Inducible β-lactam resistance in Aeromonas hydrophila: Therapeutic challenge for antimicrobial therapy

Despite the abundant amount of knowledge about inducible chromosomally mediated β-lactamases among Aeromonas species, extended-spectrum β-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 β-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 β-lactamase activity in A139 suggests that A139 is a derepressed mutant which overexpresses β-lactamases. These results call attention to the use of β-lactam agents for the treatment of invasive Aeromonas infections.