N. C. Gordon

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.

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.

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.

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.

Antimicrobial activity of the green tea polyphenol (−)-epigallocatechin-3-gallate (EGCG) against clinical isolates of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is increasingly recognised as an important nosocomial pathogen. Treatment options are limited due to intrinsic resistance to many antibiotics as well as concerns over toxicity of the mainstay of treatment, co-trimoxazole. Epigallocatechin-3-gallate (EGCG), the major catechin found in green tea, has been shown to have antimicrobial effects against a number of bacterial pathogens. We evaluated the in vitro activity of this compound against 40 clinical isolates of S. maltophilia. MIC(50/90) values (minimal inhibitory concentrations for 50% and 90% of the organisms, respectively) were 256 mg/L when determined by agar dilution and 512 mg/L by broth microdilution. MBC(50/90) values (minimal bactericidal concentrations for 50% and 90% of the organisms, respectively) were 512 mg/L. In time-kill assays, the bactericidal activity of EGCG was analysed by viable colony counts as well as a colorimetric assay for bacterial reduction of XTT. EGCG was slowly bactericidal at 4x MIC, with a 2.5 log reduction in viable bacteria at 24h. EGCG has promising in vitro antimicrobial activity against S. maltophilia. Although the mechanism of action is not yet clear, further studies to evaluate its clinical potential and role in combination with other antimicrobial agents are warranted.

In vitro activity of teicoplanin combined with colistin versus multidrug-resistant strains of Acinetobacter baumannii

OBJECTIVES Antimicrobial treatment of multidrug-resistant Acinetobacter baumannii (MDRAB) remains an important therapeutic challenge. With isolates resistant to all conventional agents now reported, clinicians are increasingly forced to turn to unorthodox combination treatments in the hope that these may be efficacious. Although a potent interaction between vancomycin and colistin has been demonstrated, there are concerns regarding the inherent toxicity of combining these agents in clinical practice. As teicoplanin has less nephrotoxic potential than vancomycin, we assessed whether a colistin/teicoplanin combination would have similar antimicrobial activities in vitro. METHODS The antimicrobial activity of colistin alone and in combination with teicoplanin was assessed versus a collection of MDRAB belonging to a number of epidemic lineages present in the UK. Synergy studies were undertaken using microtitre plate chequerboard assays, an Etest agar dilution method and standard time-kill methodology. RESULTS The combination of teicoplanin and colistin was bactericidal versus all of the strains tested. In chequerboard assays, fractional inhibitory concentration indices of <0.5 were obtained, consistent with significant in vitro synergy. Using the Etest method the MIC of teicoplanin fell from >256 mg/L to ≤2 mg/L in the presence of subinhibitory concentrations of colistin. CONCLUSIONS Significant synergy was observed when colistin was combined with teicoplanin versus MDRAB in vitro. This may represent a useful therapeutic combination for the treatment of A. baumannii infections, especially when renal toxicity is a significant concern.

Novel variants of the Smqnr family of quinolone resistance genes in clinical isolates of Stenotrophomonas maltophilia

BACKGROUND Recent analysis of Stenotrophomonas maltophilia has identified a novel family of resistance genes (Smqnr) encoding pentapeptide repeat proteins, which confer low-level resistance to quinolones. This study describes further novel variants present in clinical isolates of S. maltophilia and investigates their effect on resistance to a number of quinolones in an Escherichia coli host. METHODS PCR for Smqnr alleles was carried out on a selection of S. maltophilia from clinical specimens, and amplicons were cloned and transformed in E. coli TOP10 cells. Transformed colonies carrying the plasmid were tested for susceptibility to a range of quinolones by MIC determination. DNA sequences were determined and translated peptide sequences compared with known SmQnr sequences. RESULTS Thirteen isolates were found to contain Smqnr alleles, of which six corresponded to previously identified Smqnr sequences, while seven were novel variants. Increases in quinolone MICs compared with wild-type E. coli TOP10 were seen for all strains transformed with Smqnr alleles. CONCLUSIONS There is considerable diversity within Smqnr alleles. S. maltophilia may be a significant reservoir for the dissemination of quinolone resistance elements to Enterobacteriaceae.

Evaluation of CHROMagar Acinetobacter for detection of enteric carriage of multidrug-resistant Acinetobacter baumannii in samples from critically ill patients.

ABSTRACT CHROMagar Acinetobacter was used to screen stool and perineal swabs for enteric carriage of multidrug-resistant Acinetobacter baumannii in samples from critically ill patients. Results were compared with a molecular assay resulting in sensitivity and specificity of culture compared to PCR of 91.7% and 89.6%, respectively.

Modifications to CHROMagar Acinetobacter for improved selective growth of multi-drug resistant Acinetobacter baumannii

Performance of CHROMagar Acinetobacter was assessed for the selective isolation and identification of Acinetobacter baumannii. The medium was effective in suppressing the growth of other Gram-positive and Gram-negative species while the addition of KPC supplement ensured growth of only carbapenem resistant A baumannii.

Failure of the MicroScan WalkAway System To Detect Heteroresistance to Carbapenems in a Patient with Enterobacter aerogenes Bacteremia

ABSTRACT We report the failure of the automated MicroScan WalkAway system to detect carbapenem heteroresistance in Enterobacter aerogenes. Carbapenem resistance has become an increasing concern in recent years, and robust surveillance is required to prevent dissemination of resistant strains. Reliance on automated systems may delay the detection of emerging resistance.