Carbapenemase OXA-423: A Novel OXA-23 Variant in Acinetobacter baumannii

A novel variant of OXA-23, named OXA-423, was identified in an Acinetobacter baumannii clinical isolate. The aim of this study was to analyse the resistance phenotype of OXA-423.

The A. baumannii strain WY-0713 was isolated from an intensive care unit patient. PCR was used to detect the bla _OXA-23-like genes. Amplifying, cloning and sequencing were performed for the complete bla _OXA-23-like. The novel bla _OXA-423 and its ancestor bla _OXA-23 were cloned into the expression vector pET-28b(+), and transformed into E. coli Rosetta (DE3) for antibiotic susceptibility testing. SDS-PAGE, modified Hodge test and CarbaNP test were used for detecting the expression of OXA-423 and OXA-23.

PCR screening of A. baumannii WY-0713 was positive for bla _OXA-23-like genes. Sequencing of the PCR product identified a novel bla _OXA-23-like, named bla _OXA-423 which encoding  OXA-423. OXA-423 differed from OXA-23 by a crucial amino acid substitution (Val128Ala). The V128A substitution was located at the conserved active-site motifs SAV of OXA-23. Antibiotic susceptibility testing performed using isogenic E. coli showed that the MICs of E. coli Rosetta (pET-OXA-423) for penicillins and carbapenems were lower (reduced MICs 4-fold to 16-fold) than that of E. coli Rosetta (pET-OXA-23). The MICs of cefotaxime, ceftazidime and aztreonam for both transformants remained the same as the acceptor strain. Moreover, OXA-423 was slightly inhibited by sulbactam, clavulanic acid and tazobactam. SDS-PAGE analysis showed that OXA-423 and OXA-23 were conspicuously expressed. Modified Hodge test and CarbaNP test were positive demonstrated both of them were functional.

OXA-423, the first report of an amino acid substitution located at conserved active-site motifs of OXA-23, conferred lower MIC values of penicillins and carbapenems as compared with OXA-23, while without affecting the resistance profiles of expanded-spectrum cephalosporins and aztreonam.