Detection of intrinsic blaOXA-51-like by multiplex PCR on its own is not reliable for the identification of Acinetobacter baumannii

Three clinical A. baumannii isolates Ab-508, Ab-511, and Ab-653 were recovered from South Africa, South Korea, and Turkey, respectively. Multiplex PCR to detect OXA-type carbapenemases showed atypical blaOXA-51-like amplification products. The aim of this study was to investigate the background of changes in blaOXA-51-like PCR products. Isolates were confirmed as A. baumannii using gyrB multiplex and rpoB sequencing and were epidemiologically unrelated by rep-PCR-based DiversiLab. Sequencing of blaOXA-51-like revealed an insertion of ISAba15 in blaOXA-66 (isolate Ab-511) and an insertion of the novel ISAba19 in blaOXA-78 (isolates Ab-508 and Ab-653). Detection of the intrinsic blaOXA-51-like by OXA-multiplex PCR should not be considered a fully reliable method for identification of A. baumannii when used without an additional independent method. Other species identification methods such as gyrB multiplex PCR and rpoB sequencing should be used to reliably identify A. baumannii.

Molecular epidemiology of Acinetobacter baumannii and Acinetobacter nosocomialis in Germany over a 5-year period (2005-2009)

To investigate the species distribution within the Acinetobacter calcoaceticus-Acinetobacter baumannii complex and the molecular epidemiology of A. baumannii and Acinetobacter nosocomialis, 376 Acinetobacter isolates were collected prospectively from hospitalized patients at 15 medical centres in Germany during three surveillance studies conducted over a 5-year period. Species identification was performed by molecular methods. Imipenem minimum inhibitory concentrations (MIC) were determined by broth microdilution. The prevalence of the most common carbapenemase-encoding genes was investigated by oxacillinase (OXA) -multiplex polymerase chain reaction (PCR). The molecular epidemiology was investigated by repetitive sequence-based PCR (rep-PCR; DiversiLab™). Acinetobacter pittii was the most prevalent Acinetobacter species (n = 193), followed by A. baumannii (n = 140), A. calcoaceticus (n = 10) and A. nosocomialis (n = 8). The majority of A. baumannii was represented by sporadic isolates (n = 70, 50%) that showed unique rep-PCR patterns, 25 isolates (18%) clustered with one or two other isolates, and only 45 isolates (32%) belonged to one of the previously described international clonal lineages. The most prevalent clonal lineage was international clone (IC) 2 (n = 34) and IC 1 (n = 6). According to CLSI, 25 A. baumannii isolates were non-susceptible to imipenem (MIC ≥ 8 mg/L), all of which produced an OXA-58-like or OXA-23-like carbapenemase. The rate of imipenem susceptibility among A. baumannii isolates decreased from 96% in 2005 to 76% in 2009. All other Acinetobacter isolates were susceptible to imipenem. The population structure of carbapenem-susceptible A. baumannii in Germany is highly diverse. Imipenem non-susceptibility was strongly associated with the clonal lineages IC 2 and IC 1. These data underscore the high clonality of carbapenem-resistant A. baumannii isolates.

Conversion of OXA-66 into OXA-82 in clinical Acinetobacter baumannii isolates and association with altered carbapenem susceptibility

OBJECTIVES

Three clinical Acinetobacter baumannii isolates (A-C) were isolated from three separate patients during an outbreak in a hospital in Krakow, Poland. Isolate A was recovered first and was susceptible to carbapenems, whereas isolates B and C were resistant. The aim of this study was to investigate the differences in carbapenem susceptibility in these outbreak-related isolates.

METHODS

Clonal relatedness was determined using rep-PCR-based DiversiLab. The bla(OXA-51-like) genes and their upstream regions were sequenced. Expression of the genes encoding OXA-51-like and the three major porins CarO, OprD-like and 33-36 kDa Omp were investigated by semi-quantitative RT-PCR. Comparison of outer membrane protein (OMP) profiles was performed using SDS-PAGE. ISAba1-bla(OXA-82) was cloned into the shuttle vector pWH1266 and transferred into A. baumannii ATCC 17978.

RESULTS

The isolates were identical by rep-PCR and clustered with international clonal lineage 2. Sequencing of bla(OXA-51-like) revealed a conversion of OXA-66 (isolate A) into OXA-82 (isolates B and C). bla(OXA-82) was also associated with ISAba1. Expression analysis revealed overexpression of bla(OXA-82). There was no difference in OMP expression between the isolates. ISAba1-bla(OXA-82) conferred carbapenem resistance in ATCC 17978.

CONCLUSIONS

Carbapenem resistance in outbreak-related isolates was mediated by conversion of OXA-66 into OXA-82 and its subsequent overexpression. This further highlights the genome plasticity of A. baumannii, leading to carbapenem resistance.

Surface-associated motility, a common trait of clinical isolates of Acinetobacter baumannii, depends on 1,3-diaminopropane

While flagella-independent motility has long been described in representatives of the genus Acinetobacter, the mechanism of motility remains ambiguous. Acinetobacter baumannii, a nosocomial pathogen appearing increasingly multidrug-resistant, may profit from motility during infection or while persisting in the hospital environment. However, data on the frequency of motility skills among clinical A. baumannii isolates is scarce. We have screened a collection of 83 clinical A. baumannii isolates of different origin and found that, with the exception of one isolate, all were motile on wet surfaces albeit to varying degrees and exhibiting differing morphologies. Screening a collection of transposon mutants of strain ATCC 17978 for motility defects, we identified 2 akinetic mutants carrying transposon insertions in the dat and ddc gene, respectively. These neighbouring genes contribute to synthesis of 1,3-diaminopropane (DAP), a polyamine ubiquitously produced in Acinetobacter. Supplementing semi-solid media with DAP cured the motility defect of both mutants. HPLC analyses confirmed that DAP synthesis was abolished in ddc and dat mutants of different A. baumannii isolates and was re-established after genetic complementation. Both, the dat and ddc mutant of ATCC 17978 were attenuated in the Galleria mellonella caterpillar infection model. Taken together, surface-associated motility is a common trait of clinical A. baumannii isolates that requires DAP and may play a role in its virulence.

In vitro activity of the siderophore monosulfactam BAL30072 against meropenem-non-susceptible Acinetobacter baumannii

OBJECTIVES

The activity of BAL30072 was compared with that of anti-Acinetobacter reference drugs against meropenem-non-susceptible Acinetobacter baumannii isolates associated with up-regulation of the intrinsic OXA-51-like enzyme or an acquired OXA.

METHODS

Antimicrobial susceptibility testing was investigated by broth microdilution of 310 non-duplicate, meropenem-non-susceptible A. baumannii isolates to BAL30072, amikacin ampicillin/sulbactam, aztreonam, cefepime, colistin, imipenem, levofloxacin, meropenem, rifampicin, tigecycline and tobramycin.

RESULTS

BAL30072 showed greater activity than the β-lactam comparators, levofloxacin, amikacin, tobramycin and rifampicin. The activity of BAL30072 was comparable to that of tigecycline, with an MIC(50) of 2 mg/L. Elevated BAL30072 MICs were found, but there was no correlation with elevated MICs of the other antimicrobials.

CONCLUSIONS

BAL30072 is a promising new agent with good activity against carbapenem-non-susceptible A. baumannii.

Interlaboratory reproducibility of DiversiLab rep-PCR typing and clustering of Acinetobacter baumannii isolates

We have investigated the reproducibility of DiversiLab rep-PCR fingerprints between two laboratories with the aim of determining if the fingerprints and clustering are laboratory-specific or portable. One-hundred non-duplicate Acinetobacter baumannii isolates were used in this study. DNA isolation and rep-PCR were each performed separately in two laboratories and rep-PCR patterns generated in laboratory A were compared with those from laboratory B. Twelve A. baumannii isolates processed in laboratory A showed ≥98 % pattern similarity with the corresponding 12 isolates tested in laboratory B and were considered identical. Sixty-four isolates showed 95-97.9 % similarity with their corresponding isolates. Twenty-three isolates showed 90-94 % similarity with the corresponding isolates, while one isolate showed only 87.4 % similarity. However, intra-laboratory clustering was conserved: isolates that clustered in laboratory A also clustered in laboratory B. While clustering was conserved and reproducible at two different laboratories, demonstrating the robustness of rep-PCR, interlaboratory comparison of individual isolate fingerprints showed more variability. This comparison allows conclusions regarding clonality to be reached independent of the laboratory where the analysis is performed.

Molecular characterization of blaNDM-1 in an Acinetobacter baumannii strain isolated in Germany in 2007

OBJECTIVES

To investigate the genetic environment of the metallo-β-lactamase gene bla(NDM-1) in an Acinetobacter baumannii isolated in 2007 in a German hospital.

METHODS

Antimicrobial susceptibility testing was performed and resistance genes were characterized by PCR amplification and sequencing. Transferability of β-lactam resistance was tested by broth mating assays and transformation of plasmids. The genetic background of bla(NDM-1) was analysed by primer walking. Typing of the A. baumannii strain was performed by repetitive extragenic palindromic sequence-based PCR (rep-PCR) using the DiversiLab system.

RESULTS

The multidrug-resistant A. baumannii isolate harboured β-lactamase genes bla(NDM-1) and intrinsic bla(OXA-64), but without the insertion sequence ISAba1 often located upstream. Transfer of carbapenem resistance by conjugation and transformation failed. Hybridization of isolated plasmid DNA with bla(NDM) probes was not successful. Shotgun cloning of whole genomic DNA and sequence analyses revealed that bla(NDM-1) was located between two insertion elements of ISAba125. Furthermore, this bla(NDM-1)-containing transposon structure was integrated into a chromosomal gene encoding a putative A. baumannii major facilitator superfamily (MFS) metabolite/H+ symporter.

CONCLUSIONS

The metallo-β-lactamase gene bla(NDM-1) in this A. baumannii strain was integrated in the chromosome on a new transposon structure composed of two copies of insertion sequence ISAba125. The variability of the genetic environment of bla(NDM-1) likely facilitates the rapid dissemination of this gene within many Gram-negative bacterial species.

Outbreak of carbapenem-resistant Acinetobacter baumannii carrying the carbapenemase OXA-23 in a German university medical centre

A prolonged outbreak of carbapenem-resistant Acinetobacter baumannii in a German university medical centre in 2006 was investigated; the investigation included a descriptive epidemiological analysis, a case-control study, environmental sampling, molecular typing of A. baumannii isolates using PFGE and repetitive-sequence-based PCR (rep-PCR) typing, and detection of OXA-type carbapenemases by multiplex PCR. Thirty-two patients acquired the outbreak strain in five intensive care units (ICUs) and two regular wards at a tertiary care hospital within 10 months. The outbreak strain was resistant to penicillins, cephalosporins, ciprofloxacin, gentamicin, tobramycin, imipenem and meropenem, and carried the bla(OXA-23)-like gene. Based on PFGE and rep-PCR typing, it was shown to be related to the pan-European A. baumannii clone II. The most likely mode of transmission was cross-transmission from colonized or infected patients via the hands of health-care workers, with the severity of disease and intensity of care (therapeutic intervention scoring system 28 score >median) being independently associated with acquisition of the outbreak strain (odds ratio 6.67, 95 % confidence interval 1.55-36.56). Control of the outbreak was achieved by enforcement of standard precautions, education of personnel, screening of ICU patients for carbapenem-resistant A. baumannii and cohorting of patients. This is believed to be the first report of an outbreak of A. baumannii carrying the carbapenemase OXA-23 in Germany.

Clonal spread of carbapenem-resistant OXA-23-positive Acinetobacter baumannii in a Bulgarian university hospital

From October 1999 to September 2006, 29 carbapenem-resistant isolates of Acinetobacter baumannii were collected consecutively from patients hospitalized in different wards of the University Hospital in Pleven, Bulgaria. The bla(OXA-23) gene, associated with the upstream-located ISAba1, was identified as the mechanism responsible for carbapenem resistance in all isolates. The isolates belonged to two different clonal groups, indicating a sustained hospital outbreak. This study demonstrates both the epidemic potential of carbapenem-resistant A. baumannii and its longevity in the hospital environment.

Molecular epidemiology of clinical Acinetobacter baumannii and Acinetobacter genomic species 13TU isolates using a multilocus sequencing typing scheme

To further expand the limited multilocus sequence typing (MLST) database for Acinetobacter baumannii, 53 clinical isolates from various outbreaks in Europe and the USA, collected between 1991 and 2004, plus the A. baumannii reference strain ATCC 19606(T) and 20 clinical Acinetobacter genomic species 13TU isolates from the same period, were analyzed using a new MLST scheme based on fragments of the gltA, gyrB, gdhB, recA, cpn60, gpi and rpoD genes. Data were compared with typing results generated using pulsed-field gel electrophoresis (PFGE) and randomly amplified polymorphic DNA (RAPD)-PCR. In total, 50 sequence types (STs) were distinguished among the A. baumannii isolates investigated, and the MLST data were in high concordance with the PFGE and RAPD-PCR results. Only five clonal complexes were identified by eBURST analysis, including the 21 STs listed in a previous study, suggesting high diversity among the A. baumannii isolates. With one exception, there was no relatedness among isolates from outbreaks in different countries (Europe) or regions (USA). No intercontinental spread was revealed. Acinetobacter genomic species 13TU isolates could also be analyzed using the A. baumannii MLST scheme (18 different STs) and could be distinguished from A. baumannii isolates according to characteristic sequences. It was concluded that the MLST scheme provides a high level of resolution and is a promising tool for studying the epidemiology of A. baumannii and Acinetobacter genomic species 13TU.

A PCR-based method to differentiate between Acinetobacter baumannii and Acinetobacter genomic species 13TU

A new PCR-based method that exploits differences in gyrB gene sequences was developed to distinguish between Acinetobacter baumannii and Acinetobacter genomic sp. 13TU. Among 118 clinical and reference Acinetobacter strains, 102 of which were previously speciated by amplified rDNA restriction analysis as belonging to the Acinetobacter calcoaceticus-A. baumannii complex, the method correctly identified 31 A. baumannii and 54 Acinetobacter genomic sp. 13TU isolates to the species level. The method was rapid, specific and easy to interpret.

Multidrug efflux inhibition in Acinetobacter baumannii: comparison between 1-(1-naphthylmethyl)-piperazine and phenyl-arginine-beta-naphthylamide

OBJECTIVES

1-(1-Naphthylmethyl)-piperazine (NMP) has been shown to reverse multidrug resistance (MDR) in Escherichia coli overexpressing RND-type efflux pumps but there are no data on its activity in non-fermenters like Acinetobacter.

METHODS

Antimicrobial susceptibility in the absence and presence of NMP and, for comparison, phenyl-arginine-beta-naphthylamide (PAbetaN), another putative efflux pump inhibitor (EPI), was tested in laboratory and mutant strains with differing intracellular dye accumulation and expression of adeB, and in clinical isolates of Acinetobacter baumannii.

RESULTS

Based on a 4-fold or greater MIC reduction, the effects of both EPIs at low concentrations (25 mg/L) were limited. PAbetaN had a highly selective action on the reduction in the MIC of rifampicin and clarithromycin. At a higher concentration of the putative EPIs (100 mg/L), NMP was more active than PAbetaN. This effect was not limited to strains with adeB overexpression, but affected the susceptibility to linezolid, chloramphenicol and tetracycline most, and was enhanced in clinical isolates with reduced fluoroquinolone susceptibility.

CONCLUSION

NMP can partially reverse MDR in A. baumannii and differs substantially in its activity from that of PAbetaN.

Selection of topoisomerase mutations and overexpression of adeB mRNA transcripts during an outbreak of Acinetobacter baumannii

OBJECTIVES

To investigate the mechanism of ciprofloxacin resistance in isolates of Acinetobacter baumannii during two hospital outbreaks and to determine the expression level of the gene encoding the AdeB efflux pump.

METHODS

Isolates were previously typed by PFGE and their MICs determined by broth microdilution. The gyrA and parC genes were sequenced and the adeB gene examined by real-time reverse transcription PCR (RT-PCR).

RESULTS

Two clonal lineages were responsible for the two hospital outbreaks. In both outbreaks, ciprofloxacin susceptibility was reduced during the course of the outbreak when compared with the index isolates. Mutations in gyrA and parC were found to have occurred during the outbreak. The MICs of non-fluoroquinolone antibiotics were raised in one clonal lineage and this was associated with a >10-fold increase in mRNA transcripts for adeB.

CONCLUSIONS

We have witnessed the appearance of gyrA and parC mutations during outbreaks of A. baumannii. In parallel with these mutations, we observed up-regulation of the adeB gene associated with a decrease in susceptibility to non-fluoroquinolone antibiotics. These data illustrate the propensity for A. baumannii to develop multi-drug resistance rapidly.

In vitro activities of the beta-lactamase inhibitors clavulanic acid, sulbactam, and tazobactam alone or in combination with beta-lactams against epidemiologically characterized multidrug-resistant Acinetobacter baumannii strains

Acinetobacter baumannii is an important nosocomial pathogen usually in the context of serious underlying disease. Multidrug resistance in these organisms is frequent. The beta-lactamase inhibitors clavulanic acid, sulbactam, and tazobactam have intrinsic activity against Acinetobacter strains. To evaluate their potential therapeutic usefulness, we determined the in vitro activity of ampicillin, sulbactam, ampicillin-sulbactam, cefoperazone, cefoperazone-sulbactam, piperacillin, piperacillin-sulbactam, tazobactam, piperacillin-tazobactam, amoxicillin, clavulanic acid, amoxicillin-clavulanic acid, ticarcillin, and ticarcillin-clavulanic acid against multidrug-resistant A. baumannii. All isolates were epidemiologically characterized by RAPD [random(ly) amplified polymorphic DNA] analysis and/or pulsed-field gel electrophoresis and represented different strain types, including sporadic strains, as well as outbreak-related strains. The MICs were determined by agar dilution on Mueller-Hinton agar (using fixed concentrations, as well as fixed ratios for beta-lactamase inhibitors) and the E-test. The majority of E-test results were within two dilutions of those recorded by agar dilution, with the exception of piperacillin-tazobactam. Sulbactam was superior to clavulanic acid and tazobactam and may represent an alternative treatment option for infections due to multiresistant A. baumannii strains. beta-Lactamase inhibitors have intrinsic activity but do not enhance activity of beta-lactams against A. baumannii. Testing with the inhibitor added at a fixed concentration as recommended for piperacillin-tazobactam and ticarcillin-clavulanic acid by the National Committee for Clinical Laboratory Standards may falsely suggest high activity or gives uninterpretable results due to trailing. If combinations are used for testing, fixed ratios may give more useful results.

Neisseria meningitidis, Neisseria lactamica and Moraxella catarrhalis share cross-reactive carbohydrate antigens

Carriage of commensal bacteria species is associated with the development of natural immunity to meningococcal disease, with lipo-oligosaccharides (LOS) of meningococci being one of the main virulence factors associated with severity of meningococcal disease. Meningococcal reference strains and isolates from the commensal species Neisseria lactamica and Moraxella catarrhalis were assessed for the presence of cross-reactive glycoconjugate antigens. Binding of human blood group antibodies of the P and Ii system to meningococcal immunotype reference strains were in accordance with the presence of known LOS carbohydrate structures. Binding studies with meningococcal immunotyping antibodies and blood group phenotyping antibodies to N. lactamica strains from different European countries showed, that a greater number of isolates obtained from native Greek and Scottish adults and children bound anti-meningococcal L(3, 7, 9) immunotyping (P < 0.001), pK (P = 0.035) and paragloboside (P < 0.001) blood group typing antibodies compared to isolates obtained from children of Russian immigrants in Greece. A greater number of M. catarrhalis strains isolated from children in Scotland bound anti-L(3, 7, 9) antibodies (38.2%) compared to strains isolated from adults (22.2%) (P = 0.017). These findings provide evidence that blood group like glycoconjugate antigens found on the commensal species N. lactamica and M. catarrhalis might be involved in the development of natural immunity to meningococcal endotoxins during childhood, and might be exploited as anti-meningococcal vaccine candidates.

Mutations in GyrA, ParC, MexR and NfxB in clinical isolates of Pseudomonas aeruginosa

The target enzymes GyrA and ParC and two efflux pump regulatory genes mexR and nfxB were analysed to determine changes associated with fluoroquinolone resistance in Pseudomonas aeruginosa. Both low- and high-level ciprofloxacin resistance was associated with a Thr-83Ile substitution in GyrA. A ParC Ser-80Leu substitution was found in highly resistant isolates in tandem with the Thr-83Ile substitution in GyrA. Mutations in the efflux regulatory genes were associated with resistance only when in tandem with a mutation in GyrA or ParC. These data show that the main mechanism of fluoroquinolone resistance in P. aeruginosa is mediated primarily through mutations in GyrA, and that mutations in ParC and the efflux regulatory genes are secondary.

Fluoroquinolones: structure and target sites

The quinolones are a potent group of drugs that target the essential bacterial enzymes DNA gyrase and topoisomerase IV. DNA gyrase is the primary target of Gram negative organisms however, it is topoisomerase IV that is the primary target of Gram positive organisms. Within these enzymes is a highly conserved region centered round the active site where resistance mutations occur. These mutations are almost always identical, irrespective of organism. In spite of the homology of this region, amino acid sequence analysis shows that there are defined differences between the Gram groups, particularly in topoisomerase IV, and it is speculated that herein lies the origin of target preference. Since the first quinolone nalidixic acid was developed, the quinolones have undergone structural modifications, in particular the addition of a fluorine at position 6, to produce the fluoroquinolones. This has seen their potency and pharmakokinetic profile greatly increase. In vitro selection of resistance mutations has allowed the observation of how resistance is acquired and some of the modifications in newer fluoroquinolones have resulted in the shift of primary target from topoisomerase IV to gyrase with Gram positives. Curiously, purified topoisomerase IV is still more sensitive even if gyrase is the primary target. Gyrase remains the primary target for Gram negatives.

Activity of quinolones against gram-positive cocci: mechanisms of drug action and bacterial resistance

The quinolones are a potent class of antimicrobial agents that target two essential enzymes of bacterial cells: DNA gyrase and topoisomerase IV. Resistance is mediated chiefly through stepwise mutations in the genes that encode these enzymes, leading to alterations of the target site. These mutations occur in an area called the "quinolone resistance determining region". In gram-positive organisms, mutations occur more often in topoisomerase IV than in DNA gyrase. This target preference appears to depend upon two factors: the species of organism and the selecting drug. Resistance can be enhanced by a decrease in intracellular drug concentration, which is mediated through efflux pumps. The newer generation of fluoroquinolones and non-fluorinated quinolones exhibits enhanced activity against gram-positive organisms compared to the older members of this drug class, although development of resistance to these drugs has been demonstrated in vitro. This review gives a chronological perspective of the literature on the action of DNA gyrase and topoisomerase IV and the mechanisms of resistance to quinolones in staphylococci, streptococci and enterococci.

Bactericidal and bacteriostatic activity of gemifloxacin against Acinetobacter spp. in vitro

This study compared the in vitro bacteriostatic activity of gemifloxacin (SB-265805) and a panel of test antimicrobial agents against 100 clinical isolates of Acinetobacter spp. (47 Acinetobacter baumannii, 18 Acinetobacter anitratus, 18 Acinetobacter lwoffii, 13 Acinetobacter calcoaceticus and four other Acinetobacter spp.). Gemifloxacin (MIC(50/90) 0.06/16 mg/L) was more than eight-fold more potent than ciprofloxacin (0.5/>128 mg/L), two- to eight-fold more potent than grepafloxacin, moxifloxacin, levofloxacin, ofloxacin and gatifloxacin, and of similar potency to trovafloxacin and sparfloxacin. Cross-resistance was seen only within the quinolone group and did not extend to non-quinolone antimicrobials. The bactericidal activities of gemifloxacin and the six comparator quinolones were investigated by dose-response and time-kill studies against A. baumannii ATCC 19606 at their optimum bactericidal concentration (OBC) and at 4 x MIC. At the OBC there was no significant difference between the quinolones, but at 4 x MIC gemifloxacin showed superior activity, reducing the viable count by almost 2 log(10) in 30 min compared with a 1 log(10) reduction seen with the other drugs. This enhanced killing extended over 24 h, reducing cell numbers by >4 log(10). These data suggest that gemifloxacin has the potential to be of therapeutic value in the treatment of infection by Acinetobacter spp.