OXA-24, a novel class D beta-lactamase with carbapenemase activity in an Acinetobacter baumannii clinical strain

Genomic Surveillance Uncovers a 10-Year Persistence of an OXA-24/40 Acinetobacter baumannii Clone in a Tertiary Hospital in Northern Spain

Infections caused by carbapenem-resistant Acinetobacter baumannii are a global threat causing a high number of fatal infections. This microorganism can also easily acquire antibiotic resistance determinants, making the treatment of infections a big challenge, and has the ability to persist in the hospital environment under a wide range of conditions. The objective of this work was to study the molecular epidemiology and genetic characteristics of two blaOXA24/40Acinetobacter baumannii outbreaks (2009 and 2020-21) at a tertiary hospital in Northern Spain. Thirty-six isolates were investigated and genotypically screened by Whole Genome Sequencing to analyse the resistome and virulome. Isolates were resistant to carbapenems, aminoglycosides and fluoroquinolones. Multi-Locus Sequence Typing analysis identified that Outbreak 1 was mainly produced by isolates belonging to ST3Pas/ST106Oxf (IC3) containing blaOXA24/40, blaOXA71 and blaADC119. Outbreak 2 isolates were exclusively ST2Pas/ST801Oxf (IC2) blaOXA24/40, blaOXA66 and blaADC30, the same genotype seen in two isolates from 2009. Virulome analysis showed that IC2 isolates contained genes for capsular polysaccharide KL32 and lipooligosacharide OCL5. A 8.9 Kb plasmid encoding the blaOXA24/40 gene was common in all isolates. The persistance over time of a virulent IC2 clone highlights the need of active surveillance to control its spread.

Global Epidemiology and Mechanisms of Resistance of Acinetobacter baumannii-calcoaceticus Complex

Acinetobacter baumannii-calcoaceticus complex is the most commonly identified species in the genus Acinetobacter and it accounts for a large percentage of nosocomial infections, including bacteremia, pneumonia, and infections of the skin and urinary tract. A few key clones of A. baumannii-calcoaceticus are currently responsible for the dissemination of these organisms worldwide. Unfortunately, multidrug resistance is a common trait among these clones due to their unrivalled adaptive nature. A. baumannii-calcoaceticus isolates can accumulate resistance traits by a plethora of mechanisms, including horizontal gene transfer, natural transformation, acquisition of mutations, and mobilization of genetic elements that modulate expression of intrinsic and acquired genes.

Molecular tracking of carbapenem-resistant Acinetobacter baumannii clinical isolates: a multicentre study over a 4-year period across eastern China

Introduction. Colonization by carbapenem-resistant Acinetobacter baumannii (CRAB) causes therapeutic and economic problems for critically ill patients.Gap Statement. The analysis of CRAB in China was limited to certain regions.Aims. To investigate the antibiotic susceptibility, molecular characterization and clonal relationship among CRAB isolates from multiple hospitals of eastern China.Methodology. Isolates from 29 tertiary hospitals from September 2015 to September 2018 were recovered. All strains were analysed using antimicrobial susceptibility testing to detect their tolerance. PCR was also used to detect multiple β-lactamase genes. After multilocus sequence typing (MLST) of seven house-keeping genes. eBURST was used to assess clonal complexes and explore evolutionary relationships.Results. All isolates showed resistance to carbapenems, while remaining susceptible to colistin and tigecycline. All isolates were detected with bla _OXA-51 gene by PCR, and 80.1 % harboured the bla _OXA-23 gene. The prevalence of bla_OXA-23 gene was remarkably increased from 50.7 % in 2015 to 90.5 % in 2018. Other genes such as bla _OXA-24, bla _OXA-58, bla _IMP-2/4, bla _VIM-2, bla _SHV, bla _AmpC and bla _TEM were also obtained. While bla _KPC, bla _NDM-1, bla _IMP-4 and bla _SIM-1 were not found in these strains. MLST showed all isolates could be divided into 26 known sequence types (STs) and ten novel STs and 47.2 % isolates belong to ST195 and ST208. eBURST revealed clonal complex 92 as the major clonal complex (98.4 %), which includes 88.5 % (23/26) of known STs and 80 % (8/10) of unknown STs. Phylogenetic analysis also found that almost all CRAB isolates could cluster into one lineage, suggesting an epidemic of this CRAB lineage. This indicated severe nosocomial infections of CRAB in multiple hospitals of eastern China.Conclusion. An outbreak of ST195 and ST208 CRAB-resistant clones with bla _OXA-23 gene might be happening in multiple hospitals in eastern China.

Pharmacokinetic/Pharmacodynamic Evaluation of Aztreonam/Amoxicillin/Clavulanate Combination against New Delhi Metallo-β-Lactamase and Serine-β-Lactamase Co-Producing Escherichia coli and Klebsiella pneumoniae

This study aimed to examine specific niches and usage for the aztreonam/amoxicillin/clavulanate combination and to use population pharmacokinetic simulations of clinical dosing regimens to predict the impact of this combination on restricting mutant selection. The in vitro susceptibility of 19 New-Delhi metallo-β-lactamase (NDM)-producing clinical isolates to amoxicillin/clavulanate and aztreonam alone and in co-administration was determined based on the minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC). The fractions of a 24-h duration that the free drug concentration was within the mutant selection window (fTMSW) and above the MPC (fT>MPC) in both plasma and epithelial lining fluid were determined from simulations of 10,000 subject profiles based on regimens by renal function categories. This combination reduced the MIC of aztreonam and amoxicillin/clavulanate to values below their clinical breakpoint in 7/9 K. pneumoniae and 8/9 E. coli, depending on the β-lactamase genes detected in the isolate. In the majority of the tested isolates, the combination resulted in fT>MPC > 90% and fTMSW < 10% for both aztreonam and amoxicillin/clavulanate. Clinical dosing regimens of aztreonam and amoxicillin/clavulanate were sufficient to provide mutant restriction coverage for MPC and MIC ≤ 4 mg/L. This combination has limited coverage against NDM- and extended-spectrum β-lactamase co-producing E. coli and K. pneumoniae and is not effective against isolates carrying plasmid-mediated AmpC and KPC-2. This study offers a potential scope and limitations as to where the aztreonam/amoxicillin/clavulanate combination may succeed or fail.

Molecular Characterization of Carbapenem-Resistant Acinetobacter baumannii with Special Reference to Carbapenemases: A Systematic Review

Carbapenemases are β-lactamase enzymes that hydrolyze a variety of β-lactams including carbapenem and belong to different Ambler classes (A, B, D). These enzymes can be encoded by plasmid or chromosomal-mediated genes. The major issues associated with carbapenemases-producing organisms are compromising the activity and increasing the resistance to carbapenems which are the last resort antibiotics used in treating serious infections. The global increase of pathogen, carbapenem-resistant A. baumannii has significantly threatened public health. Thus, there is a pressing need for a better understanding of this pathogen, to know the various carbapenem resistance encoding genes and dissemination of resistance genes from A. baumannii which help in developing strategies to overcome this problem. The horizontal transfer of resistant determinants through mobile genetic elements increases the incidence of multidrug, extensive drug, and Pan-drug resistant A. baumannii. Therefore, the current review aims to know the various mechanisms of carbapenem resistance, categorize and discuss carbapenemases encoding genes and various mobile genetic elements, and the prevalence of carbapenemase genes in recent years in A. baumannii from various geographical regions.

In Vitro and In Vivo Activity of AS101 against Carbapenem-Resistant Acinetobacter baumannii

The increasing trend of carbapenem-resistant Acinetobacter baumannii (CRAB) worldwide has become a concern, limiting therapeutic alternatives and increasing morbidity and mortality rates. The immunomodulation agent ammonium trichloro (dioxoethylene-O,O′-) tellurate (AS101) was repurposed as an antimicrobial agent against CRAB. Between 2016 and 2018, 27 CRAB clinical isolates were collected in Taiwan. The in vitro antibacterial activities of AS101 were evaluated using broth microdilution, time-kill assay, reactive oxygen species (ROS) detection and electron microscopy. In vivo effectiveness was assessed using a sepsis mouse infection model. The MIC range of AS101 for 27 CRAB isolates was from 0.5 to 32 µg/mL, which is below its 50% cytotoxicity (approximately 150 µg/mL). Bactericidal activity was confirmed using a time-kill assay. The antibacterial mechanism of AS101 was the accumulation of the ROS and the disruption of the cell membrane, which, in turn, results in cell death. The carbapenemase-producing A. baumannii mouse sepsis model showed that AS101 was a better therapeutic effect than colistin. The mice survival rate after 120 h was 33% (4/12) in the colistin-treated group and 58% (7/12) in the high-dose AS101 (3.33 mg/kg/day) group. Furthermore, high-dose AS101 significantly decreased bacterial population in the liver, kidney and spleen (all p < 0.001). These findings support the concept that AS101 is an ideal candidate for further testing in future studies.

Nationwide Surveillance and Molecular Characterization of Critically Drug-Resistant Gram-Negative Bacteria: Results of the Research University Network Thailand Study

A large-scale surveillance is an important measure to monitor the regional spread of antimicrobial resistance. We prospectively studied the prevalence and molecular characteristics of clinically important Gram-negative bacilli, including Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii complex (ABC), and Pseudomonas aeruginosa, from blood, respiratory tract, urine, and sterile sites at 47 hospitals across Thailand. Among 187,619 isolates, 93,810 isolates (50.0%) were critically drug resistant, of which 12,915 isolates (13.8%) were randomly selected for molecular characterization. E. coli was most commonly isolated from all specimens, except the respiratory tract, in which ABC was predominant. Prevalence of extended-spectrum cephalosporin resistance (ESCR) was higher in E. coli (42.5%) than K. pneumoniae (32.0%), but carbapenem-resistant (CR)-K. pneumoniae (17.2%) was 4.5-fold higher than CR-E. coli (3.8%). The majority of ESCR/CR-E. coli and K. pneumoniae isolates carried bla_CTX-M (64.6% to 82.1%). bla_NDM and bla_OXA-48-like were the most prevalent carbapenemase genes in CR-E. coli/CR-K. pneumoniae (74.9%/52.9% and 22.4%/54.1%, respectively). In addition, 12.9%/23.0% of CR-E. coli/CR-K. pneumoniae cocarried bla_NDM and bla_OXA-48-like. Among ABC isolates, 41.9% were extensively drug resistant (XDR) and 35.7% were multidrug resistant (MDR), while P. aeruginosa showed XDR/MDR at 6.3%/16.5%. A. baumannii was the most common species among ABC isolates. The major carbapenemase gene in MDR-A. baumannii/XDR-A. baumannii was bla_OXA-23-like (85.8%/93.0%), which had much higher rates than other ABC species. bla_IMP, bla_VIM, bla_OXA-40-like, and bla_OXA-58-like were also detected in ABC at lower rates. The most common carbapenemase gene in MDR/XDR-P. aeruginosa was bla_IMP (29.0%/30.6%), followed by bla_VIM (9.5%/25.3%). The findings reiterate an alarming situation of drug resistance that requires serious control measures.

Arginine Modulates Carbapenem Deactivation by OXA-24/40 in Acinetobacter baumannii

The resistance of Gram-negative bacteria to β-lactam antibiotics stems mainly from β-lactamase proteins that hydrolytically deactivate the β-lactams. Of particular concern are the β-lactamases that can deactivate a class of β-lactams known as carbapenems. Carbapenems are among the few anti-infectives that can treat multi-drug resistant bacterial infections. Revealing the mechanisms of their deactivation by β-lactamases is a necessary step for preserving their therapeutic value. Here, we present NMR investigations of OXA-24/40, a carbapenem-hydrolyzing Class D β-lactamase (CHDL) expressed in the gram-negative pathogen, Acinetobacter baumannii. Using rapid data acquisition methods, we were able to study the "real-time" deactivation of the carbapenem known as doripenem by OXA-24/40. Our results indicate that OXA-24/40 has two deactivation mechanisms: canonical hydrolytic cleavage, and a distinct mechanism that produces a β-lactone product that has weak affinity for the OXA-24/40 active site. The mechanisms issue from distinct active site environments poised either for hydrolysis or β-lactone formation. Mutagenesis reveals that R261, a conserved active site arginine, stabilizes the active site environment enabling β-lactone formation. Our results have implications not only for OXA-24/40, but the larger family of CHDLs now challenging clinical settings on a global scale.

Class D β-lactamases

Class D β-lactamases are composed of 14 families and the majority of the member enzymes are included in the OXA family. The genes for class D β-lactamases are frequently identified in the chromosome as an intrinsic resistance determinant in environmental bacteria and a few of these are found in mobile genetic elements carried by clinically significant pathogens. The most dominant OXA family among class D β-lactamases is superheterogeneous and the family needs to have an updated scheme for grouping OXA subfamilies through phylogenetic analysis. The OXA enzymes, even the members within a subfamily, have a diverse spectrum of resistance. Such varied activity could be derived from their active sites, which are distinct from those of the other serine β-lactamases. Their substrate profile is determined according to the size and position of the P-, Ω- and β5-β6 loops, assembling the active-site channel, which is very hydrophobic. Also, amino acid substitutions occurring in critical structures may alter the range of hydrolysed substrates and one subfamily could include members belonging to several functional groups. This review aims to describe the current class D β-lactamases including the functional groups, occurrence types (intrinsic or acquired) and substrate spectra and, focusing on the major OXA family, a new model for subfamily grouping will be presented.

Variability in carbapenemase activity of intrinsic OxaAb (OXA-51-like) β-lactamase enzymes in Acinetobacter baumannii

OBJECTIVES

To measure the variability in carbapenem susceptibility conferred by different OxaAb variants, characterize the molecular evolution of oxaAb and elucidate the contribution of OxaAb and other possible carbapenem resistance factors in the clinical isolates using WGS and LC-MS/MS.

METHODS

Antimicrobial susceptibility tests were performed on 10 clinical Acinetobacter baumannii isolates. Carbapenem MICs were evaluated for all oxaAb variants cloned into A. baumannii CIP70.10 and BM4547, with and without their natural promoters. Molecular evolution analysis of the oxaAb variants was performed using FastTree and SplitsTree4. Resistance determinants were studied in the clinical isolates using WGS and LC-MS/MS.

RESULTS

Only the OxaAb variants with I129L and L167V substitutions, OxaAb(82), OxaAb(83), OxaAb(107) and OxaAb(110) increased carbapenem MICs when expressed in susceptible A. baumannii backgrounds without an upstream IS element. Carbapenem resistance was conferred with the addition of their natural upstream ISAba1 promoter. LC-MS/MS analysis on the original clinical isolates confirmed overexpression of the four I129L and L167V variants. No other differences in expression levels of proteins commonly associated with carbapenem resistance were detected.

CONCLUSIONS

Elevated carbapenem MICs were observed by expression of OxaAb variants carrying clinically prevalent substitutions I129L and L167V. To drive carbapenem resistance, these variants required overexpression by their upstream ISAba1 promoter. This study clearly demonstrates that a combination of IS-driven overexpression of oxaAb and the presence of particular amino acid substitutions in the active site to improve carbapenem capture is key in conferring carbapenem resistance in A. baumannii and other mechanisms are not required.

A plethora of carbapenem resistance in Acinetobacter baumannii: no end to a long insidious genetic journey

Acinetobacter baumannii, notorious for causing nosocomial infections especially in patients admitted to intensive care unit (ICU) and burn units, is best at displaying resistance to all existing antibiotic classes. Consequences of high potential for antibiotic resistance has resulted in extensive drug or even pan drug resistant A. baumannii. Carbapenems, mainly imipenem and meropenem, the last resort for the treatment of A. baumannii infections have fallen short due to the emergence of carbapenem resistant A. baumannii (CRAB). Though enzymatic degradation by production of class D β-lactamases (Oxacillinases) and class B β-lactamases (Metallo β-lactamases) is the core mechanism of carbapenem resistance in A. baumannii; however over-expression of efflux pumps such as resistance-nodulation cell division (RND) family and variant form of porin proteins such as CarO have been implicated for CRAB inception. Transduction and outer membrane vesicles-mediated transfer play a role in carbapenemase determinants spread. Colistin, considered as the most promising antibacterial agent, nevertheless faces adverse effects flaws. Cefiderocol, eravacycline, new β-lactam antibiotics, non-β-lactam-β-lactamase inhibitors, polymyxin B-derived molecules and bacteriophages are some other new treatment options streamlined.

Antisense inhibition of lpxB gene expression in Acinetobacter baumannii by peptide-PNA conjugates and synergy with colistin

BACKGROUND

LpxB is an enzyme involved in the biosynthesis pathway of lipid A, a component of LPS.

OBJECTIVES

To evaluate the lpxB gene in Acinetobacter baumannii as a potential therapeutic target and to propose antisense agents such as peptide nucleic acids (PNAs) as a tool to combat bacterial infection, either alone or in combination with known antimicrobial therapies.

METHODS

RNA-seq analysis of the A. baumannii ATCC 17978 strain in a murine pneumonia model was performed to study the in vivo expression of lpxB. Protein expression was studied in the presence or absence of anti-lpxB (KFF)3K-PNA (pPNA). Time-kill curve analyses and protection assays of infected A549 cells were performed. The chequerboard technique was used to test for synergy between pPNA and colistin. A Galleria mellonella infection model was used to test the in vivo efficacy of pPNA.

RESULTS

The lpxB gene was overexpressed during pneumonia. Treatment with a specific pPNA inhibited LpxB expression in vitro, decreased survival of the ATCC 17978 strain and increased the survival rate of infected A549 cells. Synergy was observed between pPNA and colistin in colistin-susceptible strains. In vivo assays confirmed that a combination treatment of anti-lpxB pPNA and colistin was more effective than colistin in monotherapy.

CONCLUSIONS

The lpxB gene is essential for A. baumannii survival. Anti-lpxB pPNA inhibits LpxB expression, causing bacterial death. This pPNA showed synergy with colistin and increased the survival rate in G. mellonella. The data suggest that antisense pPNA molecules blocking the lpxB gene could be used as antibacterial agents.

A comprehensive and contemporary "snapshot" of β-lactamases in carbapenem resistant Acinetobacter baumannii

Successful treatment of Acinetobacter baumannii infections require early and appropriate antimicrobial therapy. One of the first steps in this process is understanding which β-lactamase (bla) alleles are present and in what combinations. Thus, we performed WGS on 98 carbapenem-resistant A. baumannii (CR Ab). In most isolates, an acquired bla_OXA carbapenemase was found in addition to the intrinsic bla_OXA allele. The most commonly found allele was bla_OXA-23 (n = 78/98). In some isolates, bla_OXA-23 was found in addition to other carbapenemase alleles: bla_OXA-82 (n = 12/78), bla_OXA-72 (n = 2/78) and bla_OXA-24/40 (n = 1/78). Surprisingly, 20% of isolates carried carbapenemases not routinely assayed for by rapid molecular diagnostic platforms, i.e., bla_OXA-82 and bla_OXA-172; all had ISAba1 elements. In 8 CR Ab, bla_OXA-82 or bla_OXA-172 was the only carbapenemase. Both bla_OXA-24/40 and its variant bla_OXA-72 were each found in 6/98 isolates. The most prevalent ADC variants were bla_ADC-30 (21%), bla_ADC-162 (21%), and bla_ADC-212 (26%). Complete combinations are reported.

In Vitro and In Vivo Assessment of the Efficacy of Bromoageliferin, an Alkaloid Isolated from the Sponge Agelas dilatata, against Pseudomonas aeruginosa

The pyrrole-imidazoles, a group of alkaloids commonly found in marine sponges belonging to the genus Agelas, display a wide range of biological activities. Herein, we report the first chemical study of the secondary metabolites of the sponge A. dilatata from the coastal area of the Yucatan Peninsula (Mexico). In this study, we isolated eight known alkaloids from an organic extract of the sponge. We used NMR and MS analysis and comparison with existing databases to characterize the alkaloids: ageliferin (1), bromoageliferin (2), dibromoageliferin (3), sceptrin (4), nakamuric acid (5), 4-bromo-1H-pyrrole-2-carboxylic acid (6), 4,5-dibromopyrrole-2-carboxylic acid (7) and 3,7-dimethylisoguanine (8). We also evaluated, for the first time, the activity of these alkaloids against the most problematic multidrug-resistant (MDR) pathogens, i.e., the Gram-negative bacteria Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii. Bromoageliferin (2) displayed significant activity against P. aeruginosa. Comparison of the antibacterial activity of ageliferins 1–3 (of similar structure) against P. aeruginosa revealed some relationship between structure and activity. Furthermore, in in vitro assays, 2 inhibited growth and biofilm production in clinical strains of P. aeruginosa. Moreover, 2 increased the survival time in an in vivo Galleria mellonella model of infection. The findings confirm bromoageliferin (2) as a potential lead for designing new antibacterial drugs.

Antibiotic Resistance Profiles, Molecular Mechanisms and Innovative Treatment Strategies of Acinetobacter baumannii

Antibiotic resistance is one of the biggest challenges for the clinical sector and industry, environment and societal development. One of the most important pathogens responsible for severe nosocomial infections is Acinetobacter baumannii, a Gram-negative bacterium from the Moraxellaceae family, due to its various resistance mechanisms, such as the β-lactamases production, efflux pumps, decreased membrane permeability and altered target site of the antibiotic. The enormous adaptive capacity of A. baumannii and the acquisition and transfer of antibiotic resistance determinants contribute to the ineffectiveness of most current therapeutic strategies, including last-line or combined antibiotic therapy. In this review, we will present an update of the antibiotic resistance profiles and underlying mechanisms in A. baumannii and the current progress in developing innovative strategies for combating multidrug-resistant A. baumannii (MDRAB) infections.

A surface loop modulates activity of the Bacillus class D β-lactamases

The expression of β-lactamases is a major mechanism of bacterial resistance to the β-lactam antibiotics. Four molecular classes of β-lactamases have been described (A, B, C and D), however until recently the class D enzymes were thought to exist only in Gram-negative bacteria. In the last few years, class D enzymes have been discovered in several species of Gram-positive microorganisms, such as Bacillus and Clostridia, and an investigation of their kinetic and structural properties has begun in earnest. Interestingly, it was observed that some species of Bacillus produce two distinct class D β-lactamases, one highly active and the other with only basal catalytic activity. Analysis of amino acid sequences of active (BPU-1 from Bacillus pumilus) and inactive (BSU-2 from Bacillus subtilis and BAT-2 from Bacillus atrophaeus) enzymes suggests that presence of three additional amino acid residues in one of the surface loops of inefficient β-lactamases may be responsible for their severely diminished activity. Our structural and docking studies show that the elongated loop of these enzymes severely restricts binding of substrates. Deletion of the three residues from the loops of BSU-2 and BAT-2 β-lactamases relieves the steric hindrance and results in a significant increase in the catalytic activity of the enzymes. These data show that this surface loop plays an important role in modulation of the catalytic activity of Bacillus class D β-lactamases.

Carbapenemases: Transforming Acinetobacter baumannii into a Yet More Dangerous Menace

Acinetobacter baumannii is a common cause of serious nosocomial infections. Although community-acquired infections are observed, the vast majority occur in people with preexisting comorbidities. A. baumannii emerged as a problematic pathogen in the 1980s when an increase in virulence, difficulty in treatment due to drug resistance, and opportunities for infection turned it into one of the most important threats to human health. Some of the clinical manifestations of A. baumannii nosocomial infection are pneumonia; bloodstream infections; lower respiratory tract, urinary tract, and wound infections; burn infections; skin and soft tissue infections (including necrotizing fasciitis); meningitis; osteomyelitis; and endocarditis. A. baumannii has an extraordinary genetic plasticity that results in a high capacity to acquire antimicrobial resistance traits. In particular, acquisition of resistance to carbapenems, which are among the antimicrobials of last resort for treatment of multidrug infections, is increasing among A. baumannii strains compounding the problem of nosocomial infections caused by this pathogen. It is not uncommon to find multidrug-resistant (MDR, resistance to at least three classes of antimicrobials), extensively drug-resistant (XDR, MDR plus resistance to carbapenems), and pan-drug-resistant (PDR, XDR plus resistance to polymyxins) nosocomial isolates that are hard to treat with the currently available drugs. In this article we review the acquired resistance to carbapenems by A. baumannii. We describe the enzymes within the OXA, NDM, VIM, IMP, and KPC groups of carbapenemases and the coding genes found in A. baumannii clinical isolates.

Acinetobacter baumannii Resistance: A Real Challenge for Clinicians

Acinetobacter baumannii (named in honor of the American bacteriologists Paul and Linda Baumann) is a Gram-negative, multidrug-resistant (MDR) pathogen that causes nosocomial infections, especially in intensive care units (ICUs) and immunocompromised patients with central venous catheters. A. baumannii has developed a broad spectrum of antimicrobial resistance, associated with a higher mortality rate among infected patients compared with other non-baumannii species. In terms of clinical impact, resistant strains are associated with increases in both in-hospital length of stay and mortality. A. baumannii can cause a variety of infections; most involve the respiratory tract, especially ventilator-associated pneumonia, but bacteremia and skin wound infections have also been reported, the latter of which has been prominently observed in the context of war-related trauma. Cases of meningitis associated with A. baumannii have been documented. The most common risk factor for the acquisition of MDR A baumannii is previous antibiotic use, following by mechanical ventilation, length of ICU/hospital stay, severity of illness, and use of medical devices. Current efforts focus on addressing all the antimicrobial resistance mechanisms described in A. baumannii, with the objective of identifying the most promising therapeutic scheme. Bacteriophage- and artilysin-based therapeutic approaches have been described as effective, but further research into their clinical use is required.

The Current Burden of Carbapenemases: Review of Significant Properties and Dissemination among Gram-Negative Bacteria

Carbapenemases are β-lactamases belonging to different Ambler classes (A, B, D) and can be encoded by both chromosomal and plasmid-mediated genes. These enzymes represent the most potent β-lactamases, which hydrolyze a broad variety of β-lactams, including carbapenems, cephalosporins, penicillin, and aztreonam. The major issues associated with carbapenemase production are clinical due to compromising the activity of the last resort antibiotics used for treating serious infections, and epidemiological due to their dissemination into various bacteria across almost all geographic regions. Carbapenemase-producing Enterobacteriaceae have received more attention upon their first report in the early 1990s. Currently, there is increased awareness of the impact of nonfermenting bacteria, such as Acinetobacter baumannii and Pseudomonas aeruginosa, as well as other Gram-negative bacteria that are carbapenemase-producers. Outside the scope of clinical importance, carbapenemases are also detected in bacteria from environmental and zoonotic niches, which raises greater concerns over their prevalence, and the need for public health measures to control consequences of their propagation. The aims of the current review are to define and categorize the different families of carbapenemases, and to overview the main lines of their spread across different bacterial groups.

6-Arylmethylidene Penicillin-Based Sulfone Inhibitors for Repurposing Antibiotic Efficiency in Priority Pathogens

The ability of 6-(aryl)methylidene penicillin-based sulfones 1-7 to repurpose β-lactam antibiotics activity with bacterial species that carry carbapenem-hydrolyzing class D β-lactamases (OXA-23, OXA-24/40 and OXA-48), as well as with class A (TEM-1, CTX-M-2) and class C (CMY-2, DHA-1) enzymes, is reported. The combinations imipenem/3 and imipenem/4 restored almost completely the antibiotic efficacy in OXA-23 and OXA-24/40 carbapenemase-producing A. baumannii strains (1 μg mL^-1) and also provided good results for OXA-48 carbapenemase-producing K. pneumoniae strains (4 μg mL^-1). Compounds 2-6 in combinations with ceftazidime and ampicillin were also efficient in restoring antibiotic efficacy in E. coli strains carrying class C (CMY-2 and DHA-1) and class A (TEM-1 and CTX-M-2) β-lactamase enzymes, respectively. Kinetic and inhibition studies with the OXA-24/40 enzyme, protein mass spectrometry analysis and docking studies allowed us to gain an insight into the inhibition mechanism and the experimentally observed differences between the ligands.

Interactions between Avibactam and Ceftazidime-Hydrolyzing Class D β-Lactamases

Class D β-lactamases exhibit very heterogeneous hydrolysis activity spectra against the various types of clinically useful β-lactams. Similarly, and according to the available data, their sensitivities to inactivation by avibactam can vary by a factor of more than 100. In this paper, we performed a detailed kinetic study of the interactions between two ceftazidime-hydrolyzing OXA enzymes and showed that they were significantly more susceptible to avibactam than several other class D enzymes that do not hydrolyze ceftazidime. From a clinical point of view, this result is rather interesting if one considers that avibactam is often administered in combination with ceftazidime.

Structural Basis for Substrate Specificity and Carbapenemase Activity of OXA-48 Class D β-Lactamase

Carbapenem-hydrolyzing class D β-lactamases (CHDLs) are a diverse family of enzymes that are rapidly becoming the predominant cause of bacterial resistance against β-lactam antibiotics in many regions of the world. OXA-48, an atypical member of CHDLs, is one of the most frequently observed in the clinic and exhibits a unique substrate profile. We applied X-ray crystallography to OXA-48 complexes with multiple β-lactam antibiotics to elucidate this enzyme's carbapenemase activity and its preference of imipenem over meropenem and other substrates such as cefotaxime. In particular, we obtained acyl-enzyme complexes of OXA-48 with imipenem, meropenem, faropenem, cefotaxime, and cefoxitin, and a product complex with imipenem. Importantly, the product complex captures a key reaction milestone with the newly generated carboxylate group still in the oxyanion hole, and represents the first such complex with a wild-type serine β-lactamase. A potential hydrogen bond is observed between the two carboxylate groups from the product and the carbamylated Lys73, representing the stage immediately after the breakage of the acyl-enzyme bond where the product carboxylate would be neutral. The placement of the product carboxylate also illustrates the approximate transient location of the deacylation water that has long eluded structural characterization in class D β-lactamases. Additionally, comparing the product complex with the acyl-enzyme intermediates provides new insights into the various mechanisms by which specific side chain groups hinder the access of the deacylation water to the acyl-enzyme linkage, especially in meropenem. Taken together, these data offer valuable information on the substrate specificity of OXA-48 and the catalytic mechanism of CHDLs.

Assessment of Molecular Mechanism of Gallate-Polyvinylpyrrolidone-Capped Hybrid Silver Nanoparticles against Carbapenem-Resistant Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic nosocomial pathogen and causes bacteremia, urinary tract infections, meningitis, and pneumonia. The emergence of drug-resistant strain makes most of the current antibiotics ineffective. It is high time to screen some therapeutics against drug-resistant strains. Plant-based medicines have recently emerged as one of the important therapeutic choices. Therefore, in the present study, we have screened the metabolites of Phyllanthus emblica, Ocimum tenuiflorum, and Murraya koenigii for their antibacterial effect against carabapenem-resistant strain (RS-307) of A. baumannii. The result showed that the methanolic extract of P. emblica inhibits the growth of RS-307. The composition of this extract was determined using phytochemical screening and nuclear magnetic resonance (1D and 2D-NMR). The mechanism of action of the plant extract was validated by estimating reactive oxygen species (ROS), lipid peroxidation, protein carbonylation, and membrane damage. The result showed that treatment with this extract showed a significant elevation in the production of ROS generations, lipid peroxidation, and protein carbonylation. This confirms that plant extract treatment confirmed ROS-dependent membrane damage mechanism. The NMR result showed the presence of ethyl gallate, ellagic acid, chebulagic acid, quercetin, flavonoid, and alkaloid. To validate the antimicrobial activity of the secondary metabolite (i.e., gallic acid), we synthesized gallate-polyvinylpyrrolidone-capped hybrid silver nanoparticles (G-PVP-AgNPs) and characterized using Fourier transform infrared spectroscopy (FTIR). G-PVP-AgNPs showed good antimicrobial activity against RS-307, and its mechanism of action was investigated using fluorescence and transmission electron microscopy and FTIR that confirmed ROS-dependent killing mechanism. Therefore, the present study highlighted and recommended the use of G-PVP-AgNPs as suitable therapeutics against carbapenem-resistant A. baumannii.

Prevalence of carbapenem-hydrolyzing OXA-type β-lactamases among Acinetobacter baumannii in patients with severe urinary tract infection

Acinetobacter baumannii produces carbapenemase-hydrolyzing class D β-lactamases (CHDLs) as one of the major drug resistance mechanisms. This investigation is thus aimed to assess the prevalence and to characterize the CHDL-producing strains of A. baumannii by both phenotypic assays and genotypic characterization. A total of 73 isolates of A. baumannii were phenotypically and genotypically characterized from patients (N = 1,000) with severe urinary tract infection. Tested strains were subjected to double disk synergy testing by Kirby-Bauer disk diffusion method with modified Hodge test (MHT) for carbapenemase production. Plasmid DNA was molecularly screened for CHDL-encoding blaoxa-51, blaoxa-23, and blaoxa-143 genes by polymerase chain reaction. Carbapenem-resistant profile showed 100%, 61.64%, and 67.12% resistance by Kirby-Bauer disk diffusion method that correlated with MHT positivity for 100% (n = 73), 80% (n = 36), and 78% (n = 38) of the isolates against imipenem, doripenem, and meropenem, respectively. The blaoxa-51 and blaoxa-23 were observed in 41.09% (n = 30) and 35.61% (n = 26) with co-occurrence in 4.10% (n = 3) of the isolates. MHT-positive isolates showed 100%, 91.66%, and 71.4% for blaoxa-51 and 91.78%, 51.11%, and 34.69% for blaoxa-23 with imipenem, doripenem, and meropenem resistance, respectively. None of the strains yielded blaoxa-143 gene. The findings of this study showed prevalence of carbapenem resistance and high frequency of blaoxa-51 and blaoxa-23 among A. baumannii.

Comparative genomic analysis of four multidrug-resistant isolates of Acinetobacter baumannii from Georgia

OBJECTIVES

This study reports the draft genomes of four newly isolated multidrug-resistant (MDR) Acinetobacter baumannii (A. baumannii) isolates (0830, 0365, 4022, and 2846) from western Georgia to identify putative antimicrobial resistance genes (ARGs) and to determine the clonal subtypes of local clinical isolates.

METHODS

An Illumina MiSeq sequencer was used to perform whole-genome sequencing (WGS). The Vitek 2 automated system was used for microbial identification and antimicrobial resistance profiling.

RESULTS

Taxonomical identification as A. baumannii was confirmed by WGS. In silico analyses resolved their ARG content and clonal relatedness using the Oxford (Oxf) and Pasteur (Pas) multi-locus sequence typing schemes. Isolates 0365 and 4022 displayed similar allelic profiles corresponding to ST944Oxf/ST78Pas. Isolate 2846 displayed a different allelic profile consistent with ST19Pas/IC 1 (International or European Clone I) and exhibited a novel Oxford ST that was designated as 1868. Isolate 0830 displayed the ST78Pas allelic profile, similar to isolates 0365 and 4022, and also possessed a single allelic mismatch in the gpi gene, resulting in an ST1104Oxf allele profile in the Oxford typing scheme.

CONCLUSION

Circulating MDR A. baumannii exhibited genetic heterogeneity with variations in the structure and content of genomic A. baumannii resistance islands and encoded multiple putative ARGs. This report represents the first clonal subtype information and genomic characterization of MDR A. baumannii in Georgia and may inform future epidemiological investigations.

Emergence of Carbapenem-Hydrolyzing Oxacillinases in Acinetobacter baumannii in Children from Croatia

INTRODUCTION

Carbapenem resistance in Acinetobacter baumannii can be mediated by carbapenemases of class A, class B metallo-β-lactamases (MBLs), and class D carbapenem-hydrolyzing oxacillinases (CHDL). The aim of the study was to investigate the antimicrobial susceptibility and β-lactamase production of carbapenem-resistant A. baumannii isolates (CRAB) from the Children's Hospital Zagreb, Croatia.

METHODS

A total of 12 A. baumannii isolates collected between August 2016 and March 2018 were analyzed. Antibiotic susceptibility was determined by the broth microdilution method. The presence of MBLs was explored by combined disk test with EDTA. The presence of carbapenemases of class A, B, and D was explored by PCR. The occurrence of the ISAba1 upstream of the blaOXA-51-like or blaOXA-23-like was determined by PCR mapping. Epidemiological typing was performed by determination of sequence groups (SG). Genotyping was performed by SG determination, rep-PCR, and MLST.

RESULTS

All CRAB were resistant to piperacillin/tazobactam, ceftazidime, cefotaxime, ceftriaxone, cefepime, imipenem, meropenem, gentamicin, and ciprofloxacin. Moderate resistance rates were observed for ampicillin/sulbactam (67%) and tigecycline (42%). The isolates were uniformly susceptible to colistin. PCR revealed the presence of genes encoding OXA-24-like CHDL in nine and OXA-23-like CHDL in three isolates. blaOXA-51 genes were preceded by ISAba1. PCR for the common MBLs in Acinetobacter was negative. All isolates belonged to SG 1 corresponding to ICL (International Clonal Lineage) II. Rep-PCR identified four major clones.

CONCLUSIONS

The study found OXA-24-like β-lactamase to be the dominant CHDL among children'sCRAB. The predominant spread of OXA-24-like is in contrast with the recent global dissemination of OXA-23 reported all over the world. In contrast to the previous studies in which emergency of OXA-24-like positive isolates was monoclonal, we found considerable genetic diversity of the isolates.

Structural Insights into the Mechanism of Carbapenemase Activity of the OXA-48 β-Lactamase

Carbapenem-hydrolyzing class D carbapenemases (CHDLs) are enzymes that produce resistance to the last-resort carbapenem antibiotics, severely compromising the available therapeutic options for the treatment of life-threatening infections. A broad variety of CHDLs, including OXA-23, OXA-24/40, and OXA-58, circulate in Acinetobacter baumannii, while the OXA-48 CHDL is predominant in Enterobacteriaceae Extensive structural studies of A. baumannii enzymes have provided important information regarding their interactions with carbapenems and significantly contributed to the understanding of the mechanism of their carbapenemase activity. However, the interactions between carbapenems and OXA-48 have not yet been elucidated. We determined the X-ray crystal structures of the acyl-enzyme complexes of OXA-48 with four carbapenems, imipenem, meropenem, ertapenem, and doripenem, and compared them with those of known carbapenem complexes of A. baumannii CHDLs. In the A. baumannii enzymes, acylation by carbapenems triggers significant displacement of one of two conserved hydrophobic surface residues, resulting in the formation of a channel for entry of the deacylating water into the active site. We show that such a channel preexists in apo-OXA-48 and that only minor displacement of the conserved hydrophobic surface residues occurs upon the formation of OXA-48 acyl-enzyme intermediates. We also demonstrate that the extensive hydrophobic interactions that occur between a conserved hydrophobic bridge of the A. baumannii CHDLs and the carbapenem tails are lost in OXA-48 in the absence of an equivalent bridge structure. These data highlight significant differences between the interactions of carbapenems with OXA-48 and those with A. baumannii enzymes and provide important insights into the mechanism of carbapenemase activity of the major Enterobacteriaceae CHDL, OXA-48.

Molecular epidemiology of carbapenem-resistant Acinetobacter baumannii isolates reveals the emergence of blaOXA-23 and blaNDM-1 encoding international clones in India

Acinetobacter baumannii is a nosocomial pathogen increasingly affecting the critically ill patients and represents a major public health challenge. Carbapenem-resistant A. baumannii (CRAB) is found to be associated with International Clones (ICs) and different classes of carbapenemases. The objective of the present study was to investigate the prevalence of carbapenem resistance genes, clonal relationship and genetic structure of clinical isolates of A. baumannii. In the present study, multi-locus sequence typing (MLST^OX) and analysis were carried out using Oxford scheme for 86 clinical isolates of CRAB along with 11 carbapenem sensitive A. baumannii (CSAB) collected over a period of two years (2014-2016) from two tertiary care hospitals of North India. We observed a high prevalence of the bla_OXA-23-like (97.7%) among the CRAB followed by bla_NDM-1 (29.1%) and bla_OXA58-like (3.5%). Forty-seven Sequence Types (STs) were represented by all 97 isolates, out of which, 28 (59.6%) were novel STs that were assigned to 41 isolates. STs 451 (13%), 447 (7%), 195 (6%) and 848 (5%) were the most common STs. The majority of CRAB isolates (44.3%) belonged to the CC92, followed by the CC447 (15.1%), CC109 (9.3%) and CC110 (3.4%), which corresponds to the IC2, 8, 1 and 7 respectively. Phylogenetic and recombination analysis suggested two major and one minor lineage in the population. Further linkage disequilibrium analysis suggested clonal nature of the population as recombination was noticed at a low frequency, which was not enough to split the clonal relationship. The knowledge of genetic structure of CRAB from this study will be invaluable to illustrate epidemiology, surveillance and understanding its global diversity.

Specific and Selective Bacteriophages in the Fight against Multidrug-resistant Acinetobacter baumannii

Acinetobacter baumannii causes serious infections especially in immunocompromised and/or hospitalized patients. Several A. baumannii strains are multidrug resistant and infect wounds, bones, and the respiratory tract. Current studies are focused on finding new effective agents against A. baumannii. Phage therapy is a promising means to fight this bacterium and many studies on procuring and applying new phages against A. baumannii are currently being conducted. As shown in animal models, phages against multidrug-resistant A. baumannii may control bacterial infections caused by this pathogen and may be a real hope to solve this dangerous health problem.

COMPARISON OF TWO DIFFERENT METHODS FOR TIGECYCLINE SUSCEPTIBILITY TESTING IN ACINETOBACTER BAUMANNII

SUMMARY – Tigecycline susceptibility testing (TST) presents a tremendous challenge for clinical microbiologists. Previous studies have shown that the Epsilometer test (E-test) and Vitek 2 automated system significantly overestimate the minimum inhibitory concentrations for tigecycline resistance compared to the broth microdilution method (BMM). This leads to very major errors or false susceptibility (i.e. the isolate is called susceptible when it is actually resistant). The aim of this study was to compare E-test against BMM for TST in carbapenem-resistant and carbapenem-susceptible Acinetobacter (A.) baumannii and to analyze changes in tigecycline susceptibility between two time periods (2009-2012 and 2013-2014), with BMM as the gold standard. Using the EUCAST criteria, the rate of resistance to tigecycline for the OXA-23 MBL-positive, OXA-23 MBL-negative and carbapenemase-negative strains for BMM was 54.5% (6/11), 29.4% (5/17) and 2.7% (1/37), respectively; the OXA-24/40 and OXA-58 producing organisms did not exhibit any resistance. With E-test, all OXA-23 MBL-positive organisms (11/11), 23.5% (4/17) of OXA-23 MBL-negative, and 4.1% of OXA-24/40 (3/74) strains displayed tigecycline resistance; there were no resistant strains among the OXA-58 and carbapenemase-negative isolates. Resistance emerged in the bacterial isolates from 2013 to 2014. Although tigecycline does not display cross-resistance, the highest rates of resistant A. baumannii isolates were observed among those producing VIM MBL, regardless of the testing method. These findings suggest that the commercial E-test does not provide reliable results for TST of A. baumannii. Further confirmation with the dilution method should be recommended, particularly in cases of serious infections.

Molecular mechanisms of antimicrobial resistance in Acinetobacter baumannii, with a special focus on its epidemiology in Lebanon

Acinetobacter baumannii is an opportunistic bacterium involved in several types of infection with high mortality and morbidity, especially in intensive care units. Treatment of these infections remains a challenge due to the worldwide emergence of broad-spectrum resistance to many antibiotics. Following the implementation of molecular techniques to study A. baumannii outbreaks, it has been shown that they are mainly caused by specific clones such as international clones I, II and III. The present work aims to review the available data on the mechanisms underlying antimicrobial resistance in A. baumannii, with a special focus on the molecular epidemiology of this species in Lebanon.

High prevalence and clonal dissemination of OXA-72-producing Acinetobacter baumannii in a Chinese hospital: a cross sectional study

BACKGROUND

Carbapenem resistance in Acinetobacter baumannii in China was mainly mediated by OXA-23-like carbapenemases, while OXA-24/40-like carbapenemases were rarely identified. OXA-72 is one variant of OXA-24/40-like carbapenemases. This study aimed to demonstrate the epidemiology and characterizations of OXA-72-producing A. baumannii in a Chinese hospital.

METHODS

A total of 107 clinical A. calcoaceticus-A. baumannii (Acb) complex isolates were collected in a Chinese hospital during between 2014 and 2016. These isolates were identified using Vitek 2 system and gyrB multiplex PCR. Vitek 2 system was used for antibiotic susceptibility testing. Genes encoding for major classes of carbapenemases were investigated by PCR. Rep-PCR was used for genotyping of all the A. baumannii isolates. The risk factors for carriage of OXA-72-producing or OXA-23-producing A. baumannii were analyzed through univariate and multivariate logistic regression.

RESULTS

Of the 107 Acb isolates collected, 101 isolates (94.4%) and 6 isolates (5.6%) were identified as A. baumannii and A. pittii, respectively. 78 A. baumannii isolates (77.2%) were carbapenem resistant and mainly cultured from intensive care unit (ICU). blaOXA-72 and blaOXA-23 genes were identified in 45(57.7%) and 33(42.3%) carbapenem-resistant A. baumannii (CRAB), respectively. Multivariate risk factor analyses showed that prior carbapenem usage and nasogastric intubation were significantly associated with carriage of OXA-72-producing A. baumannii or OXA-23-producing A. baumannii. Rep-PCR analysis showed that 9 and 22 Rep-PCR types were assigned to 78 CRAB isolates and 23 carbapenem-susceptible A. baumannii (CSAB) isolates, respectively. A higher diverstiy of Rep-PCR patterns was observed among OXA-72-producing A. baumannii isolates than OXA-23-producing A. baumannii isolates, but all of them belonged to the same clone complex. MLST analysis suggested that the OXA-72 isolates from this study correspond to CC92/CC2 clone complex.

CONCLUSIONS

This study demonstrates high prevalence and potential clonal spread of closely related genotypes of OXA-72-producing A. baumannii within a Chinese hospital. Continuous surveillance is necessary to monitor the dissemination of these strains in other healthcare settings to guide infection control policies in order to curb the spread of this bacterium.

Strategic Approaches to Overcome Resistance against Gram-Negative Pathogens Using β-Lactamase Inhibitors and β-Lactam Enhancers: Activity of Three Novel Diazabicyclooctanes WCK 5153, Zidebactam (WCK 5107), and WCK 4234

Limited treatment options exist to combat infections caused by multidrug-resistant (MDR) Gram-negative bacteria possessing broad-spectrum β-lactamases. The design of novel β-lactamase inhibitors is of paramount importance. Here, three novel diazabicyclooctanes (DBOs), WCK 5153, zidebactam (WCK 5107), and WCK 4234 (compounds 1-3, respectively), were synthesized and biochemically characterized against clinically important bacteria. Compound 3 inhibited class A, C, and D β-lactamases with unprecedented k₂/ K values against OXA carbapenemases. Compounds 1 and 2 acylated class A and C β-lactamses rapidly but not the tested OXAs. Compounds 1-3 formed highly stable acyl-complexes as demonstrated by mass spectrometry. Crystallography revealed that 1-3 complexed with KPC-2 adopted a "chair conformation" with the sulfate occupying the carboxylate binding region. The cefepime-2 and meropenem-3 combinations were effective in murine peritonitis and neutropenic lung infection models caused by MDR Acinetobacter baumannii. Compounds 1-3 are novel β-lactamase inhibitors that demonstate potent cross-class inhibition, and clinical studies targeting MDR infections are warranted.

Activity of the β-Lactamase Inhibitor LN-1-255 against Carbapenem-Hydrolyzing Class D β-Lactamases from Acinetobacter baumannii

The number of infections caused by Gram-negative pathogens carrying carbapenemases is increasing, and the group of carbapenem-hydrolyzing class D β-lactamases (CHDLs) is especially problematic. Several clinically important CHDLs have been identified in Acinetobacter baumannii, including OXA-23, OXA-24/40, OXA-58, OXA-143, OXA-235, and the chromosomally encoded OXA-51. The selection and dissemination of carbapenem-resistant A. baumannii strains constitutes a serious global threat. Carbapenems have been successfully utilized as last-resort antibiotics for the treatment of multidrug-resistant A. baumannii infections. However, the spread of OXA carbapenemases is compromising the continued use of these antimicrobials. In response to this clinical issue, it is necessary and urgent to design and develop new specific inhibitors with efficacy against these enzymes. The aim of this work was to characterize the inhibitory activity of LN-1-255 (a 6-alkylidene-2-substituted penicillin sulfone) and compare it to that of two established inhibitors (avibactam and tazobactam) against the most relevant enzymes of each group of class D carbapenemases in A. baumannii The β-lactamase inhibitor LN-1-255 demonstrated excellent microbiological synergy and inhibition kinetics parameters against all tested CHDLs and a significantly higher activity than tazobactam and avibactam. A combination of carbapenems and LN-1-255 was effective against A. baumannii class D carbapenemases. Docking assays confirmed the affinity of LN-1-255 for the active site of these enzymes. LN-1-255 represents a potential new β-lactamase inhibitor that may have a significant role in eradicating infections caused by A. baumannii isolates carrying CHDLs.

Emergence of different Acinetobacter baumannii clones in a Croatian hospital and correlation with antibiotic susceptibility

OBJECTIVES

During routine diagnostic laboratory work, the clinical microbiologist observed an increase of Acinetobacter baumannii isolates with three different carbapenem susceptibility patterns: susceptible, intermediate and resistant. Isolates belonging to the same carbapenem susceptibility phenotype exhibited identical susceptibility/resistance patterns to non-β-lactam antibiotics. This prompted us to analyse the mechanisms of carbapenem-resistance and the molecular epidemiology of the isolates. A total of 59 A. baumannii isolates were analysed and grouped according to their susceptibility to imipenem: group 1 were susceptible (N=24), group 2 were intermediate (N=8) and group 3 were resistant (N=27) to imipenem.

MATERIAL AND METHODS

PCR and sequencing was used to detect resistance genes. Genotyping of the isolates was performed by PFGE and MLST.

RESULTS

Out of 27 resistant isolates, 20 harboured bla_OXA-40-like and 7 bla_OXA-23-like genes. ISAba1 was found upstream of bla_OXA-51 and bla_OXA-23 genes. PFGE genotyping demonstrated the existence of three major A. baumannii clones in GH Pula and determination of sequence groups showed that the isolates belonged to international clones commonly associated with multidrug-resistance. MLST (performed on six isolates) showed diverse population structure of isolates belonging to the same cluster, including ST 195, ST 231, ST 775 and ST 1095.

CONCLUSIONS

A previous study conducted in 2009-2010 showed that reduced susceptibility to carbapenems in GH Pula was only associated with upregulation of the intrinsic OXA-51 β-lactamase. In this study a shift to isolates with acquired oxacillinases, belonging to two major clones was reported.

First Report of the Carbapenemase Gene blaOXA-499 in Acinetobacter pittii

We identified the carbapenemase gene bla_OXA-499, a variant of bla_OXA-143, from a clinical isolate of Acinetobacter pittii for the first time. OXA-499 shared 93.1% amino acid identity with OXA-143, and the gene was located on the chromosome. By cloning the OXA-499-encoding gene into the pWH1266 vector and transforming it into susceptible Acinetobacter spp., we were able to show that OXA-499 confers resistance to carbapenems.

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.

Spread of the blaOXA-23-Containing Tn2008 in Carbapenem-Resistant Acinetobacter baumannii Isolates Grouped in CC92 from China

The rapid expansion of carbapenem-resistant Acinetobacter baumannii (CRAB) clinical isolates is a big issue. We investigated the antibiotic susceptibility, molecular epidemiology and resistance gene of A. baumannii collected at two hospitals in Shanghai, China. Besides, the A. baumannii PCR-based replicon typing method (AB-PBRT) was conducted to categorize the plasmids into homogeneous groups on the basis of replicase genes. Most CRAB isolates showed high-level resistance to almost all antibiotics but retain susceptibility to colistin and tigecycline. A total of 101 isolates carried bla_OXA-51-like gene. Sequencing identified the presence of bla_OXA-66 for CRAB isolates. bla_OXA–23 gene were discovered in all CRAB isolates. Each CRAB isolate contained 1–3 of 19 different plasmid replicase (rep) gene homology groups (GRs) and the GR6 (repAci6) was ubiquitous. Genotyping by Multilocus Sequence Typing (MLST) showed seven defined MLST patterns and three novel STs were found. eBURST analysis indicated they were all grouped in CC92 (GCII) with the most frequent ST208 (50%). Two bla_OXA–23-bearing transposons were found: Tn2006 and Tn2008. Tn2008 were detected in 54 (96.4%) isolates and Tn2006 in two remaining isolates. The bla_OXA–23 carbapenem gene was vitally associated with repAci6 plasmid belong to CC92 clonal group. Our survey revealed severe drug resistance in A. baumannii isolates. Tn2008-containing CC92 A. baumannii were endemic, which may facilitate the bla_oxa23 dissemination.

The epidemiology of carbapenemases in Latin America and the Caribbean

INTRODUCTION

Enterobacteriaceae, Pseudomonas spp., and Acinetobacter spp. infections are major causes of morbidity and mortality, especially due to the emergence and spread of β-lactamases. Carbapenemases, which are β-lactamases with the capacity to hydrolyze or inactivate carbapenems, have become a serious concern as they have the largest hydrolytic spectrum and therefore limit the utility of most β-lactam antibiotics. Areas covered: Here, we present an update of the current status of carbapenemases in Latin America and the Caribbean. Expert commentary: The increased frequency of reports on carbapenemases in Latin America and the Caribbean shows that they have successfully spread and have even become endemic in some countries. Countries such as Brazil, Colombia, Argentina, and Mexico account for the majority of these reports. Early suspicion and detection along with implementation of antimicrobial stewardship programs in all healthcare settings are crucial for the control and prevention of carbapenemase-producing bacteria.

Insights on the Horizontal Gene Transfer of Carbapenemase Determinants in the Opportunistic Pathogen Acinetobacter baumannii

Horizontal gene transfer (HGT) is a driving force to the evolution of bacteria. The fast emergence of antimicrobial resistance reflects the ability of genetic adaptation of pathogens. Acinetobacter baumannii has emerged in the last few decades as an important opportunistic nosocomial pathogen, in part due to its high capacity of acquiring resistance to diverse antibiotic families, including to the so-called last line drugs such as carbapenems. The rampant selective pressure and genetic exchange of resistance genes hinder the effective treatment of resistant infections. A. baumannii uses all the resistance mechanisms to survive against carbapenems but production of carbapenemases are the major mechanism, which may act in synergy with others. A. baumannii appears to use all the mechanisms of gene dissemination. Beyond conjugation, the mostly reported recent studies point to natural transformation, transduction and outer membrane vesicles-mediated transfer as mechanisms that may play a role in carbapenemase determinants spread. Understanding the genetic mobilization of carbapenemase genes is paramount in preventing their dissemination. Here we review the carbapenemases found in A. baumannii and present an overview of the current knowledge of contributions of the various HGT mechanisms to the molecular epidemiology of carbapenem resistance in this relevant opportunistic pathogen.

Synergy between Colistin and the Signal Peptidase Inhibitor MD3 Is Dependent on the Mechanism of Colistin Resistance in Acinetobacter baumannii

Synergy between colistin and the signal peptidase inhibitor MD3 was tested against isogenic mutants and clinical pairs of Acinetobacter baumannii isolates. Checkerboard assays and growth curves showed synergy against both colistin-susceptible strains (fractional inhibitory concentration index [FICindex] = 0.13 to 0.24) and colistin-resistant strains with mutations in pmrB and phosphoethanolamine modification of lipid A (FICindex = 0.14 to 0.25) but not against colistin-resistant Δlpx strains with loss of lipopolysaccharide (FICindex = 0.75 to 1). A colistin/MD3 combination would need to be targeted to strains with specific colistin resistance mechanisms.

OXA β-lactamases

The OXA β-lactamases were among the earliest β-lactamases detected; however, these molecular class D β-lactamases were originally relatively rare and always plasmid mediated. They had a substrate profile limited to the penicillins, but some became able to confer resistance to cephalosporins. From the 1980s onwards, isolates of Acinetobacter baumannii that were resistant to the carbapenems emerged, manifested by plasmid-encoded β-lactamases (OXA-23, OXA-40, and OXA-58) categorized as OXA enzymes because of their sequence similarity to earlier OXA β-lactamases. It was soon found that every A. baumannii strain possessed a chromosomally encoded OXA β-lactamase (OXA-51-like), some of which could confer resistance to carbapenems when the genetic environment around the gene promoted its expression. Similarly, Acinetobacter species closely related to A. baumannii also possessed their own chromosomally encoded OXA β-lactamases; some could be transferred to A. baumannii, and they formed the basis of transferable carbapenem resistance in this species. In some cases, the carbapenem-resistant OXA β-lactamases (OXA-48) have migrated into the Enterobacteriaceae and are becoming a significant cause of carbapenem resistance. The emergence of OXA enzymes that can confer resistance to carbapenems, particularly in A. baumannii, has transformed these β-lactamases from a minor hindrance into a major problem set to demote the clinical efficacy of the carbapenems.