Abrp, a new gene, confers reduced susceptibility to tetracycline, glycylcine, chloramphenicol and fosfomycin classes in Acinetobacter baumannii

Molecular Analysis of Tigecycline Resistance in Carbapenem-Resistant Enterobacterales (CRE) in Mthatha and Surrounding Hospitals

Background: The emergence of carbapenem-resistant Enterobacterales is prevalent and poses a significant threat to health systems worldwide. This study aimed to conduct a molecular analysis of tigecycline resistance in 100 CRE isolates from Mthatha Hospital and surrounding hospitals. Methods: A retrospective study among patients who attended Nelson Mandela Academic Hospital (NMAH) and Mthatha Regional Hospital (MRH), Eastern Cape, South Africa. Enterobacterales isolates were identified using the Vitek2® system (bioMérieux); an E-test was performed on 100 CRE isolates according to the manufacturer's instructions. PCR assays for rapid detection of tet(X) and its variants, including tet(X1) and tet(X2), and high-level tigecycline resistance genes tet(X3), tet(X4), and tet(X5) were developed. Results: The results show a notably high prevalence of CRE infections in neonatal, male surgical, and maternal and pediatric wards, predominantly driven by Klebsiella species (53.4%), followed by Enterobacter species (20.5%) and then Escherichia coli (6.7%), and 7.2% of CRE isolates were resistant to tigecycline (E-test). In this study, tet(X) genes were not identified as the primary mechanism of tigecycline resistance. The risk factors associated with tigecycline resistance in CRE include age, pre-exposure to antibiotics, prolonged hospitalization, and undergoing invasive procedures, indicated by strong r = 0.9501. Conclusions: CRE gradually evolves, posing a significant threat to patients of all ages; early detection of carbapenemase production in clinical infections, carriage states, or both is essential to prevent hospital-based outbreaks.

Identification of a novel chromosome-encoded fosfomycin resistance gene fosC3 in Aeromonas caviae

Background

Owing to the rapid emerging of multidrug-, even pandrug-resistant pathogens, and lack of new antibiotics, the older antibiotic, fosfomycin, has been reused in recent years in the clinical practice, especially for treatment of uropathogen infections. With the increased use of fosfomycin, bacterial resistance to it has also increased drastically. Elucidating the resistance mechanism to the antimicrobial has become an urgent task.

Methods

The putative fosfomycin resistance gene fosC3 was cloned, and minimal inhibitory concentrations were determined by the agar dilution method. Enzyme kinetic parameters were measured by high-performance liquid chromatography. Bioinformatics analysis was applied to understand the evolutionary characteristics of FosC3.

Results

The A. caviae strain DW0021 exhibited high level resistance to several antimicrobials including kanamycin, streptomycin, chloramphenicol, florfenicol, tetracycline, and especially higher to fosfomycin (> 1,024 μg/mL), while genome annotation indicated that no function-characterized resistance gene was associated with fosfomycin resistance. A novel functional gene designated fosC3 responsible for fosfomycin resistance was identified in the chromosome of A. caviae DW0021. Among the function-characterized proteins, FosC3 shared the highest amino acid similarity of 58.65% with FosC2. No mobile genetic element (MGE) was found surrounding the fosC3 gene. The recombinant pMD19-fosC3/DH5α displayed a MIC value of 32 μg/mL to fosfomycin, which revealed a 128-fold increase of MIC value to fosfomycin compared to the control pMD19/E. coli DH5α (0.25 μg/mL). FosC3 was phylogenetically close to FosC2 and exhibited a k cat and K m of 82,442 ± 1,475 s-1, 70.99 ± 4.31 μM, respectively, and a catalytic efficiency of (1.2 ± 0.3) × 103 μM-1·s-1.

Conclusion

In this work, a novel functional fosfomycin thiol transferase, FosC3, which shared the highest protein sequence similarity with FosC2, was identified in A. caviae. The fosfomycin inactivation enzyme FosC3 could effectively inactivate fosfomycin by chemical modification. It is implied that such mechanism facilitates A. caviae to respond to fosfomycin exposure, thereby enhancing survival. However, fosC3 was not related with any MGE, which differs from many other fosfomycin thiol transferase genes. As a result, fosC3 is not expected to be transmitted to other species through horizontal gene transfer mechanism. Our findings will contribute to the resistance mechanism of the common pathogenic A. caviae.

The Prevalence and the Underlying Mechanisms of Fosfomycin Resistance of Escherichia coli and Salmonella spp. Among Cattle in Japan

To investigate fosfomycin resistance rates in cattle across Japan, we carried out susceptibility tests. To identify the genes contributing to fosfomycin resistance, we performed whole-genome sequencing on the fosfomycin-resistant strains. Escherichia coli were sampled from healthy cattle (n = 292, combined total from 2017, 2020, 2021, and 2022) and diseased cattle (n = 73, from 2021 to 2022). Salmonella spp. were obtained from diseased cattle (n = 74 from 2021 to 2022). These samples originated from different and non-duplicated farms. The MICs to fosfomycin were measured using an agar dilution method with a breakpoint of 256 μg/mL. We conducted whole-genome sequencing with a MiSeq, followed by in silico analysis of the acquired draft genomes. The resistance rates were 0.3% (95% CI [0-1.9%]), 6.8% (95% CI [2.3-15.3%]), and 1.4% (95% CI [0-7.3%]). The FosA3 gene was detected in five out of six fosfomycin-resistant E. coli strains and one Salmonella spp. strain. The fosfomycin-resistant Salmonella spp. strain also has a fosA7 gene. One E. coli strain showed resistance to fosfomycin without having the fosA3 gene, and with the mutations of glpT, uhpT, uhpT and ptsI, and with the existence of efflux pumps. The nationwide scale of resistance rates to fosfomycin in E. coli isolated from healthy and diseased cattle and that of Salmonella spp. from diseased cattle were revealed for the first time, and the resistance rates were low. In addition, genes linked to the mechanism of fosfomycin resistance were identified.

The tigecycline resistance mechanisms in Gram-negative bacilli

Tigecycline, hailed as a pivotal agent in combating multidrug-resistant bacterial infections, confronts obstacles posed by the emergence of resistance mechanisms in Gram-negative bacilli. This study explores the complex mechanisms of tigecycline resistance in Gram-negative bacilli, with a particular focus on the role of efflux pumps and drug modification in resistance. By summarizing these mechanisms, our objective is to provide a comprehensive understanding of tigecycline resistance in Gram-negative bacilli, thereby illuminating the evolving landscape of antimicrobial resistance. This review contributes to the elucidation of current existing tigecycline resistance mechanisms and provides insights into the development of effective strategies to manage the control of antimicrobial resistance in the clinical setting, as well as potential new targets for the treatment of tigecycline-resistant bacterial infections.

Genome analysis of tigecycline-resistant Acinetobacter baumannii reveals nosocomial lineage shifts and novel resistance mechanisms

OBJECTIVES

To investigate the characteristics and clonal dynamics of tigecycline-resistant Acinetobacter baumannii (TRAB) isolates from a Chinese hospital from 2016 to 2021.

METHODS

A total of 64 TRAB isolates were screened and WGS was performed. Phylogenetic analysis and non-polymorphic mutation analysis were used to analyse their clonal dynamics and tigecycline resistance-related mutations. RT-PCR was used to analyse the expression of the resistance-nodulation cell-division (RND) efflux pump genes adeB and adeJ. Gene cloning was used to explore the effect of tet(39) variants on tigecycline resistance.

RESULTS

Most TRAB isolates were found to be MDR, with 95% (61/64) of the isolates showing resistance to carbapenems. These TRAB isolates were classified into three primary genetic clusters based on core-genome SNPs. The KL2 cluster persisted throughout the study period, whereas the KL7 cluster emerged in 2019 and became the dominant clone. The KL7 cluster carried more antimicrobial resistance genes than the other two clusters. The predominant tigecycline resistance mechanism of the KL2 cluster and KL7 cluster was IS insertion in adeN (82.1%, 23/28) and genetic alterations in adeS (76.2%, 16/21), respectively. Eleven novel AdeS mutations were identified associated with elevated AdeB expression and tigecycline resistance. Moreover, we characterized a plasmid-borne tet(39) variant with an Ala-36-Thr substitution that synergizes with the RND efflux pump to confer high-level tigecycline resistance.

CONCLUSIONS

This work provides important insights into the diverse mechanisms associated with tigecycline resistance in A. baumannii, highlighting a pressing need for further monitoring of ST2-KL7 A. baumannii in clinical settings.

Understanding the mechanisms of antimicrobial resistance and potential therapeutic approaches against the Gram-negative pathogen Acinetobacter baumannii

Globally, the emergence of anti-microbial resistance in pathogens has become a serious threat to human health and well-being. Infections caused by drug-resistant microorganisms in hospitals are associated with increased morbidity, mortality, and healthcare costs. Acinetobacter baumannii is a Gram-negative bacterium belonging to the ESKAPE group and is widely associated with nosocomial infections. It persists in hospitals and survives antibiotic treatment, prompting acute infections such as urinary tract infections, pneumonia, bacteremia, meningitis, and wound-related infections. An innovation void in drug discovery and the lack of new therapeutic measures against A. baumannii continue to afflict infection control against the rising drug-resistant cases. The emergence of drug-resistant A. baumannii strains has also led to the incessant collapse of newly discovered antibiotics. Therefore exploring novel strategies is requisite to give impetus to A. baumannii drug discovery. The present review discusses the bacterial research community's efforts in the field of A. baumannii, focusing on the strategies adapted to identify potent scaffolds and novel targets to bolster and diversify the chemical space available for drug discovery. Firstly, we have discussed existing chemotherapy and various anti-microbial resistance mechanisms in A. baumannii bacterial strains. Next, we elaborate on multidisciplinary approaches and strategies that may be the way forward to combat the current menace caused by the drug-resistant A. baumannii strains. The review highlights the recent advances in drug discovery, including combinational therapy, high-throughput screening, drug repurposing, nanotechnology, and anti-microbial peptides, which are imperative tools to fight bacterial pathogens in the future.

Acinetobacter baumannii: an evolving and cunning opponent

Acinetobacter baumannii is one of the most common multidrug-resistant pathogens causing nosocomial infections. The prevalence of multidrug-resistant A. baumannii infections is increasing because of several factors, including unregulated antibiotic use. A. baumannii drug resistance rate is high; in particular, its resistance rates for tigecycline and polymyxin-the drugs of last resort for extensively drug-resistant A. baumannii-has been increasing annually. Patients with a severe infection of extensively antibiotic-resistant A. baumannii demonstrate a high mortality rate along with a poor prognosis, which makes treating them challenging. Through carbapenem enzyme production and other relevant mechanisms, A. baumannii has rapidly acquired a strong resistance to carbapenem antibiotics-once considered a class of strong antibacterials for A. baumannii infection treatment. Therefore, understanding the resistance mechanism of A. baumannii is particularly crucial. This review summarizes mechanisms underlying common antimicrobial resistance in A. baumannii, particularly those underlying tigecycline and polymyxin resistance. This review will serve as a reference for reasonable antibiotic use at clinics, as well as new antibiotic development.

Alteration of adeS Contributes to Tigecycline Resistance and Collateral Sensitivity to Sulbactam in Acinetobacter baumannii

The treatment of extensively drug-resistant (XDR) A. baumannii has emerged as a major problem. Tigecycline (TGC) and sulbactam (SUL) are both effective antibiotics against XDR A. baumannii. Here, we investigated the in-host evolution and mechanism of collateral sensitivity (CS) phenomenon in development of tigecycline resistance accompanied by a concomitant increase of sulbactam susceptibility. A total of four XDR A. baumannii strains were sequentially isolated from the same patient suffering from bacteremia. Core-genome multilocus sequence typing separated all the strains into two clusters. Comparative analysis of isolate pair 1 revealed that multiplication of blaOXA-23 within Tn2006 on the chromosome contributed to the change in the antimicrobial susceptibility phenotype of isolate pair 1. Additionally, we observed the emergence of CS to sulbactam in isolate pair 2, as demonstrated by an 8-fold increase in the TGC MIC with a simultaneous 4-fold decrease in the SUL MIC. Compared to the parental strain Ab-3557, YZM-0406 showed partial deletion in the two-component system sensor adeS. Reconstruction of the adeS mutant in Ab-3557 in situ suggested that TGC resistance and CS to SUL were mainly caused by the mutation of adeS. Overall, our study reported a novel CS combination of TGC and SUL in A. baumannii and further revealed a mechanism of CS attributed to the mutation of adeS. This study provides a valuable foundation for developing effective regimens and sequential combinations of tigecycline and sulbactam against XDR A. baumannii. IMPORTANCE Collateral sensitivity (CS) has become an increasingly common evolutionary trade-off during adaptive bacterial evolution. Here, we report a novel combination of tigecycline (TGC) resistance and CS to sulbactam (SUL) in A. baumannii. TGC and SUL are both effective antibiotics against XDR A. baumannii, and it is essential to reveal the mechanism of CS between TGC and SUL. In our study, the partial deletion of adeS, a two-component system sensor, was confirmed to be the key factor contributing to this CS phenomenon. This study provides a valuable foundation for developing effective regimens and sequential combinations of tigecycline and sulbactam against XDR A. baumannii.

The Mechanism of Tigecycline Resistance in Acinetobacter baumannii Revealed by Proteomic and Genomic Analysis

The mechanism of tigecycline resistance in A. baumannii remains largely unclear. In this study, we selected a tigecycline-resistant and a tigecycline-susceptible strain from a tigecycline-susceptible and a resistant strain, respectively. Proteomic and genomic analyses were performed to elucidate the variations associated with tigecycline resistance. Our study showed proteins associated with efflux pump, biofilm formation, iron acquisition, stress response, and metabolic ability are upregulated in tigecycline resistant strains, and efflux pump should be the key mechanism for tigecycline resistance. By genomic analysis, we found several changes in the genome that can explain the increased level of efflux pump, including the loss of the global negative regulator hns in the plasmid and the disruption of the hns gene and acrR gene on the chromosome by the insertion of IS5. Collectively, we not only revealed the phenomenon that the efflux pump is mainly responsible for tigecycline resistance, but also highlighted the mechanism at the genomic level, which will help in understanding the resistance mechanism in detail and provide clues for the treatment of clinical multiple drug-resistant A. baumannii.

Resistance mechanisms of tigecycline in Acinetobacter baumannii

Acinetobacter baumannii is widely distributed in nature and in hospital settings and is a common pathogen causing various infectious diseases. Currently, the drug resistance rate of A. baumannii has been persistently high, showing a worryingly high resistance rate to various antibiotics commonly used in clinical practice, which greatly limits antibiotic treatment options. Tigecycline and polymyxins show rapid and effective bactericidal activity against CRAB, and they are both widely considered to be the last clinical line of defense against multidrug resistant A. baumannii. This review focuses with interest on the mechanisms of tigecycline resistance in A. baumannii. With the explosive increase in the incidence of tigecycline-resistant A. baumannii, controlling and treating such resistance events has been considered a global challenge. Accordingly, there is a need to systematically investigate the mechanisms of tigecycline resistance in A. baumannii. Currently, the resistance mechanism of A. baumannii to tigecycline is complex and not completely clear. This article reviews the proposed resistance mechanisms of A. baumannii to tigecycline, with a view to providing references for the rational clinical application of tigecycline and the development of new candidate antibiotics.

Acinetobacter baumannii: Pathogenesis, virulence factors, novel therapeutic options and mechanisms of resistance to antimicrobial agents with emphasis on tigecycline

WHAT IS KNOWN AND OBJECTIVE

Acinetobacter baumannii is one of the most important nosocomial pathogens with the ability to cause infections such as meningitis, pneumonia, urinary tract, septicaemia and wound infections. A wide range of virulence factors are responsible for pathogenesis and high mortality of A. baumannii including outer membrane proteins, lipopolysaccharide, capsule, phospholipase, nutrient- acquisition systems, efflux pumps, protein secretion systems, quarom sensing and biofilm production. These virulence factors contribute in pathogen survival in stressful conditions and antimicrobial resistance.

COMMENT

According to the World Health Organization (WHO), A. baumannii is one of the most resistant pathogens of ESKAPE group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, A. baumannii, Pseudomonas aeruginosa and Enterobacter spp.). In recent years, resistance to a wide range of antibiotics in A. baumannii has significantly increased and the high emergence of extensively drug resistant (XDR) isolates is challenging. Among therapeutic antibiotics, resistance to tigecycline as a last resort antibiotic has become a global concern. Several mechanisms are involved in tigecycline resistance, the most important of which is RND (Resistance-Nodulation-Division) family efflux pumps overexpression. The development of new therapeutic strategies to confront A. baumannii infections has been very promising in recent years.

WHAT IS NEW AND CONCLUSION

In the present review we highlight microbiological and virulence traits in A. baumannii and peruse the tigecycline resistance mechanisms and novel therapeutic options. Among the novel therapeutic strategies we focus on combination therapy, drug repurposing, novel antibiotics, bacteriophage therapy, antimicrobial peptides (AMPs), human monoclonal antibodies (Hu-mAbs), nanoparticles and gene editing.

Dissemination and prevalence of plasmid-mediated high-level tigecycline resistance gene tet (X4)

With the large-scale use of antibiotics, antibiotic resistant bacteria (ARB) continue to rise, and antibiotic resistance genes (ARGs) are regarded as emerging environmental pollutants. The new tetracycline-class antibiotic, tigecycline is the last resort for treating multidrug-resistant (MDR) bacteria. Plasmid-mediated horizontal transfer enables the sharing of genetic information among different bacteria. The tigecycline resistance gene tet(X) threatens the efficacy of tigecycline, and the adjacent ISCR2 or IS26 are often detected upstream and downstream of the tet(X) gene, which may play a crucial driving role in the transmission of the tet(X) gene. Since the first discovery of the plasmid-mediated high-level tigecycline resistance gene tet(X4) in China in 2019, the tet(X) genes, especially tet(X4), have been reported within various reservoirs worldwide, such as ducks, geese, migratory birds, chickens, pigs, cattle, aquatic animals, agricultural field, meat, and humans. Further, our current researches also mentioned viruses as novel environmental reservoirs of antibiotic resistance, which will probably become a focus of studying the transmission of ARGs. Overall, this article mainly aims to discuss the current status of plasmid-mediated transmission of different tet(X) genes, in particular tet(X4), as environmental pollutants, which will risk to public health for the "One Health" concept.

JMM Profile: Fosfomycin: a potential antibiotic for multi- and extensively resistant bacteria

Fosfomycin (FOF) is the first antimicrobial of the epoxide class. It is commercially available in oral and parenteral formulations. Oral FOF is widely used to treat uncomplicated cystitis in women, while parenteral FOF is extensively utilized for upper urinary tract infections. FOF has a broad-spectrum bactericidal activity with a low risk of cross-resistance to other antimicrobial classes. Therefore, parenteral FOF is increasingly prescribed adjunctive therapy to treat extra-urinary tract infections caused by multidrug-resistant, Gram-negative bacteria.

Clinical Perspective of Antimicrobial Resistance in Bacteria

Antimicrobial resistance (AMR) has become a global clinical problem in recent years. With the discovery of antibiotics, infections were not a deadly problem for clinicians as they used to be. However, worldwide AMR comes with the overuse/misuse of antibiotics and the spread of resistance is deteriorated by a multitude of mobile genetic elements and relevant resistant genes. This review provides an overview of the current situation, mechanism, epidemiology, detection methods and clinical treatment for antimicrobial resistant genes in clinical important bacteria including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), penicillin-resistant Streptococcus pneumoniae (PRSP), extended-spectrum β-lactamase-producing Enterobacteriaceae, acquired AmpC β-lactamase-producing Enterobacteriaceae, carbapenemase-producing Enterobacteriaceae (CPE), multidrug-resistant (MDR) Acinetobacter baumannii and Pseudomonas aeruginosa.

Acinetobacter baumannii Antibiotic Resistance Mechanisms

Acinetobacter baumannii is a Gram-negative ESKAPE microorganism that poses a threat to public health by causing severe and invasive (mostly nosocomial) infections linked with high mortality rates. During the last years, this pathogen displayed multidrug resistance (MDR), mainly due to extensive antibiotic abuse and poor stewardship. MDR isolates are associated with medical history of long hospitalization stays, presence of catheters, and mechanical ventilation, while immunocompromised and severely ill hosts predispose to invasive infections. Next-generation sequencing techniques have revolutionized diagnosis of severe A. baumannii infections, contributing to timely diagnosis and personalized therapeutic regimens according to the identification of the respective resistance genes. The aim of this review is to describe in detail all current knowledge on the genetic background of A. baumannii resistance mechanisms in humans as regards beta-lactams (penicillins, cephalosporins, carbapenems, monobactams, and beta-lactamase inhibitors), aminoglycosides, tetracyclines, fluoroquinolones, macrolides, lincosamides, streptogramin antibiotics, polymyxins, and others (amphenicols, oxazolidinones, rifamycins, fosfomycin, diaminopyrimidines, sulfonamides, glycopeptide, and lipopeptide antibiotics). Mechanisms of antimicrobial resistance refer mainly to regulation of antibiotic transportation through bacterial membranes, alteration of the antibiotic target site, and enzymatic modifications resulting in antibiotic neutralization. Virulence factors that may affect antibiotic susceptibility profiles and confer drug resistance are also being discussed. Reports from cases of A. baumannii coinfection with SARS-CoV-2 during the COVID-19 pandemic in terms of resistance profiles and MDR genes have been investigated.

Peptidoglycan recycling contributes to intrinsic resistance to fosfomycin in Acinetobacter baumannii

Background

Acinetobacter baumannii is intrinsically resistant to fosfomycin; however, the mechanisms underlying this resistance are poorly understood.

Objectives

To identify and characterize genes that contribute to intrinsic fosfomycin resistance in A. baumannii.

Methods

More than 9000 individual transposon mutants of the A. baumannii ATCC 17978 strain (fosfomycin MIC ≥1024 mg/L) were screened to identify mutations conferring increased susceptibility to fosfomycin. In-frame deletion mutants were constructed for the identified genes and their susceptibility to fosfomycin was characterized by MIC determination and growth in the presence of fosfomycin. The effects of these mutations on membrane permeability and peptidoglycan integrity were characterized. Susceptibilities to 21 antibiotics were determined for the mutant strains.

Results

Screening of the transposon library identified mutants in the ampD and anmK genes, both encoding enzymes of the peptidoglycan recycling pathway, that demonstrated increased susceptibility to fosfomycin. MIC values for in-frame deletion mutants were ≥42-fold (ampD) and ≥8-fold (anmK) lower than those for the parental strain, and growth of the mutant strains in the presence of 32 mg/L fosfomycin was significantly reduced. Neither mutation resulted in increased cell permeability; however, the ampD mutant demonstrated decreased peptidoglycan integrity. Susceptibility to 21 antibiotics was minimally affected by mutations in ampD and anmK.

Conclusions

This study demonstrates that AmpD and AnmK of the peptidoglycan recycling pathway contribute to intrinsic fosfomycin resistance in A. baumannii, indicating that inhibitors of these enzymes could be used in combination with fosfomycin as a novel treatment approach for MDR A. baumannii.

Dual Role of gnaA in Antibiotic Resistance and Virulence in Acinetobacter baumannii

Acinetobacter baumannii is an important Gram-negative pathogen in hospital-related infections. However, treatment options for A. baumannii infections have become limited due to multidrug resistance. Bacterial virulence is often associated with capsule genes found in the K locus, many of which are essential for biosynthesis of the bacterial envelope. However, the roles of other genes in the K locus remain largely unknown. From an in vitro evolution experiment, we obtained an isolate of the virulent and multidrug-resistant A. baumannii strain MDR-ZJ06, called MDR-ZJ06^M, which has an insertion by the ISAba16 transposon in gnaA (encoding UDP-N-acetylglucosamine C-6 dehydrogenase), a gene found in the K locus. The isolate showed an increased resistance toward tigecycline, whereas the MIC decreased in the case of carbapenems, cephalosporins, colistin, and minocycline. By using knockout and complementation experiments, we demonstrated that gnaA is important for the synthesis of lipooligosaccharide and capsular polysaccharide and that disruption of the gene affects the morphology, drug susceptibility, and virulence of the pathogen.

Resistance to fosfomycin: Mechanisms, Frequency and Clinical Consequences

Fosfomycin has been used for the treatment of infections due to susceptible and multidrug-resistant (MDR) bacteria. It inhibits bacterial cell wall synthesis through a unique mechanism of action at a step prior to that inhibited by β-lactams. Fosfomycin enters the bacterium through membrane channels/transporters and inhibits MurA, which initiates peptidoglycan (PG) biosynthesis of the bacterial cell wall. Several bacteria display inherent resistance to fosfomycin mainly through MurA mutations. Acquired resistance involves, in order of decreasing frequency, modifications of membrane transporters that prevent fosfomycin from entering the bacterial cell, acquisition of plasmid-encoded genes that inactivate fosfomycin, and MurA mutations. Fosfomycin resistance develops readily in vitro but less so in vivo. Mutation frequency is higher among Pseudomonas aeruginosa and Klebsiella spp. compared with Escherichia coli and is associated with fosfomycin concentration. Mutations in cAMP regulators, fosfomycin transporters and MurA seem to be associated with higher biological cost in Enterobacteriaceae but not in Pseudomonas spp. The contribution of fosfomycin inactivating enzymes in emergence and spread of fosfomycin resistance currently seems low-to-moderate, but their presence in transferable plasmids may potentially provide the best means for the spread of fosfomycin resistance in the future. Their co-existence with genes conferring resistance to other antibiotic classes may increase the emergence of MDR strains. Although susceptibility rates vary, rates seem to increase in settings with higher fosfomycin use and among multidrug-resistant pathogens.

Resistance to pentamidine is mediated by AdeAB, regulated by AdeRS, and influenced by growth conditions in Acinetobacter baumannii ATCC 17978

In recent years, effective treatment of infections caused by Acinetobacter baumannii has become challenging due to the ability of the bacterium to acquire or up-regulate antimicrobial resistance determinants. Two component signal transduction systems are known to regulate expression of virulence factors including multidrug efflux pumps. Here, we investigated the role of the AdeRS two component signal transduction system in regulating the AdeAB efflux system, determined whether AdeA and/or AdeB can individually confer antimicrobial resistance, and explored the interplay between pentamidine resistance and growth conditions in A. baumannii ATCC 17978. Results identified that deletion of adeRS affected resistance towards chlorhexidine and 4',6-diamidino-2-phenylindole dihydrochloride, two previously defined AdeABC substrates, and also identified an 8-fold decrease in resistance to pentamidine. Examination of ΔadeA, ΔadeB and ΔadeAB cells augmented results seen for ΔadeRS and identified a set of dicationic AdeAB substrates. RNA-sequencing of ΔadeRS revealed transcription of 290 genes were ≥2-fold altered compared to the wildtype. Pentamidine shock significantly increased adeA expression in the wildtype, but decreased it in ΔadeRS, implying that AdeRS activates adeAB transcription in ATCC 17978. Investigation under multiple growth conditions, including the use of Biolog phenotypic microarrays, revealed resistance to pentamidine in ATCC 17978 and mutants could be altered by bioavailability of iron or utilization of different carbon sources. In conclusion, the results of this study provide evidence that AdeAB in ATCC 17978 can confer intrinsic resistance to a subset of dicationic compounds and in particular, resistance to pentamidine can be significantly altered depending on the growth conditions.

The role of the type VI secretion system vgrG gene in the virulence and antimicrobial resistance of Acinetobacter baumannii ATCC 19606

The Type VI Secretion System (T6SS) is an important virulence system that exists in many bacterial pathogens, and has emerged as a potent mediator of pathogenicity in Acinetobacter baumannii. In this study, we inactivated one of the T6SS components vgrG (valine–glycine repeat G) gene in A. baumannii ATCC 19606 and constructed a complementation strain. BEAS-2b human alveolar epithelial cells was adopted to assess bacterial adhesion, and wild female BALB/c mice were used for in vivo experiments to assess the bacterial killing ability to host. Upon deletion of the vgrG gene, increased antimicrobial resistance to ampicillin/sulbactam, but reduced resistance to chloramphenicol were observed. The vgrG mutant strain showed lower growth rate, reduced eukaryotic cell adherence and impaired lethality in mice. However, the vgrG mutant strain is not implicated in biofilm formation. Our study suggests that the Type VI Secretion System core component VgrG contributes to both virulence and antimicrobial resistance in A. baumannii ATCC 19606.

Biology of Acinetobacter baumannii: Pathogenesis, Antibiotic Resistance Mechanisms, and Prospective Treatment Options

Acinetobacter baumannii is undoubtedly one of the most successful pathogens responsible for hospital-acquired nosocomial infections in the modern healthcare system. Due to the prevalence of infections and outbreaks caused by multi-drug resistant A. baumannii, few antibiotics are effective for treating infections caused by this pathogen. To overcome this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. In this review, we summarize current studies on the virulence factors that contribute to A. baumannii pathogenesis, including porins, capsular polysaccharides, lipopolysaccharides, phospholipases, outer membrane vesicles, metal acquisition systems, and protein secretion systems. Mechanisms of antibiotic resistance of this organism, including acquirement of β-lactamases, up-regulation of multidrug efflux pumps, modification of aminoglycosides, permeability defects, and alteration of target sites, are also discussed. Lastly, novel prospective treatment options for infections caused by multi-drug resistant A. baumannii are summarized.