In Vitro antimicrobial synergy testing of Acinetobachter Baumannii

[摘要] ENGLISH ABSTRACT: Acinetobacter baumannii has emerged as one of the most troublesome nosocomial pathogensglobally. This organism causes infections that are often extremely difficult to treat because of thewidespread resistance to the major antibiotic groups. Colonization or infection with multidrugresistantA. baumannii is associated with the following risk factors: prolonged hospital stay,admission to an intensive care unit (ICU), mechanical ventilation, and exposure to broad spectrumantibiotics, recent surgery, invasive procedures, and severe underlying disease.A. baumannii has been isolated as part of the skin flora, mostly in moist regions such as axillae,groin and toe webs. It has also been isolated from the oral cavity and respiratory tract of healthyadults. Debilitated hospitalized patients have a high rate of colonization, especially duringnosocomial Acinetobacter outbreaks. This organism is an opportunistic pathogen as it containsfew virulence factors. Clinical manifestations of A. baumannii include nosocomial pneumonia,nosocomial bloodstream infections, traumatic battlefield and other wound infections, urinary tractinfections, and post-neurological surgery meningitis. Fulminant community-acquired pneumoniahas recently been reported, indicating that this organism can be highly pathogenic.The number of multidrug-resistant A. baumannii strains has been increasing worldwide in the pastfew years. Therefore the selection of empirical antibiotic treatment is very challenging. Antibioticcombinations are used mostly as empirical therapy in critically ill patients. One rationale for theuse of combination therapy is to achieve synergy between agents.The checkerboard and time-kill methods are two traditional methods that have been used forsynergy testing. These methods are labor intensive, cumbersome, costly, and time consuming.The E-test overlay method is a modification of the E-test method to determine synergy betweenthe different antibiotics. This method is easy to perform, flexible and time efficient.The aim of this study was to assess the in vitro activity of different combinations of colistin,rifampicin, imipenem, and tobramycin against selected clinical strains of A. baumannii using thecheckerboard and the E-test synergy methods. The MICs obtained with the E-test and brothmicrodilution method were compared. The results of the disk diffusion for imipenem andtobramycin as tested in the routine microbiology laboratory were presented for comparison.Overall good reproducibility was obtained with all three methods of sensitivity testing. Theagreement of MICs between the broth dilution and E-test methods was good with not more thantwo dilution differences in MIC values for all isolates, except one in which the rifampicin E-test MICdiffered with three dilutions from the MIC obtained with the microdilution method. However, thecategorical agreement between the methods for rifampicin was poor. Although MICs did not differwith more than two dilutions in most cases, many major errors occurred because the MICsclustered around the breakpoints.The combinations of colistin + rifampicin, colistin + imipenem, colistin + tobramycin, rifampicin +tobramycin, and imipenem + tobramycin all showed indifferent or additive results by the E-testmethod. No results indicating synergy were obtained for all the above-mentioned combinations.There was one result indicating antagonistic effect for the combination of colistin + tobramycin.The results of the checkerboard method showed results indicating synergy in four of the sixisolates for which the combination of colistin and rifampicin was tested. The other two isolatesshowed indifferent/additive results. All the other combinations showed indifferent/additive resultsfor all isolates except isolate 30 (col + tob) and isolate 25 (rif + tob) which showed synergism. Noantagonistic results were observed by the checkerboard method.When the results obtained with the E-test and checkerboard methods were compared, it wasnoted that for most antibiotic combinations an indifferent/additive result was obtained. However,for the colistin + rifampicin combination, the checkerboard method showed synergism for 4 of 6isolates, whereas the E-test method showed indifference and an additive result in one. For therifampicin + tobramycin, and colistin + tobramycin combinations, synergism was also shown withthe checkerboard method in one isolate for each combination. The E-test method howevershowed an indifferent and additive result respectively..The E-test method was found to be a rapid, reproducible, easy-to-perform, and flexible method todetermine synergistic antibiotic activity. This study was however limited by low numbers ofisolates. This might explain why no synergistic results were obtained with the E-test method andfew synergistic results with the checkerboard method. Genotypic analysis using pulse-field gelelectrophoresis (PFGE) may be considered in future studies to determine relatedness of the isolates which will facilitate the selection of different strains for synergy testing. Furthermore,clinical studies are needed to establish whether in vitro synergy testing is useful in the clinicalsetting and whether the results of synergy testing will have any bearing on the clinical outcome ofpatients infected with multidrug resistant A. baumannii.

[发布机构] Stellenbosch University