Beta-lactamase of Mycobacterium fortuitum: kinetics of production and relationship with resistance to beta-lactam antibiotics

Abstract

The kinetics of both intracellular and extracellular beta-lactamase production and the relationship between extracellular enzyme and in vitro susceptibility of Mycobacterium fortuitum to beta-lactam antibiotics have been studied. To this end we used a panel of stable nitrosoguanidine-induced mutants of M. fortuitum derived from the parental strain ATCC 19542 and differing in beta-lactamase production from 0.0001 to 278 U/liter in Mueller-Hinton broth. For overproducers of beta-lactamase (mutants A188, B180, C207, D316, and E31), MICs of benzylpenicillin, amoxicillin, ampicillin, and cephaloridine progressively increased with the amount of enzyme released into the medium, whereas MICs of imipenem and cefoxitin did not. The resistance of the mutants to amoxicillin was reduced up to 32-fold by clavulanic acid, whereas that to ampicillin was reduced 8-fold by sulbactam. These data suggest that the enzyme participated in the mechanisms of resistance to the beta-lactam antibiotics. However, for a mutant of M. fortuitum (gamma 27) with virtually nonexistent beta-lactamase production, the antibiotics still had relatively high MICs (for instance, benzylpenicillin and cephaloridine had MICs of 64 and 32 micrograms/ml, respectively). This suggests that, aside from beta-lactamase production, other mechanisms such as cell wall permeability and/or affinity for penicillin-binding proteins could coexist in M. fortuitum and explain its natural resistance to beta-lactam antibiotics.

Antimicrobial Susceptibility Testing, Drug Resistance Mechanisms, and Therapy of Infections with Nontuberculous Mycobacteria

“…However, it is less clear whether they play a significant role in either intrinsic resistance or acquired resistance to ␤-lactams in NTM. Several studies of the ␤-lactamases of M. fortuitum found that the level of these enzymes did correlate with susceptibility to some ␤-lactams, such as amoxicillin, ampicillin, and benzylpenicillin (107,260,303). Similar results were found with M. smegmatis (113).…”

“…Porin loss has a significant fitness cost (362), which may outweigh the resistance benefit to the organism. Despite this, there is indirect evidence that the permeability barrier of the cell wall may play a critical role in the intrinsic resistance of mycobacteria to ␤-lactams (107,177,252,303).…”

“…An explanation for such discrepancies may come from the finding that at least some mycobacterial ␤-lactamases are inducible (260). That said, a common finding has been that ␤-lactamases of NTM appear to have little, if any, effect on susceptibility to imipenem and cefoxitin (106,107,113,303), the ␤-lactams with perhaps the most significant clinical utility against NTM.…”

“…The most studied ␤-lactamase inhibitor of mycobacteria is clavulanic acid, and this inhibitor has been shown to increase the susceptibility of NTM to ␤-lactamase substrates such as amoxicillin (27,106,107). Ethambutol.…”

Resistance to beta-lactams in Mycobacterium fortuitum

“…1-Lactamase activity (released and cell bound) was determined spectrophotometrically with cephaloridine as the substrate (5). Susceptibility tests were conducted by using a twofold agar dilution method (5).…”