David W. Wareham

Multidrug-resistant Acinetobacter baumannii: mechanisms of virulence and resistance

Infection due to Acinetobacter baumannii has become a significant challenge to modern healthcare systems. The organism shows a formidable capacity to develop antimicrobial resistance, yet the clinical impact of A. baumannii infection remains unclear. Much is known about the processes involved in multidrug resistance, but those underlying the pathogenicity and virulence potential of the organism are only beginning to be elucidated. In this article, we provide an overview of current knowledge, focusing on mechanisms of pathogenesis, the molecular basis of resistance and options for treatment in the absence of novel therapeutic agents.

Imaging bacterial infection with 99mTc-ciprofloxacin (Infecton)

Aims: The diagnosis of deep seated bacterial infections, such as intra-abdominal abscesses, endocarditis, and osteomyelitis, can be difficult and delayed, thereby compromising effective treatment. This study assessed the efficacy of a new radioimaging agent, Tc-99m ciprofloxacin (Infecton), in accurately detecting sites of bacterial infection. Methods: Eight hundred and seventy nine patients with suspected bacterial infection underwent Infecton imaging and microbiological evaluation. The sensitivity and specificity of Infecton in detecting sites of bacterial infection were determined with respect to Centres of Disease Control, World Health Organisation, and Dukes’s criteria. Results: Five hundred and seventy four positive and 295 negative images were produced. These included 528 true positives, 46 false positives, 205 true negatives and 90 false negatives, giving an overall sensitivity of 85.4% and a specificity of 81.7% for detecting infective foci. Sensitivity was higher (87.6%) in microbiologically confirmed infections. Conclusions: Infecton is a sensitive technique, which aids in the earlier detection and treatment of a wide variety of deep seated bacterial infections. The ability to localise infective foci accurately is also important for surgical intervention, such as drainage of abscesses. In addition, serial imaging with Infecton might be useful in monitoring clinical response and optimising the duration of antimicrobial treatment.

A Novel Variant, NDM-5, of the New Delhi Metallo-β-Lactamase in a Multidrug-Resistant Escherichia coli ST648 Isolate Recovered from a Patient in the United Kingdom

ABSTRACT A new variant of the New Delhi metallo-enzyme (NDM) carbapenemase was identified in a multidrug-resistant Escherichia coli ST648 isolate recovered from the perineum and throat of a patient in the United Kingdom with a recent history of hospitalization in India. NDM-5 differed from existing enzymes due to substitutions at positions 88 (Val→Leu) and 154 (Met→Leu) and reduced the susceptibility of E. coli TOP10 transformants to expanded-spectrum cephalosporins and carbapenems when expressed under its native promoter.

Potent synergy and sustained bactericidal activity of a vancomycin-colistin combination versus multidrug-resistant strains of Acinetobacter baumannii.

ABSTRACT Multidrug-resistant Acinetobacter baumannii (MDRAB) presents an increasing challenge to health care. Although colistin has been used as a treatment of last resort, there is concern regarding its potential for toxicity and the emergence of resistance. The mechanism of action of colistin, however, raises the possibility of synergy with compounds that are normally inactive against Gram-negative organisms by virtue of the impermeability of the bacterial outer membrane. This study evaluated the effect of colistin combined with vancomycin on 5 previously characterized epidemic strains and 34 MDRAB clinical isolates by using time-kill assay, microdilution, and Etest methods. For all the isolates, significant synergy was demonstrated by at least one method, with reductions in the MIC of vancomycin from >256 μg/ml to ≤48 μg/ml for all strains after exposure to 0.5 μg/ml colistin. This raises the possibility of the clinical use of this combination for infections due to MDRAB, with the potential for doses lower than those currently used.

Antimicrobial resistance in community and nosocomial Escherichia coli urinary tract isolates, London 2005-2006.

BackgroundEscherichia coli is the commonest cause of community and nosocomial urinary tract infection (UTI). Antibiotic treatment is usually empirical relying on susceptibility data from local surveillance studies. We therefore set out to determine levels of resistance to 8 commonly used antimicrobial agents amongst all urinary isolates obtained over a 12 month period.MethodsAntimicrobial susceptibility to ampicillin, amoxicillin/clavulanate, cefalexin, ciprofloxacin, gentamicin, nitrofurantoin, trimethoprim and cefpodoxime was determined for 11,865 E. coli urinary isolates obtained from community and hospitalised patients in East London.ResultsNitrofurantoin was the most active agent (94% susceptible), followed by gentamicin and cefpodoxime. High rates of resistance to ampicillin (55%) and trimethoprim (40%), often in combination were observed in both sets of isolates. Although isolates exhibiting resistance to multiple drug classes were rare, resistance to cefpodoxime, indicative of Extended spectrum β-lactamase production, was observed in 5.7% of community and 21.6% of nosocomial isolates.ConclusionWith the exception of nitrofurantoin, resistance to agents commonly used as empirical oral treatments for UTI was extremely high. Levels of resistance to trimethoprim and ampicillin render them unsuitable for empirical use. Continued surveillance and investigation of other oral agents for treatment of UTI in the community is required.

A review of clinical and microbiological outcomes following treatment of infections involving multidrug-resistant Acinetobacter baumannii with tigecycline

OBJECTIVES Multidrug-resistant Acinetobacter baumannii (MRAB) is an increasing problem in UK hospitals, with many strains now resistant to all available antibiotics except polymyxins. Tigecycline has been used for the treatment of MRAB as it demonstrates activity in vitro, but there are limited data on its clinical efficacy in Gram-negative infections, especially those involving the lower respiratory tract or bacteraemia. PATIENTS AND METHODS A retrospective study of the clinical and microbiological outcomes of all patients treated with tigecycline for MRAB over an 18 month period was undertaken. RESULTS Thirty-four patients received tigecycline for MRAB or polymicrobial infection involving MRAB. Twenty-three (68%) had a positive clinical outcome: microbiological clearance was demonstrated in 10 of these. The overall mortality was 41% (n = 14), with nine deaths directly attributable to sepsis. Three patients had episodes of Gram-negative bacteraemia while receiving treatment with tigecycline, with documented resistance occurring in one patient. Overall, the correlation between microbiological and clinical outcomes was poor. CONCLUSIONS While tigecycline retains excellent in vitro activity against MRAB, its clinical efficacy remains uncertain. A prospective study, including the use of tigecycline in combination with other antimicrobial agents, should be undertaken to define its role in the treatment of MRAB.

Carbapenemase-producing Enterobacteriaceae and non-Enterobacteriaceae from animals and the environment: an emerging public health risk of our own making?

Acquired carbapenemases pose one of the most pressing public health threats relating to antibiotic resistance. In most countries, the number of carbapenemase-producing bacteria from human clinical specimens is rising, and the epidemiological status of these multiresistant bacteria is progressively worsening. Furthermore, there is a growing number of reports of carbapenemases found either in bacteria isolated from non-human sources or in Salmonella enterica subsp. enterica, a zoonotic species. However, carbapenemases are not yet systematically sought in bacteria from non-human sources, reports of them are largely observational, and there is limited investigation of carbapenemase-positive bacteria in animals and possible links with people who may have acted as potential sources. Active surveillance and monitoring for carbapenem-resistant bacteria in the food chain and other non-human sources is urgently needed, with an enhanced and rigorous follow-up of all positive results. The carbapenems are currently our last good defence against multiresistant Gram-negative bacteria. Our ability to limit the rise and spread of carbapenemase producers, which occur only at basal levels in many countries at present, should serve as a key performance indicator for the success or failure of the efforts that have been called for by international organizations and governments to reduce the impact of antibiotic resistance.

Bloodstream infection due to Acinetobacter spp: epidemiology, risk factors and impact of multi-drug resistance

Acinetobacter spp. are increasingly reported as important causes of human infection. Many isolates exhibit multi-drug resistance, raising concerns over our ability to treat serious infections with these organisms. The impact of infection on clinical outcome as well as the importance of multi-drug resistance is poorly defined. A descriptive retrospective observational study was undertaken of all episodes of Acinetobacter bacteremia occurring in a UK tertiary care centre from 1998–2006. Demographics of infected patients, characteristics and antimicrobial susceptibility of infecting strains were recorded and the impact of antimicrobial therapy on all causes of 30-day mortality assessed. Three hundred ninety-nine episodes of Acinetobacter bacteremia were identified, with A. baumannii being the most frequently isolated species. Most episodes occurred in critical care and were associated with multidrug resistance, with carbapenem resistance rising from 0% in 1998 to 55% in 2006. Although bacteremia due to carbapenem-resistant Acinetobacter and a requirement for critical care were associated with a higher mortality, mortality was not reduced by the administration of appropriate empirical antimicrobial therapy. A prospective study is required to identify both the most effective intervention and those most likely to benefit from treatment.

AdeABC-mediated efflux and tigecycline MICs for epidemic clones of Acinetobacter baumannii

OBJECTIVES Tigecycline non-susceptibility in individual Acinetobacter baumannii isolates has been associated with up-regulation of the resistance-nodulation-division (RND)-type efflux system, AdeABC. We sought to relate variation in the expression of this system to differences in modal tigecycline MIC among prevalent A. baumannii clones. The role of AdeABC in the emergence of tigecycline resistance during therapy was also investigated for two representatives of the prevalent UK lineage, OXA-23 clone 1. METHODS Clonal type was defined by PFGE and expression of adeABC by real-time RT-PCR. Laboratory mutants were selected in vitro by exposing a susceptible clinical isolate to increasing tigecycline concentrations. The adeB gene was inactivated by the directed integration of a suicide plasmid containing an internal fragment of the target gene. RESULTS Higher modal tigecycline MICs for particular clones correlated with elevated expression of adeABC. Expression of this operon was also increased in the two post-therapy, tigecycline-resistant clinical isolates and in a laboratory mutant as compared with their pre-exposure, tigecycline-susceptible counterparts. Interruption of adeB in a tigecycline-resistant clinical isolate restored full susceptibility to tigecycline. CONCLUSIONS Differences in expression of adeABC contribute to both inter- and intra-clone variation in tigecycline MICs. Tigecycline resistance can arise during therapy, mediated by up-regulation of AdeABC.

Whole-genome comparison of two Acinetobacter baumannii isolates from a single patient, where resistance developed during tigecycline therapy

OBJECTIVES The whole genomes of two Acinetobacter baumannii isolates recovered from a single patient were sequenced to gain insight into the nature and extent of genomic plasticity in this important nosocomial pathogen over the course of a short infection. The first, AB210, was recovered before tigecycline therapy and was susceptible to this agent; the second, AB211, was recovered after therapy and was resistant. METHODS DNA from AB210 was sequenced by 454 GS FLX pyrosequencing according to the standard protocol for whole-genome shotgun sequencing, producing ∼250 bp fragment reads. AB211 was shotgun sequenced using the Illumina Genetic Analyzer to produce fragment reads of exactly 36 bp. Single nucleotide polymorphisms (SNPs) and large deletions detected in AB211 in relation to AB210 were confirmed by PCR and DNA sequencing. RESULTS Automated gene prediction detected 3850 putative coding sequences (CDSs). Sequence analysis demonstrated the presence of plasmids pAB0057 and pACICU2 in both isolates. Eighteen putative SNPs were detected between the pre- and post-therapy isolates, AB210 and AB211. Three contigs in AB210 were not covered by reads in AB211, representing three deletions of ∼15, 44 and 17 kb. CONCLUSIONS This study demonstrates that significant differences were detectable between two bacterial isolates recovered 1 week apart from the same patient, and reveals the potential of whole-genome sequencing as a tool for elucidating the processes responsible for changes in antibiotic susceptibility profiles.