Beta-lactamase of Mycobacterium fortuitum: Introduction to the problem and first experimental data on purification and characterization

Over the last years an increase of infections caused by non-tuberculous mycobacteria was reported. Among fast growing mycobacteria, M. fortuitum is known to be resistant to all beta-lactam antibiotics but cefoxitin, cefmetazole and N-formimidoylthienamycin (imipenem). M. smegmatis and M. avium-intracellulare complex were demonstrated to have targets for beta-lactams (penicillin-binding proteins); for this reason it is possible that the poor mycobacterial susceptibility to these drugs is due to various mechanisms (different penicillin-binding protein affinities, permeability barrier, beta-lactamase and others). As far as beta-lactamases are concerned, it is known that these enzymes are produced by almost all mycobacterial species and that in M. fortuitum, M. smegmatis and M. phlei are constitutive, intracellular and active both against penicillins and cephalosporins. Furthermore, recently, extracellular beta-lactamase activity in M. fortuitum and M. smegmatis grown in different media has recently been reported. Beta-lactamases might play a role in beta-lactam resistance (but this mechanism does not appear to be the most important) as suggested by the observation that MIC values of beta-lactams against M. fortuitum and M. smegmatis can be lowered by using an irreversible inhibitor of beta-lactamase like clavulanic acid. The beta-lactamases of M. smegmatis and M. butyricum have already been purified. In this study the beta-lactamase from a pathogenic mycobacterial species, M. fortuitum strain NCTC Cow 18, has been partially purified. The extracellular enzyme was concentrated from the culture filtrate by ion-exchange chromatography and chromatofocusing techniques. Molecular weight was determined to be about 30 kDa. The partially purified beta-lactamase hydrolysed benzylpenicillin, cephalotin, cephaloridin, but it was inactive against methicillin, cefoxitin and cefotaxime. The kinetic parameters were not, in general, dissimilar from those of the intracellular beta-lactamases produced by M. smegmatis and M. butyricum.