beta-Lactamase activity in slow-growing nonpigmented mycobacteria and their sensitivity to certain beta-lactam antibiotics.

It has been reported that certain nonpigmented, slow-growing mycobacteria do not have a constitutive beta-lactamase and are more sensitive to benzyl penicillin than M. tuberculosis. Thirty-nine isolates of these organisms, most of them belonging to the M. avium-intracellulare complex, were found to be sensitive to less than 10 micrograms of benzyl penicillin per ml in vitro. Approximately half of these isolates were inhibited by 1 microgram per ml. By contrast of 32 isolates of M. tuberculosis, only one was inhibited by 10 micrograms of benzyl penicillin. Sensitivity studies with cephalothin yielded similar results. Analysis of beta-lactamase activity in some of these species showed no activity, confirming earlier observations of this deficit.