Beta-lactamase synthesis in Mycobacterium leprae.

Beta-lactam antibiotics are not active against Mycobacterium leprae. The enzyme beta-lactamase mediates the most common form of bacterial resistance to penicillins and cephalosporins. Cell-free extracts of purified suspensions of M. leprae were examined for beta-lactamase. The bacteria were prepared from the tissues of experimentally-infected nine-banded armadillos. Most of the suspensions were inactive. However, the bacteria obtained from the tissues of armadillos treated with penicillin G benzathine (bicillin) 6 months or more prior to sacrifice had beta-lactamase. If the organisms had been exposed to the antibiotic only for a few days, they were negative. Attempts to induce beta-lactamase in the bacteria in vitro did not succeed. Interestingly M. leprae separated from untreated armadillos, infected with the bacilli derived from treated animals contained the enzyme activity. Apparently, the M. leprae genome contains the operon for beta-lactamase, and once it is stimulated to express the enzyme, it continues to do so, even after the inducer is withdrawn.