Different activities of antimicrobial combinations against multidrug resistant Acinetobacter baumanniiin vitro

Abstract

Objective To evaluate the activities of 18 pairs of antimicrobials combinations against non - duplicate clinical isolates of multidrug resistant Acinetobacter baumanrrii （MDRAB） in vitro . Methods Collect isolates of Acinetobacter baumannii from different patients from October 2009 to May 2010, which were isolated in Clinical Laboratory Center of Beijing Friendship Hospital, Capital Medical University. Use broth microdilution method to detect MIC of mono-antimicrobial, and checkerboard broth microdilution method to detect combinatied MIC, and calculate fractional inhibitory concentration （FIC） index to determine drug combinations effects. When the performance of the same drug combinations conflicted, appropriate strains were selected for screening of drug-resistant mechanisms by polymerase chain reaction（PCR） , including effiux pump genes. Results In tests in vitro, rifampicin and polymyxin B, imipenem and gentamicin, cefepime and levofloxacin showed synergy at high proportion, 68.1% , 45.5% , 40.9% , respectively. Minocycline and rifampicin, ampicillin/sulbactam and tobramycin. Ceftazidime and ciprofloxacin showed additive effect at high proportion, 81.8% , 68.2% , 68.2% , respectively. There were several combinations which appeared the opposite effects to tested strains. Strains No. 19 corresponding reaction was synergy and No. 21, No. 26 corresponding reactions were antagonism. The three strains above were selected for screening resistant mechanisms. The difference is that genotypes of adeS were negative in No. 19 and positive in No. 21 and No. 26. Conclusion Rifampicin and polymyxin B combination showed synergy against the MDRAB in vitro, which can be considered as the treatment choice for critical infections caused by MDRAB. Imipenem and gentamicin, cefepime and levofloxacin also showed synergy in vitro, but in some isolates showed antagonism. This phenomenon may be due to the gene adeS activated by certain antibiotics, and the activated adeSdrived efflux pump express or overexpress, which made the drugs in bacterial ceils pumped out,causing antagonistic effect. The individual differences in strains should be considered when clinic strain apply these two combinations above. Key words: Multi-drug resistant Acinetobacter baumannii; Antimicrobial combination