Distribution and expression of β-lactamase genes among Aeromonas spp

Abstract

Clinical and environmental isolates of Aeromonas spp. (20 Aeromonas caviae, 33 Aeromonas veronii and 21 Aeromonas hydrophila) were examined for their ability to yield mutants derepressed for β-lactamase production and for the distribution of the three chromosomally encoded β-lactamase genes, ampS and cepS from A. veronii bv. sobria and cphA from A. hydrophila. Of these isolates, 100% and 96% of the isolates from A. hydrophila and A. caviae, respectively, yielded β-lactarslase derepressed mutants but only 38% of A. veronii isolates yielded the resistant phenotype at 37°C. However, when tested at 30°C, all isolates gave rise to the derepressed mutants, indicating a temperature effect on the control mechanism. All mutants had significantly higher β-lactamase activity against ampicillin, oxacillin, cephaloridine and imipenem. Hybridization studies with cloned aeromonas β-lactamase genes indicated that the cephalosporinase gene, cepS, is almost ubiquitous for the three veronii and A. hydrophila but not to A. caviae isolates. In contrast, hybridization studies using ampS revealed that only 25% of A. caviae, 45% of A. veronii and 38% of A. hydrophila tested carried the ampS gene or one closely homologous to it. Nonetheless, strains that failed to hybridize with ampS showed two serine β-lactamases when analysed by isoelectric focusing.