DISTRIBUTION AND EXPRESSION OF BETA-LACTAMASE GENES AMONG AEROMONAS SPP

Clinical and environmental isolates of Aeromonas spp. (20 Aeromonas ca viae, 33 Aeromonas veronii and 21 Aeromonas hydrophila) were examined for their ability to yield mutants derepressed for beta-lactamase prod uction and for the distribution of the three chromosomally encoded bet a-lactamase genes, ampS and cepS from A. veronii bv. sobria and cphA f rom A. hydrophila. Of these isolates, 100% and 96% of the isolates fro m A. hydrophila and A. caviae, respectively, yielded beta-lactamase de repressed mutants but only 38% of A. veronii isolates yielded the resi stant phenotype at 37 degrees C. However, when tested at 30 degrees C, all isolates gave rise to the derepressed mutants, indicating a tempe rature effect on the control mechanism. All mutants had significantly higher beta-lactamase activity against ampicillin, oxacillin, cephalor idine and imipenem. Hybridization studies with cloned aeromonas beta-l actamase genes indicated that the cephalosporinase gene, cepS, is almo st ubiquitous for the three species tested. The cphA gene cross-hybrid ized with all isolates of A. veronii and A. hydrophila but not to A. c aviae isolates. In contrast, hybridization studies using ampS revealed that only 25% of A. caviae, 45% of A. veronii and 38% of A. hydrophil a tested carried the ampS gene or one closely homologous to it. Noneth eless, strains that failed to hybridize with ampS showed two serine be ta-lactamases when analysed by isoelectric focusing.