and levofloxacin on the selection of resistance among P. aeruginosa isolates at our medical center.

Methods

233 P. aeruginosa clinical isolates were included in this study. Antimicrobial susceptibility testing (AST) was done using disk diffusion and broth microdilution assays. Random Amplification of Polymorphic DNA (RAPD) was done to determine the genetic relatedness between the isolates. Induction of resistance against ciprofloxacin and levofloxacin was done on 19 isolates. Fitness cost assay was done on the 38 induced mutants and their parental isolates. Finally, whole genome sequencing was done on 16 induced mutants and their 8 parental isolates.

Results

AST results showed that aztreonam had the highest non-susceptibility. RAPD results identified 18 clusters. The 19 P. aeruginosa isolates that were induced against ciprofloxacin and levofloxacin yielded MICs ranging between 16 and 256 μg/mL. Levofloxacin required fewer passages in 10 isolates and the same number of passages in 9 isolates as compared to ciprofloxacin to reach their breakpoints. Fitness cost results showed that 12 and 10 induced mutants against ciprofloxacin and levofloxacin, respectively, had higher fitness cost when compared to their parental isolates. Whole genome sequencing results showed that resistance to ciprofloxacin and levofloxacin in sequenced mutants were mainly associated with alterations in gyrA, gyrB and parC genes.

Conclusion

Understanding resistance patterns and risk factors associated with infections is crucial to decrease the emerging threat of antimicrobial resistance.

Novel approaches to overcome Colistin resistance in Acinetobacter baumannii: Exploring quorum quenching as a potential solution

Acinetobacter baumannii is responsible for a variety of infections, such as nosocomial infections. In recent years, this pathogen has gained resistance to many antibiotics, and thus, carbapenems were used to treat infections with MDR A. baumannii strains in clinical settings. However, as carbapenem-resistant isolates are becoming increasingly prevalent, Colistin is now used as the last line of defense against resistant A. baumannii strains. Unfortunately, reports are increasing on the presence of Colistin-resistant phenotypes in infections caused by A. baumannii, creating an urgent need to find a substitute way to combat these resistant isolates. Quorum sensing inhibition, also known as quorum quenching, is an efficient alternative way of reversing resistance in different Gram-negative bacteria. Quorum sensing is a mechanism used by bacteria to communicate with each other by secreting signal molecules. When the population of bacteria increases and the concentration of signal molecules reaches a certain threshold, bacteria can implement mechanisms to adapt to a hostile environment, such as biofilm formation. Biofilms have many advantages for pathogens, such as antibiotic resistance. Different studies have revealed that disrupting the biofilm of A. baumannii makes it more susceptible to antibiotics. Although very few studies have been conducted on the biofilm disruption through quorum quenching in Colistin-resistant A. baumannii, these studies and similar studies bring hope in finding an alternative way of treating the Colistin-resistant isolates. In conclusion, quorum quenching has the potential to be used against Colistin-resistant A. baumannii.

Application of antimicrobial peptides in plant protection: making use of the overlooked merits

Pathogen infection is one of the major causes of yield loss in the crop field. The rapid increase of antimicrobial resistance in plant pathogens has urged researchers to develop both new pesticides and management strategies for plant protection. The antimicrobial peptides (AMPs) showed potential on eliminating plant pathogenic fungi and bacteria. Here, we first summarize several overlooked advantages and merits of AMPs, which includes the steep dose-response relations, fast killing ability, broad synergism, slow resistance selection. We then discuss the possible application of AMPs for plant protection with above merits, and highlight how AMPs can be incorporated into a more efficient integrated management system that both increases the crop yield and reduce resistance evolution of pathogens.

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.

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.

Emergence of high colistin resistance in carbapenem resistant Acinetobacter baumannii in Pakistan and its potential management through immunomodulatory effect of an extract from Saussurea lappa

Carbapenem resistant Acinetobacter baumannii has emerged as one of the most difficult to treat nosocomial bacterial infections in recent years. It was one of the major causes of secondary infections in Covid-19 patients in developing countries. The polycationic polypeptide antibiotic colistin is used as a last resort drug to treat carbapenem resistant A. baumannii infections. Therefore, resistance to colistin is considered as a serious medical threat. The purpose of this study was to assess the current status of colistin resistance in Pakistan, a country where carbapenem resistant A. bumannii infections are endemic, to understand the impact of colistin resistance on virulence in mice and to assess alternative strategies to treat such infections. Out of 150 isolates collected from five hospitals in Pakistan during 2019–20, 84% were carbapenem resistant and 7.3% were additionally resistant to colistin. There were two isolates resistant to all tested antibiotics and 83% of colistin resistant isolates were susceptible to only tetracycline family drugs doxycycline and minocycline. Doxycycline exhibited a synergetic bactericidal effect with colistin even in colistin resistant isolates. Exposure of A. baumannii 17978 to sub inhibitory concentrations of colistin identified novel point mutations associated with colistin resistance. Colistin tolerance acquired independent of mutations in lpxA, lpxB, lpxC, lpxD, and pmrAB supressed the proinflammatory immune response in epithelial cells and the virulence in a mouse infection model. Moreover, the oral administration of water extract of Saussuria lappa, although not showing antimicrobial activity against A. baumannii in vitro, lowered the number of colonizing bacteria in liver, spleen and lung of the mouse model and also lowered the levels of neutrophils and interleukin 8 in mice. Our findings suggest that the S. lappa extract exhibits an immunomodulatory effect with potential to reduce and cure systemic infections by both opaque and translucent colony variants of A. baumannii.

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.

Genetic Dissection of Antibiotic Adjuvant Activity

Small molecule adjuvants that enhance the activity of established antibiotics represent promising agents in the battle against antibiotic resistance. Adjuvants generally act by inhibiting antibiotic resistance processes, and specifying the process acted on is a critical step in defining an adjuvant's mechanism of action. This step is typically carried out biochemically by identifying molecules that bind adjuvants and then inferring their roles in resistance. Here, we present a complementary genetic strategy based on identifying mutations that both sensitize cells to antibiotic and make them "adjuvant blind." We tested the approach in Acinetobacter baumannii AB5075 using two adjuvants: a well-characterized β-lactamase inhibitor (avibactam) and a compound enhancing outer membrane permeability (aryl 2-aminoimidazole AI-1). The avibactam studies showed that the adjuvant potentiated one β-lactam (ceftazidime) through action on a single β-lactamase (GES-14) and a second (meropenem) by targeting two different enzymes (GES-14 and OXA-23). Mutations impairing disulfide bond formation (DsbAB) also reduced potentiation, possibly by impairing β-lactamase folding. Mutations reducing AI-1 potentiation of canonical Gram-positive antibiotics (vancomycin and clarithromycin) blocked lipooligosaccharide (LOS/LPS) synthesis or its acyl modification. The results indicate that LOS-mediated outer membrane impermeability is targeted by the adjuvant and show the importance of acylation in the resistance. As part of the study, we employed Acinetobacter baylyi as a model to verify the generality of the A. baumannii results and identified the principal resistance genes for ceftazidime, meropenem, vancomycin, and clarithromycin in A. baumannii AB5075. Overall, the work provides a foundation for analyzing adjuvant action using a comprehensive genetic approach. IMPORTANCE One strategy to confront the antibiotic resistance crisis is through the development of adjuvant compounds that increase the efficacy of established drugs. A key step in the development of a natural product adjuvant as a drug is identifying the resistance process it undermines to enhance antibiotic activity. Previous procedures designed to accomplish this have relied on biochemical identification of cell components that bind adjuvant. Here, we present a complementary strategy based on identifying mutations that eliminate adjuvant activity.

Colistin Resistance Mechanisms in Human Salmonella enterica Strains Isolated by the National Surveillance Enter-Net Italia (2016–2018)

Background: A collection of human-epidemiologically unrelated S. enterica strains collected over a 3-year period (2016 to 2018) in Italy by the national surveillance Enter-Net Italia was analysed. Methods: Antimicrobial susceptibility tests, including the determination of minimal inhibitory concentrations (MICs) for colistin, were performed. Colistin resistant strains were analysed by PCR to detect mobile colistin resistance (mcr) genes. In mcr-negative S. enterica serovar Enteritidis strains, chromosomal mutations potentially involved in colistin resistance were identified by a genomic approach. Results: The prevalence of colistin-resistant S. enterica strains was 7.7%, the majority (87.5%) were S. Enteritidis. mcr genes were identified only in one strain, a S. Typhimurium monophasic variant, positive for both mcr-1.1 and mcr-5.1 genes in an IncHI2 ST4 plasmid. Several chromosomal mutations were identified in the colistin-resistant mcr-negative S. Enteritidis strains in proteins involved in lipopolysaccharide and outer membrane synthesis and modification (RfbN, LolB, ZraR) and in a component of a multidrug efflux pump (MdsC). These mutated proteins were defined as possible candidates for colistin resistance in mcr-negative S. Enteritidis of our collection. Conclusions: The colistin national surveillance in Salmonella spp. in humans, implemented with genomic-based surveillance, permitted to monitor colistin resistance, determining the prevalence of mcr determinants and the study of new candidate mechanisms for colistin resistance.

Colistin- phage combinations decrease antibiotic resistance in A. baumannii via changes in envelope architecture

Multidrug-resistant bacterial infections are becoming increasingly common, with only few last-resort antibiotics such as colistin available for clinical therapy. An alternative therapeutic strategy gaining momentum is phage therapy, which has the advantage of not being affected by bacterial resistance to antibiotics. However, a major challenge in phage therapy is the rapid emergence of phage-resistant bacteria. In this work, our main aim was to understand the mechanisms of phage-resistance used by the top priority pathogen Acinetobacter baumannii. We isolated the novel phage Phab24, capable of infecting colistin-sensitive and -resistant strains of A. baumannii. After co-incubating Phab24 with its hosts, we obtained phage-resistant mutants which were characterized on both genotypic and phenotypic levels. Using whole genome sequencing, we identified phage-resistant strains that displayed mutations in genes that alter the architecture of the bacterial envelope at two levels: the capsule and the outer membrane. Using an adsorption assay, we confirmed that phage Phab24 uses the bacterial capsule as its primary receptor, with the outer membrane possibly serving as the secondary receptor. Interestingly, the phage-resistant isolates were less virulent compared to the parental strains in a Galleria mellonella infection model. Most importantly, we observed that phage-resistant bacteria that evolved in the absence of antibiotics exhibited an increased sensitivity to colistin, even though the antibiotic resistance mechanism per se remained unaltered. This increase in antibiotic sensitivity is a direct consequence of the phage-resistance mechanism, and could potentially be exploited in the clinical setting.

Diversity of Sequence Types and Impact of Fitness Cost among Carbapenem-Resistant Acinetobacter baumannii Isolates from Tripoli, Libya

We investigated the molecular epidemiology of 21 carbapenem-resistant Acinetobacter baumannii isolates from Libya and assessed their relative fitness. Core genome multilocus sequence typing (MLST) revealed five interhospital transmission clusters. Three clusters were associated with the international clones (IC) IC1, IC2, and IC7. Carbapenem-resistance was associated with bla_OXA-23, bla_GES-11, or bla_NDM-1. Compared to that of A. baumannii DSM 30008, the doubling time was similar over 10 h, but after 16 h, half the isolates grew to higher densities, suggesting a fitness advantage.

Characterisation of mcr-4.3 in a colistin-resistant Acinetobacter nosocomialis clinical isolate from Cape Town, South Africa

OBJECTIVES

Colistin resistance in Acinetobacter spp. is increasing, resulting in potentially untreatable nosocomial infections. Plasmid-mediated colistin resistance is of particular concern due to its low fitness cost and potential transferability to other bacterial strains and species. This study investigated the colistin resistance mechanism in a clinical Acinetobacter nosocomialis isolate from Cape Town, South Africa.

METHODS

A colistin-resistant A. nosocomialis isolate was identified from a blood culture in 2017. PCR and Illumina whole-genome sequencing (WGS) were performed to identify genes and mutations conferring resistance to colistin. Plasmid sequencing was performed on an Oxford Nanopore platform. mcr functionality was assessed by broth microdilution after cloning the mcr gene into pET-48b(+) and expressing it in SHuffle® T7 Escherichia coli and after curing the plasmid using 62.5 mg/L acridine orange.

RESULTS

The colistin minimum inhibitory concentration (MIC) of the A. nosocomialis isolate was 16 mg/L. The mcr-4.3 gene was detected by PCR and WGS. No other previously described colistin resistance mechanism was found by WGS. The mcr-4.3 gene was identified on a 24 024-bp RepB plasmid (pCAC13a). Functionality studies showed that recombinant mcr-4.3 did not confer colistin resistance in E. coli. However, plasmid curing of pCAC13a restored colistin susceptibility in A. nosocomialis.

CONCLUSION

We describe the first detection of a plasmid-mediated mcr-4.3 gene encoding colistin resistance in A. nosocomialis and the first detection of mcr-4.3 in a clinical isolate in Africa. Recombinant expression of mcr-4.3 did not confer colistin resistance in E. coli, suggesting that its functionality may be RepB plasmid-dependent or species-specific.

Acinetobacter baumannii: Its Clinical Significance in Human and Veterinary Medicine

Acinetobacter baumannii is a Gram-negative, opportunistic pathogen, causing severe infections difficult to treat. The A. baumannii infection rate has increased year by year in human medicine and it is also considered as a major cause of nosocomial infections worldwide. This bacterium, also well known for its ability to form biofilms, has a strong environmental adaptability and the characteristics of multi-drug resistance. Indeed, strains showing fully resistant profiles represent a worrisome problem in clinical therapeutic treatment. Furthermore, A. baumannii-associated veterinary nosocomial infections has been reported in recent literature. Particularly, carbapenem-resistant A. baumannii can be considered an emerging opportunistic pathogen in human medicine as well as in veterinary medicine.

Molecular Characterization of Carbapenem/Colistin-Resistant Acinetobacter baumannii Clinical Isolates from Egypt by Whole-Genome Sequencing

PURPOSE

The rise of carbapenem-resistant A. baumannii (CRAB) is considered a public health problem limiting the treatment options. Our current work studied the emergence and mechanisms of colistin-resistance among CRAB isolates in Egypt.

MATERIALS AND METHODS

Seventeen clinically recovered A. baumannii were identified and screened for their antimicrobial susceptibilities using VITEK-2 system. Colistin susceptibility was evaluated using broth microdilution, and characterization of carbapenem/colistin resistance determinants was performed using whole-genome sequencing (Illumina MiSeq).

RESULTS

About 52.9% (9/17) were colistin-resistant. PCR results revealed that all isolates carried bla OXA-51-like genes, bla OXA-23-like was detected in 82.3% (14/17) and bla NDM in 23.5% (4/17). Two isolates harboured bla GES-35 and bla OXA-23. Furthermore, genome analysis of seven isolates revealed six belonged to international clone 2 (IC2) while the remaining isolate was a singleton (ST158), representing a clone circulating in Mediterranean/Middle Eastern countries.

CONCLUSION

The emergence and high incidence of colistin-resistance among CRAB clinical isolates in Egypt are alarming because it further limits therapy options and requires prudent antimicrobial stewardship and stringent infection control measures. Whole-genome sequence analyses suggest that the resistance to colistin was associated with multiple mutations in the pmrCAB genes. The high incidence of the high-risk lineage IC2 harbouring bla OXA-23-like as well as bla NDM is also of concern.

Treatment Outcomes of Colistin- and Carbapenem-resistant Acinetobacter baumannii Infections: An Exploratory Subgroup Analysis of a Randomized Clinical Trial

BACKGROUND

We evaluated the association between mortality and colistin resistance in Acinetobacter baumannii infections and the interaction with antibiotic therapy.

METHODS

This is a secondary analysis of a randomized controlled trial of patients with carbapenem-resistant gram-negative bacterial infections treated with colistin or colistin-meropenem combination. We evaluated patients with infection caused by carbapenem-resistant A. baumannii (CRAB) identified as colistin susceptible (CoS) at the time of treatment and compared patients in which the isolate was confirmed as CoS with those whose isolates were retrospectively identified as colistin resistant (CoR) when tested by broth microdilution (BMD). The primary outcome was 28-day mortality.

RESULTS

Data were available for 266 patients (214 CoS and 52 CoR isolates). Patients with CoR isolates had higher baseline functional capacity and lower rates of mechanical ventilation than patients with CoS isolates. All-cause 28-day mortality was 42.3% (22/52) among patients with CoR strains and 52.8% (113/214) among patients with CoS isolates (P = .174). After adjusting for variables associated with mortality, the mortality rate was lower among patients with CoR isolates (odds ratio [OR], 0.285 [95% confidence interval {CI}, .118-.686]). This difference was associated with treatment arm: Mortality rates among patients with CoR isolates were higher in those randomized to colistin-meropenem combination therapy compared to colistin monotherapy (OR, 3.065 [95% CI, 1.021-9.202]).

CONCLUSIONS

Colistin resistance determined by BMD was associated with lower mortality among patients with severe CRAB infections. Among patients with CoR isolates, colistin monotherapy was associated with a better outcome compared to colistin-meropenem combination therapy.

CLINICAL TRIALS REGISTRATION

NCT01732250.

Colistin resistance of carbapenem-resistant Klebsiella pneumoniae strains: molecular mechanisms and bacterial fitness

The increasing use of colistin in the clinic has led to the emergence and spread of colistin resistance. According to the literature, antibiotic resistance can have a metabolic cost, resulting in poor adaptation and survival, i.e. reduced bacterial fitness. The aim of this study was to investigate molecular mechanisms underlying resistance to colistin and their effect on the bacterial fitness of carbapenem-resistant (carba-R) strains of K. pneumoniae isolated from the patients of Moscow hospitals in 2012–2017. Of 159 analyzed carba-R isolates, 71 (45%) were resistant to colistin (minimum inhibitory concentration over 2 mg/L). By conducting Sanger sequencing, we were able to identify the mechanisms underlying colistin resistance in 26 (37%) isolates. Growth curves were constructed by measuring optical density at 600 nm wavelength for 15 hours. The competitive growth of colistin-resistant (col-R) K. pneumoniae isolates was assessed relative to the colistin-susceptible (col-S) isolate. Col-R and col-S cultures harvested in the exponential phase were combined at the ratio of 1:1, incubated in the Luria-Bertani medium and plated onto Luria-Bertani agar plates with 10 mg/L colistin and without it. The competition index was calculated as the ratio of grown col-R and col-S colonies. Resistance to colistin did not affect the growth kinetics of K. pneumoniae, but did reduce the competitive ability of the bacteria as compared to the col-S isolates. However, some col-R isolates were more competitive than the col-S strains of the same sequence type. Further research is needed to elucidate the effects of colistin resistance on bacterial fitness.

Comparison of Fitness Cost and Virulence in Chromosome- and Plasmid-Mediated Colistin-Resistant Escherichia coli

Five types of Escherichia coli strains were obtained and sequenced: colistin-susceptible (CL-S) strains, in vitro induced colistin-resistant (CL-IR) strains, mcr-1-negative colistin-resistant strains from livestock (CL-chrR), mcr-1-positive colistin-resistant strains (CL-mcrR), and mcr-1-transferred transconjugants (TC-mcr). Amino acid alterations of PmrAB, PhoPQ, and EptA were identified, and their mRNA expression was measured. Their growth rate was evaluated, and an in vitro competition assay was performed. Virulence was compared through serum resistance and survival in macrophages and Drosophila melanogaster. CL-IR and CL-chrR strains were colistin-resistant due to amino acid alterations in PmrAB, PhoPQ, or EptA, and their overexpression. All colistin-resistant strains did not show reduced growth rates compared with CL-S strains. CL-IR and CL-chrR strains were less competitive than the susceptible strain, but CL-mcrR strains were not. In addition, TC-mcr strains were also significantly more competitive than their respective parental susceptible strain. CL-IR strains had similar or decreased survival rates in human serum, macrophages, and fruit flies, compared with their parental, susceptible strains. CL-chrR strains were also less virulent than CL-S strains. Although CL-mcrR strains showed similar survival rates in human serum and fruit fly to CL-S strains, the survival rates of TC-mcr strains decreased significantly in human serum, macrophages, and fruit flies, compared with their susceptible recipient strain (J53). Chromosome-mediated, colistin-resistant E. coli strains have a fitness cost, but plasmids bearing mcr-1 do not increase the fitness burden of E. coli. Along with high usage of polymyxins, the no fitness cost of mcr-1-positive strains may facilitate rapid spread of colistin resistance.

Lipopolysaccharide-Deficient Acinetobacter baumannii Due to Colistin Resistance Is Killed by Neutrophil-Produced Lysozyme

Acinetobacter baumannii causes nosocomial infections due to its multidrug resistance and high environmental adaptability. Colistin is a polypeptide antibacterial agent that targets lipopolysaccharide (LPS) and is currently used to control serious multidrug-resistant Gram-negative bacterial infections, including those caused by A. baumannii. However, A. baumannii may acquire colistin resistance by losing their LPS. In mouse models, LPS-deficient A. baumannii have attenuated virulence. Nevertheless, the mechanism through which the pathogen is cleared by host immune cells is unknown. Here, we established colistin-resistant A. baumannii strains and analyzed possible mechanisms through which they are cleared by neutrophils. Colistin-resistant, LPS-deficient strains harbor mutations or insertion sequence (IS) in lpx genes, and introduction of intact lpx genes restored LPS deficiency. Analysis of interactions between these strains and neutrophils revealed that compared with wild type, LPS-deficient A. baumannii only weakly stimulated neutrophils, with consequent reduced levels of reactive oxygen species (ROS) and inflammatory cytokine production. Nonetheless, neutrophils preferentially killed LPS-deficient A. baumannii compared to wild-type strains. Moreover, LPS-deficient A. baumannii strains presented with increased sensitivities to antibacterial lysozyme and lactoferrin. We revealed that neutrophil-secreted lysozyme was the antimicrobial factor during clearance of LPS-deficient A. baumannii strains. These findings may inform the development of targeted therapeutics aimed to treat multidrug-resistant infections in immunocompromised patients who are unable to mount an appropriate cell-mediated immune response.

Acquisition of Colistin Resistance Links Cell Membrane Thickness Alteration with a Point Mutation in the lpxD Gene in Acinetobacter baumannii

Acinetobacter baumannii is one of the most common causes of nosocomial infections in intensive care units. Its ability to acquire diverse mechanisms of resistance limits the therapeutic choices for its treatment. This especially concerns colistin, which has been reused recently as a last-resort drug against A. baumannii. Here, we explored the impact of gaining colistin resistance on the susceptibility of A. baumannii to other antibiotics and linked colistin resistance acquisition to a gene mutation in A. baumannii. The susceptibility of 95 A. baumannii isolates revealed that 89 isolates were multi-drug resistance (MDR), and nine isolates were resistant to colistin. Subsequently, three isolates, i.e., MS48, MS50, and MS64, exhibited different resistance patterns when colistin resistance was induced and gained resistance to almost all tested antibiotics. Upon TEM examination, morphological alterations were reported for all induced isolates and a colistin-resistant clinical isolate (MS34Col-R) compared to the parental sensitive strains. Finally, genetic alterations in PmrB and LpxACD were assessed, and a point mutation in LpxD was identified in the MS64Col-R and MS34Col-R mutants, corresponding to Lys117Glu substitution in the lipid-binding domain. Our findings shed light on the implications of using colistin in the treatment of A. baumannii, especially at sub-minimum inhibitory concentrations concentrations, since cross-resistance to other classes of antibiotics may emerge, beside the rapid acquisition of resistance against colistin itself due to distinct genetic events.

In vitro reduction of colistin susceptibility and comparative genomics reveals multiple differences between MCR-positive and MCR-negative colistin-resistant Escherichia coli

Objectives: Although resistance to colistin is increasingly reported from clinical settings, the genetic mechanisms that lead to colistin resistance in Escherichia coli have not been fully characterized. Here, we assess the evolution of colistin resistance in clinical isolates of mobilized colistin resistance (MCR)-negative and MCR-positive Escherichia coli. Methods: Spontaneously mutated colistin-resistant progeny were evolved using a step-wise reduction of colistin susceptibility. Resistance phenotypes were confirmed by minimum inhibitory concentration (MIC) determination, and the probable resistance mechanisms were investigated using PCR and reverse transcription-quantitative PCR. Mutated genes of the laboratory-evolved mutants were identified by whole-genome sequencing and comparative genomics. Fitness costs and serum resistance of the mutants were also compared to the corresponding wild types. Results: MCR-negative isolates displayed higher increases in MICs than did MCR-positive isolates following colistin exposure. Upregulation of pmrAB and associated genes was evident among MCR-negative isolates but not MCR-positive isolates. Comparative genomic analysis of mutants and their corresponding wild-types (WTs) revealed numerous mutations in genes encoding membrane transporters and two-component systems. Additionally, MCR-negative mutants exhibited higher fitness costs than MCR-positive mutants compared with their corresponding WTs but displayed similar serum resistance. Conclusion: Our findings reveal multiple differences between MCR-positive and MCR-negative E. coli strains following colistin exposure, which provide reference values for clinical medication.

Dose Optimization of Colistin Combinations against Carbapenem-Resistant Acinetobacter baumannii from Patients with Hospital-Acquired Pneumonia in China by Using an In Vitro Pharmacokinetic/Pharmacodynamic Model

Colistin-based combination therapy has become an important strategy to combat the carbapenem-resistant Acinetobacter baumannii (CRAB). However, the optimal dosage regimen selection for the combination with maximum efficacy is challenging. Checkerboard assay was employed to evaluate the synergy of colistin in combination with meropenem, rifampin, fosfomycin, and minocycline against nine carbapenem-resistant A. baumannii isolates (MIC of meropenem [MIC_MEM], ≥32 mg/liter) isolated from Chinese hospital-acquired pneumonia (HAP) patients. A static time-kill assay, in vitro dynamic pharmacokinetic/pharmacodynamic (PK/PD) model, and semimechanistic PK/PD modeling were conducted to predict and validate the synergistic effect of the most efficacious combination. Both checkerboard and static time-kill assays demonstrated the superior synergistic effect of the colistin-meropenem combination against all CRAB isolates. In the in vitro PK/PD model, the dosage regimen of 2 g meropenem daily via 3-h infusion combined with steady-state 1 mg/liter colistin effectively suppressed the bacterial growth at 24 h with a 2-log₁₀ decrease, compared with the initial inocula against two CRAB isolates. The semimechanistic PK/PD model predicted that more than 2 mg/liter colistin combined with meropenem (2 g, 3-h infusion) was required to achieve the killing below the limit of detection (<LOD; i.e., 1 log₁₀CFU/ml) at 24 h with an MIC_MEM of ≥32 mg/liter. Colistin combined with meropenem exerted synergistic killing against CRAB even with an MIC_MEM of ≥32 mg/liter and MIC of colistin (MIC_CST) of ≤1 mg/liter. However, it is predicted that a higher concentration of colistin combined with meropenem was crucial to kill bacteria to <LOD. Our study provides important PK/PD information for optimization of the colistin and meropenem combination against CRAB.

Relationship Between Different Resistance Mechanisms and Virulence in Acinetobacter baumannii

Aim: This study analyzed the virulence of several Acinetobacter baumannii strains expressing different resistance mechanisms using the Caenorhabditis elegans infection model. Results: Strains susceptible/resistant to carbapenems (presenting class D (OXA-23, OXA-24), class B metallo-β-lactamase (MBL) (NDM-1), penicillin binding protein (PBP) altered and decreased expression of Omp 33-36 kDa) and isogenic A. baumannii strains susceptible/resistant to colistin (presenting loss of lipopolysaccharide (LPS) and pmrA mutations) were included to evaluate the virulence using the C. elegans infection model. The nematode killing assay, bacterial ingestion in worms, and bacterial lawn avoidance assay were performed with the Fer-15 mutant line. A. baumannii strains generally presented low virulence, showing no difference between carbapenem-resistant strains (expressing class D, MBLs, or altered PBP) and their isogenic susceptible strains. In contrast, the absence of the Omp 33-36 kDa protein in the knockout was associated with a decrease of virulence, and a significant difference was observed between colistin-resistant mutants and their susceptible counterpart when the mechanism of resistance was associated with the loss of LPS but not with its modification. Conclusions: Resistance to carbapenems in A. baumannii associated with the production of OXA-type or NDM-type enzymes does not seem to affect their virulence in the C. elegans infection model. In contrast, the presence of Omp 33-36 kDa, and high level resistance to colistin related with the loss of LPS, might contribute with the virulence profile in A. baumannii.

Genomic epidemiology of multidrug-resistant Gram-negative organisms

The emergence and spread of antibiotic-resistant Gram-negative bacteria (rGNB) across global healthcare networks presents a significant threat to public health. As the number of effective antibiotics available to treat these resistant organisms dwindles, it is essential that we devise more effective strategies for controlling their proliferation. Recently, whole-genome sequencing has emerged as a disruptive technology that has transformed our understanding of the evolution and epidemiology of diverse rGNB species, and it has the potential to guide strategies for controlling the evolution and spread of resistance. Here, we review specific areas in which genomics has already made a significant impact, including outbreak investigations, regional epidemiology, clinical diagnostics, resistance evolution, and the study of epidemic lineages. While highlighting early successes, we also point to the next steps needed to translate this technology into strategies to improve public health and clinical medicine.

Phenotypic changes associated with Colistin resistance due to Lipopolysaccharide loss in Acinetobacter baumannii

Acinetobacter baumannii can acquire resistance to colistin via complete loss of lipopolysaccharide (LPS) biosynthesis due to mutations in the lpxA, lpxC and lpxD genes. However, although colistin is increasingly being used for the treatment of multidrug resistant infections, very few A. baumannii clinical isolates develop colistin resistance through loss of LPS biosynthesis. This may suggest that LPS loss affects virulence traits that play a role in the transmission and pathogenesis of A. baumannii. In this study we characterize multiple virulence phenotypes of colistin resistant, LPS-deficient derivatives of the ATCC 19606 strain and five multidrug resistant clinical isolates and their colistin resistant, LPS-deficient derivatives. Our results indicate that LPS loss results in growth defects compared to the parental strain in vitro both in laboratory media and human serum (competition indices of 0.58 and 7.0 × 10⁻⁷, respectively) and reduced ability to grow and disseminate in vivo (competition index 6.7 × 10⁻⁸). Infection with the LPS-deficient strain resulted in lower serum levels of pro-inflammatory cytokines TNF-α and IL-6 compared to the parent strain, and was less virulent in a mouse model of disseminated sepsis. LPS loss also significantly affected biofilm production, surface motility, growth under iron limitation and susceptibility to multiple disinfectants used in the clinical setting. These results demonstrate that LPS loss has a significant effect on multiple virulence traits, and may provide insight into the low incidence of colistin resistant strains lacking LPS that have been reported in the clinical setting.

Diversity of virulence level phenotype of hypervirulent Klebsiella pneumoniae from different sequence type lineage

BACKGROUND

Hypervirulent Klebsiella pneumoniae (hvKP) is emerging around the Asian-Pacific region and it is the major cause of the community-acquired pyogenic liver abscesses. Multidrug-resistant hypervirulent Klebsiella pneumoniae (MDR-hvKP) isolates were reported in France, China and Taiwan. However, the international-ally agreed definition for hvKP and the virulence level of hvKP are not clear.

RESULTS

In this study, 56 hvKP isolates were collected from March 2008 to June 2012 and investigated by string test, capsule serotyping, multilocus sequence typing (MLST), virulence gene detection and serum resistance assay. Among the 56 K. pneumoniae isolates, 64.3% had the hypermucoviscosity phenotype, meanwhile, 64.3% were the K1 serotype and 19.6% were the K2 serotype. Within the K1 serotype, 94.4% were ST23, and within the K2 serotype, ST65, ST86 and ST375 accounted for the same percentage 27.3%. The serum resistance showed statistically normal distribution. According to the 50% lethal dose of Galleria. mellonella infection model, hvKP isolates were divided into high virulence level group and moderate virulence level group. The ability of each method evaluating the virulence level of hvKP was assessed using the area under the receiver operating characteristic curve.

CONCLUSIONS

K1 ST23 K. pneumoniae was the most prevalent clone of the hvKP. However, K1 ST23 K. pneumoniae was the dominant clone in the moderate virulence level group. MLST was a relatively reliable evaluation method to discriminate the virulence level of hvKP in our study.

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

Acinetobacter baumannii is an opportunistic pathogen associated with nosocomial and community infections of great clinical relevance. Its ability to rapidly develop resistance to antimicrobials, especially carbapenems, has re-boosted the prescription and use of polymyxins. However, the emergence of strains resistant to these antimicrobials is becoming a critical issue in several regions of the world because very few of currently available antibiotics are effective in these cases. This review summarizes the most up-to-date knowledge about chromosomally encoded and plasmid-mediated polymyxins resistance in A. baumannii. Different mechanisms are employed by A. baumannii to overcome the antibacterial effects of polymyxins. Modification of the outer membrane through phosphoethanolamine addition, loss of lipopolysaccharide, symmetric rupture, metabolic changes affecting osmoprotective amino acids, and overexpression of efflux pumps are involved in this process. Several genetic elements modulate these mechanisms, but only three of them have been described so far in A. baumannii clinical isolates such as mutations in pmrCAB, lpxACD, and lpsB. Elucidation of genotypic profiles and resistance mechanisms are necessary for control and fight against resistance to polymyxins in A. baumannii, thereby protecting this class for future treatment.

Growth Rate and Biofilm Formation Ability of Clinical and Laboratory-Evolved Colistin-Resistant Strains of Acinetobacter baumannii

Two different mechanisms of resistance to colistin in Acinetobacter baumannii have been described. The first involves the total loss of lipopolysaccharide (LPS) due to mutations in the lpxACD operon, which is involved in the lipid A biosynthesis pathway. The second entails the addition of ethanolamine to the lipid A of the LPS resulting from mutations in the PmrAB two-component system. To evaluate the impact of colistin resistance-associated mutations on antimicrobial resistance and virulence properties, four pairs of clinical and laboratory-evolved colistin-susceptible/colistin-resistant (Col^S/Col^R) A. baumannii isolates were used. Antimicrobial susceptibility, surface motility, in vitro and in vivo biofilm-forming capacity, in vitro and in vivo expression levels of biofilm-associated genes, and in vitro growth rate were analyzed in these strains. Growth rate, in vitro and in vivo biofilm formation ability, as well as expression levels of biofilm-associated gene were reduced in Col^R LPS-deficient isolate (the lpxD mutant) when compared with its Col^S partner, whereas there were not such differences between LPS-modified isolates (the pmrB mutants) and their parental isolates. Mutation in lpxD was accompanied by a greater reduction in minimum inhibitory concentrations of azithromycin, vancomycin, and rifampin than mutation in pmrB. Besides, loss of LPS was associated with a significant reduction in surface motility without any change in expression of type IV pili. Collectively, colistin resistance through loss of LPS causes a more considerable cost in biological features such as growth rate, motility, and biofilm formation capacity relative to LPS modification. Therefore, Col^R LPS-modified strains are more likely to spread and transmit from one patient to another in hospital settings, which results in more complex treatment and control.

Interplay between Colistin Resistance, Virulence and Fitness in Acinetobacter baumannii

Acinetobacter baumannii is an important opportunistic nosocomial pathogen often resistant to multiple antibiotics classes. Colistin, an “old” antibiotic, is now considered a last-line treatment option for extremely resistant isolates. In the meantime, resistance to colistin has been reported in clinical A. baumannii strains. Colistin is a cationic peptide that disrupts the outer membrane (OM) of Gram-negative bacteria. Colistin resistance is primarily due to post-translational modification or loss of the lipopolysaccharide (LPS) molecules inserted into the outer leaflet of the OM. LPS modification prevents the binding of polymyxin to the bacterial surface and may lead to alterations in bacterial virulence. Antimicrobial pressure drives the evolution of antimicrobial resistance and resistance is often associated with a reduced bacterial fitness. Therefore, the alterations in LPS may induce changes in the fitness of A. baumannii. However, compensatory mutations in clinical A. baumannii may ameliorate the cost of resistance and may play an important role in the dissemination of colistin-resistant A. baumannii isolates. The focus of this review is to summarize the colistin resistance mechanisms, and understand their impact on the fitness and virulence of bacteria and on the dissemination of colistin-resistant A. baumannii strains.

Exogenous polyunsaturated fatty acids (PUFAs) alter phospholipid composition, membrane permeability, biofilm formation and motility in Acinetobacter baumannii

Acinetobacter baumannii is a ubiquitous multidrug-resistant bacteria that is found on a variety of surfaces, including skin, hair and soil. During the past decade, A. baumannii has emerged as a significant cause of nosocomial infections in the United States. Recent studies have highlighted the ability of some bacteria to utilize a wide variety of fatty acids as a membrane remodelling strategy. Considering this, we hypothesized that fatty acids may have an effect on the emerging pathogen A. baumannii. Thin-layer chromatography indicated structural alterations to major phospholipids. Liquid chromatography/mass spectrometry confirmed the assimilation of numerous exogenous polyunsaturated fatty acids (PUFAs) into the phospholipid species of A. baumannii. The incorporation of fatty acids affected several bacterial phenotypes, including membrane permeability, biofilm formation, surface motility and antimicrobial peptide resistance.

Rapid and Consistent Evolution of Colistin Resistance in Extensively Drug-Resistant Pseudomonas aeruginosa during Morbidostat Culture

Colistin is a last-resort antibiotic commonly used against multidrug-resistant strains of Pseudomonas aeruginosa To investigate the potential for in situ evolution of resistance against colistin and to map the molecular targets of colistin resistance, we exposed two P. aeruginosa isolates to colistin using a continuous-culture device known as a morbidostat. As a result, colistin resistance reproducibly increased 10-fold within 10 days and 100-fold within 20 days, along with highly stereotypic yet strain-specific mutation patterns. The majority of mutations hit the pmrAB two-component signaling system and genes involved in lipopolysaccharide (LPS) synthesis, including lpxC, pmrE, and migA We tracked the frequencies of all arising mutations by whole-genome deep sequencing every 3 to 4 days to obtain a detailed picture of the dynamics of resistance evolution, including competition and displacement among multiple resistant subpopulations. In 7 out of 18 cultures, we observed mutations in mutS along with a mutator phenotype that seemed to facilitate resistance evolution.

Colistin Resistance in Acinetobacter baumannii MDR-ZJ06 Revealed by a Multiomics Approach

Acinetobacter baumannii has emerged as an important opportunistic pathogen due to its ability to acquire resistance to most currently available antibiotics. Colistin is often considered as the last line of therapy for infections caused by multidrug-resistant A. baumannii (MDRAB). However, colistin-resistant A. baumannii strain has recently been reported. To explore how multiple drug-resistant A. baumannii responded to colistin resistance, we compared the genomic, transcriptional and proteomic profile of A. baumannii MDR-ZJ06 to the induced colistin-resistant strain ZJ06-200P5-1. Genomic analysis showed that lpxC was inactivated by ISAba1 insertion, leading to LPS loss. Transcriptional analysis demonstrated that the colistin-resistant strain regulated its metabolism. Proteomic analysis suggested increased expression of the RND efflux pump system and down-regulation of FabZ and β-lactamase. These alterations were believed to be response to LPS loss. In summary, the lpxC mutation not only established colistin resistance but also altered global gene expression.