Ultrastructural characteristics of Mycobacterium bovis BCG and Mycobacterium leprae

Freeze-fracture studies (FF) and ultrathin sectioning (UTS) have been performed on Mycobacterium bovis (BCG) and Mycobacterium leprae (ML). FF of BCG revealed the presence of two populations of cells: one group having four fracture faces and the other showing only two; the latter showed a preferential cleavage along a plane across the cell wall (CW) having a smooth appearance devoid of particles, on both protoplasmic (PF) and exoplasmic (EF) fracture faces. On the other hand, FF of the plasma membrane of ML often showed rather large smooth areas with particles present in clusters. Linear depressions and ridges were more frequently observed in CWEF and CWPF faces of ML as compared to those of BCG. Similarly, UTS of BCG revealed two populations of cells: the majority with cell envelopes consisting of three substructures and the rest with only two. UTS of ML showed similar substructures as for BCG, except that the electron-dense zone appeared more translucent. We postulate that mycobacteria of the same genus may exist in heterogeneous forms.