Efflux pump overexpression in conjunction with alternation of outer membrane protein may induce Acinetobacter baumannii resistant to imipenem.

BACKGROUND: To investigate the role of outer membrane proteins in Acinetobacter baumannii resistant to imipenem, 2 strains were procured from the same patient. METHODS: The imipenem-resistant strain was obtained following a period of imipenem treatment in vivo. The multilocus sequence typing and repetitive extragenic palindromic PCR results indicated that the imipenem-resistant strain originated from the sensitive one. RESULTS: Isoelectric focusing detected no carbapenemases, with neither OXA carbapenemases nor metallo-β-lactamases found. Mass spectrophotometric analysis revealed that 3 outer membrane proteins were expressed differentially in the 2 strains: 2 downregulated proteins (OprD and CarO) and 1 upregulated the 34-kDa efflux pump protein in the resistant strain. A 32-fold decrease in the MIC for imipenem in the presence of Phe-Arg-β-naphthylamide in the same strain indicated a possible involvement of the efflux pump mechanism in its resistance, which was consistent with the findings that the mRNA expression of the 34-kDa efflux pump gene was almost fivefold upregulated in the imipenem-resistant strain compared with that in the imipenem-sensitive strain. Such a significant difference, however, was not found in the expression of AdeB and AdeJ between the 2 strains, and AdeE was not detected. CONCLUSIONS: Our results suggested that downregulation of outer membrane proteins in conjunction with efflux pump overexpression might contribute to imipenem resistance induced in vivo in A. baumannii.