Relatively stable L-phase colonies were isolated from old cultures of a selected clone of . The colonies grew at 52°C and were composed of rod-shaped, oval or spherical cells. Large amoeba-like cells were occasionally present. These were usually limited by a double-layered membrane and devoid of normal cell-wall components such as bacteriophage receptors. The large amoeba-like bodies sometimes showed both outer and inner double-layered membranes, especially in pseudopodium-like cellular extensions. An unusual feature of rod-shaped cells was retention of the original shape despite the loss of their cell walls.

Two types of walled cells occurred during successive transfers of L colonies. One was the true revertant which had characteristics in common with the wild-type , such as growth at 52°C and ultrastructural organisation. The other, designated as the “atypical-cell-wall variant”, was characterised by growth at 52°C, thick cell walls, and disordered septation. Wild-type , L variants, revertants and atypical-cell-wall variants released mycobacteriophage particles. These bacteriophages were almost identical in respect to morphology, host range, and neutralisation by antiserum. The results obtained suggest strongly that all types of cells examined were derived from .


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