Analysis Of Antibiotics Resistance Among Acinetobacter Baumannii And Contribution Of Efflux Pump Mechanism In Carbapenem-resistant Acinetobacter Baumannii

In recent years, as the extensive use of antibiotics, the resistance of A.baumanii isworrying. Initially, clinical A.baumanii is susceptible to most of the drugs, but now, itbecoms muti-drug resistant (MDR), even extensive-drug resistant (XDR) A.baumaniiand pan-drug resistant (PDR) A.baumanii. MDR A.baumanii is resistant to at least threeantibiotics out of the five Anti-pseudomonas drugs, including aminoglycosides,quinolones, cephalosporins, beta lactamase inhibitor compound and carbapenems. Lastyear, NDM-1-producing A.baumanii had been reported in China, which means thatA.baumanii is one of “the Super-resistant bacteria”, like Klebsiella pneumoniae andEscherichia coli.As new type of β-lactam antibiotics, carbapenems is the most powerful antibiotics totreat Gram-negative bacilli infection. However, the data from CHINET surveillanceshowed that in the year of2004-2005,31.5%and40.7%A.baumanii strains wereresistant to imipenem and meropenem,37.6%and42.7%in2007, and57.1%and58.3%A.baumanii strains were resistant to imipenem and meropenem in2010. So, wecan conclude that the resistance of carbapenems in A.baumanii is growing years byyears. Carbapenems is the last defensive-line to treat the infection caused by MDRA.baumanii which is resistant to non-carbapenems antibiotics. Once A.baumaniibecomes resistant to carbapenems, it means that it’s resistant to other antibiotics, too.This will make clinical treatment fall into the plight of the "no cure", and also will makepeople face the panic brought about by the "post-antibiotic era".The mechanisms of resistance to carbapenems in A. baumannii have been ascribedto the acquisition of carbapenemases, outer membrane protein lower expression or lack, and overexpression of efflux pumps. Among these resistance mechanisms, production ofcarbapenemase is best studied, while efflux pump is less.ObjectiveTo investigate the distribution and resistance to clinical drugs of clinical A.baumaniiisolates, to understand the distribution of the efflux pump genes and the contribution ofefflux pump mechanism to carbapenems-resistant A.baumanii, providing basis forclinical anti-infective therapy and hospital infection control.MethodsThe sensitivity of383A.baumanii strains to15antibiotics were tested bydisc-diffusion method; the minimum inhibitory concentrations (MIC) of five antibiotics,including imipenem, meropenem, ciprofloxacin gentamicin, and ceftazidime with orwithout efflux pump inhibitor PAβN for74carbapenems-resistant and31sensitiveA.baumanii strains were determined by agar dilution method.Amplify efflux pumpgenes adeB, adeJ, abeM, and regulatory gene shot fragment adeR, adeS by PolymeraseChain Reaction; Expression of adeB, adeJ, abeM gene mRNA were detected byquantitative PCR. Full-length fragment of regulatory genes adeR and adeS wereamplified by Polymerase Chain Reaction, then sent to be sequenced.ResultsThe antimicrobial susceptibility of383A.baumanii strains revealed that theresistance rate of A.baumanii to Aztreonam (85.3%) was higher than other antimicrobialagents, while the resistance rate of A.baumanii to cefoperazone-sulbactam (28.0%) wasthe lowest. About57.3%and61.8%of A.baumanii were resistant to imipenem andmeropenem respectively. Above60%of A.baumanii strains were resistant to the restantimicrobial agents except Minocycline (31.9%).In addition of PAβN, MIC of60.81%of A.baumanii strains to imipenem, and94.59%of A.baumanii strains to meropenem decreased more than4folds. In74carbapenem-resistant A.baumanii strains,100%strains were detected with all the threeadeB, adeJ, abeM genes, and95%strains with adeR,93%strains with adeS. The expression of adeB mRNA in carbapenems-resistant A.baumanii strains wassignificantly higher than sensitive A.baumanii, while the expression of adeJ and abeMmRNA in carbapenems-resistant A.baumanii strains had no significant difference withsensitive A.baumanii.After Full-length fragment of adeR in three carbapenems-resistantA.baumanii strains were sequenced, a single mutation A2639→G，leading to an Lys219→Glu amino acid replacement were identified in all the three strains, while adeS havenonsense mutation in carbapenems-resistant A.baumanii.ConclusionsMost clinical A.baumanii strains are recovered from sputum, and25.1%ofA.baumanii strains are recovered form ICU department which is higher than othersdepartments. A.baumanii is highly resistant to multi-drugs, and cefoperazone-sulbactamor Minocycline could be effective to treat infection caused by A.baumanii. Efflux pumpplays a role in decreasing the susceptibility of both meropenem and imipenem. Effluxpump genes are widespread in A.baumanii, and the detaction rate incarbapenems-resistant A.baumanii has significant difference with sensitive A.baumanii.Overexpression of adeABC is probably the cause of resisistance of carbapenems inA.baumanii, and an Lys219→Glu amino acid replacement in regulatory gene adeR maylead to its overexpression. AdeIJK and abeM do not correlate with reducedsusceptibility to carbapenems.