Group 1 β-lactamases of Aeromonas caviae and their resistance to β-lactam antibiotics

Abstract

The contribution of β-lactamase production to β-lactam antibiotic resistance was examined in an Aeromonas caviae mutant strain, selected in vitro by cefotaxime and derived from a wild-type strain isolated in our laboratory from crude sewage. Both strains produced β-lactamase. The mutant strain (AC7m) produced β-lactamase constitutively, in contrast to the parental strain (AC7), which was inducible by cefoxitin. AC7m was regarded as a mutant from AC7, which over-expressed β-lactamase. The mutant strain showed a remarkable reduction in sensitivity to most of the β-lactam antibiotics tested, such as (i) aminopenicillins and their combinations with clavulanic acid and sulbactam, (ii) carboxypenicillins, (iii) ureidopenicillins, and (iv) cephalosporins. This strain remained susceptible to ceftazidime, imipenem, and aztreonam. Isoelectric focusing of sonic extracts revealed that both strains AC7 and AC7m shared a common major β-lactamase band at pI 6.5. The plasmid DNA assays showed that the β-lactamases expressed by each A. caviae strain were chromosomally encoded. Based on substrate and inhibitor profiles determined in sonic extracts for AC7 and AC7m, the enzymes displayed on isoelectric focusing at pI 6.5 were assigned to chromosomal Group 1 β-lactamases. Imipenem would therefore be the appropriate choice for therapy of infections caused by A. caviae β-lactamase over-expressing mutants.