beta-lactamase expression in Plesiomonas shigelloides

We have examined the production of beta-lactamases by 11 clinical and nine environmental isolates of Plesiomonas shigelloides from Czechoslovakia, the Czech Republic and Cuba. Ten isolates (50%) expressed detectable amounts of a chromosomally encoded, non-inducible beta-lactamase, though all isolates showed a broadly similar resistance profile: low-level resistance to ampicillin and higher-level resistance to carbenicillin. All strains were susceptible to cephalosporins and meropenem. Three clinical isolates expressed a beta-lactamase similar to a class 2c carbenicillinase, with a pI of 5.2 and three expressed an enzyme similar to a class 2d oxacillinase, with a pI of 5.3. The environmental isolates produced a variety of penicillinases, indicating that there is a reservoir of heterogeneous beta-lactamase genes in this species.