Characterization of beta-lactamases in Mycobacterium fortuitum including a role in beta-lactam resistance and evidence of partial inducibility.

The beta-lactamases from the 3 biovariants of M. fortuitum were compared on the basis of substrate profiles, susceptibility to enzyme inhibitors, and inducibility in the presence of selected beta-lactams. Despite differences in the distribution of beta-lactamase bands observed when enzymes from different isolates were subjected to isoelectric focusing, substrate profiles for the 3 biovariants were similar. All demonstrated a comparable broad spectrum hydrolytic activity for both cephalosporins and penicillins. The MIC for amoxicillin were reduced 4- to 16-fold when combined with the beta-lactamase inhibitor clavulanic acid, but not to a clinically susceptible range. The degree of reduction in MIC for amoxicillin correlated well with the susceptibility of enzyme to inhibition by clavulanic acid as determined in an in vitro assay. Although all M. fortuitum strains produce beta-lactamase under routine growth conditions, 90% of strains demonstrated an increase in the amount of this enzyme when cultured in the presence of selected beta-lactams as potential inducers. Quantitative assays and isoelectric focusing further indicated that this apparent induction of beta-lactamase is a simple enhancement of the same enzyme(s) produced in the absence of a known inducer. This is the first demonstration of any inducibility among mycobacterial beta-lactamases and suggests that synthesis of these enzymes in M. fortuitum is under some form of regulatory control. These results indicate that the beta-lactamases have a role in resistance of M. fortuitum to the beta-lactams. Other factors, such as permeability and penicillin-binding proteins, were not evaluated.