Antibiotic resistance determinants in nosocomial strains of multidrug-resistant Acinetobacter baumannii

Jennifer K Mak, Mi-Jurng Kim, Jeanette Pham, John Tapsall, Peter A White
Journal of Antimicrobial Chemotherapy 2009, 63 (1): 47-54

OBJECTIVES: To investigate the presence of resistance genes in nosocomial multidrug-resistant (MDR) Acinetobacter baumannii isolated from outbreak and sporadic settings.

METHODS: Thirty-two A. baumannii isolates were collected, 13 of which were involved in two outbreaks from different hospitals, which resulted in four deaths. The remaining 19 isolates were collected sporadically over 5 years from two other hospitals. The MICs of 25 antibiotics were determined for each isolate. PCR screening was carried out to identify possible genes that contributed to each resistance phenotype. Repetitive extragenic palindromic-PCR (REP-PCR) was performed to assess isolate clonality in conjunction with genotype data.

RESULTS: Between eight and 12 resistance determinants were detected in the 32 MDR A. baumannii isolates examined. These resistance determinants included the genes blaOXA-23 and ampC, with the upstream element ISAba1 promoting increased gene expression and subsequent resistance to carbapenems and cephalosporins, respectively. In all isolates, resistance to quinolones and fluoroquinolones was conferred by an S83L mutation in GyrA. Twenty-eight of the 32 isolates were also positive for tet(B), a tetracycline resistance determinant, blaTEM-1, which contributed to beta-lactam resistance, and strB, which contributed to aminoglycoside resistance. Class 1 integrons that harboured aacC1, aadA1, qacEDelta1 and sul1 were identified in 10 of the 32 isolates (31%) together with the kanamycin resistance gene, aphA1. A putative trimethoprim resistance gene, folA, was also identified in all isolates. REP-PCR together with genotyping identified three main clonal types.

CONCLUSIONS: Isolates of A. baumannii from both outbreak and sporadic cases possess at least eight resistance gene determinants that give rise to the MDR phenotype.

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