Antigenic properties and immunoelectron microscopic localization of Mycobacterium fortuitum β-lactamase

Mycobacterium fortuitum is a fast-growing Mycobacterium species which produces a β-lactamase involved in the intrinsic resistance of the microorganism to β-lactam antibiotics. An anti-β-lactamase serum against the purified enzyme was raised in rabbits. Antibody binding was specific for native β-lactamase, and enzyme activity was partially inhibited by the serum; furthermore, cross-reactions with denatured class A β-lactamases were observed. This serum was used as a probe in immunogold labeling for the localization of the cell-bound β-lactamase in both the low-level producer ATCC 19542 (parental strain) and the overproducer mutant D316. By the combination of preembedding immunogold labeling and replica technique, it was shown that the β-lactamase was uniformly distributed on the whole external cell surface, where it appeared to be associated with a Tween 80-removable capsule-like material. Compared with the parental strain, a much higher level of expression of surface enzyme was observed in strain D316. Surface labeling was more intense in the stationary phase of growth than in exponentially growing cells. The data obtained are interpreted in the context of the intrinsic resistance of M. fortuitum to β-lactam antibiotics.