Inducible beta-lactam resistance in Aeromonas hydrophila: therapeutic challenge for antimicrobial therapy

Abstract

Despite the abundant amount of knowledge about inducible chromosomally mediated beta-lactamases among Aeromonas species, extended-spectrum beta-lactam-resistant A. hydrophila strains selected in clinical practice were rarely reported. In the present study, two strains of A. hydrophila, A136 and A139, with markedly different susceptibilities to extended-spectrum cephalosporins were isolated from blood and the tip segment of an arterial catheter of a burn patient. Another strain (A136m) was selected in vitro by culturing A136 in a subinhibitory concentration of cefotaxime, the beta-lactam agent administered for the treatment of Aeromonas bacteremia in this patient. Typing studies by arbitrarily primed PCR and pulsed-field gel electrophoresis indicated a clonal relationship among strains A136, A136m, and A139. These strains were identified to be of DNA hybridization group 1. Wild-type strain A136 was resistant only to ampicillin and cephamycins, but A136m and A139 were highly resistant to the expanded- and broad-spectrum cephalosporins. The presence of increased beta-lactamase activity in A139 suggests that A139 is a derepressed mutant which overexpresses beta-lactamases. These results call attention to the use of beta-lactam agents for the treatment of invasive Aeromonas infections.

FIG. 1

AP-PCR profiles for six strains of A. hydrophila obtained with primer ERIC-1R. Lane 1, A136; lane 2, A136m; lane 3, A139; lanes 4 and 5, two clinical strains of A. hydrophila, A2866 and A4252, respectively; lane 6, CCRC 13018; lane 7, CCRC 13881; lane M, 100-bp DNA ladder used as a molecular size standard.

FIG. 2

PFGE separation of SpeI-digested chromosomal DNA from A. hydrophila. Lane 1, A136; lane 2, A136m; lane 3, A139; lanes 4 and 5, two clinical strains of A. hydrophila, A2866 and A4252, respectively; lane 6, CCRC 13018; lane 7, CCRC 13881; lane M, bacteriophage lambda DNA concatemers.