RESUMO
In order to elucidate the role of beta-lactamase in the resistance of M. fortuitum to beta-lactams, M. fortuitum ATCC 19542 and three mutants, strains D 316, D 319 and D 170 obtained from it by nitrosoguanidine treatment, were studied. Furthermore the kinetics of the beta-lactamase production during the bacterial growth and many biochemical and enzymatic characteristics of parent and mutant strains were investigated. Amoxicillin MICs well correlated with the beta-lactamase production in the high producer strains D 316 and D 319; on the contrary in strain D 170 a high MIC was joined with a moderate production of the enzyme showing that not only beta-lactamase but also other mechanisms can be effective in the resistance of M. fortuitum to beta-lactams. Clavulanic acid, an inhibitor of beta-lactamase, reduced MICs to amoxicillin in high and in low producer strains. The production of extracellular beta-lactamase occurred in a M. fortuitum mutant strain mainly during the stationary phase, indicating that a cell wall damage or initial autolysis could be responsible for the release of enzyme. Enzymatic and biochemical characteristics were not affected by nitrosoguanidine treatment except for nitrate test which showed only a weak positivity in high producer strains.