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Background  Antibiotic resistant strains of  Acinetobacter baumannii  have been responsible for an increasing number of nosocomial infections including bacteremia and ventilator-associated pneumonia. In this study, we analyzed 38 isolates of  A. baumannii  obtained from two hospital outbreaks in Los Angeles County for the molecular epidemiology, antimicrobial susceptibility and resistance determinants.    Methods  Pulsed field gel electrophoresis, tri-locus multiplex PCR and multi-locus sequence typing (Pasteur scheme) were used to examine clonal relationships of the outbreak isolates. Broth microdilution method was used to determine antimicrobial susceptibility of these isolates. PCR and subsequent DNA sequencing were employed to characterize antibiotic resistance genetic determinants.    Results  Trilocus multiplex PCR showed these isolates belong to Global Clones I and II, which were confirmed to ST1 and ST2, respectively, by multi-locus sequence typing. Pulsed field gel electrophoresis analysis identified two clonal clusters, one with 20 isolates (Global Clone I) and the other with nine (Global Clone II), which dominated the two outbreaks. Antimicrobial susceptibility testing using 14 antibiotics indicated that all isolates were resistant to antibiotics belonging to four or more categories of antimicrobial agents. In particular, over three fourth of 38 isolates were found to be resistant to both imipenem and meropenem. Additionally, all isolates were found to be resistant to piperacillin, four cephalosporin antibiotics, ciprofloxacin and levofloxacin. Resistance phenotypes of these strains to fluoroquinolones were correlated with point mutations in  gyrA  and  parC  genes that render reduced affinity to target proteins. IS Aba1  was detected immediately upstream of the  bla  OXA–23  gene present in those isolates that were found to be resistant to both carbapenems. Class 1 integron-associated resistance gene cassettes appear to contribute to resistance to aminoglycoside antibiotics.    Conclusion  The two outbreaks were found to be dominated by two clonal clusters of  A. baumannii  belonging to MLST ST1 and ST2. All isolates were resistant to antibiotics of at least four categories of antimicrobial agents, and their antimicrobial susceptibility profiles correlate well with genetic determinants. The results of this study will facilitate our understanding of the molecular epidemiology, antimicrobial susceptibility and mechanisms of resistance of  A. baumannii  obtained from Los Angeles hospitals.

bmc infectious diseases

Molecular characterization and antimicrobial susceptibility of Acinetobacter baumannii isolates obtained from two hospital outbreaks in Los Angeles County, California, USA