Stress-induced L-forms of Mycobacterium bovis: a challenge to survivability.

This study addressed the ability of Mycobacterium bovis to produce unusual extreme morphologic forms (cell wall-deficient or L-forms) under stress conditions. Models using nutrient starvation and cryogenic stress treatments of Mycobacterium bovis, as well as the filtration technique followed by cultivation in semisolid medium, were used for isolation of L-form variants. Morphological transformations and developmental stages, typical for the bacterial L-cycle were observed by electron microscopy. Of special interest was the formation of giant filaments and common extremely thick membranous structures enveloping the entire L-form population. Following collapse of giant filamentous structures small viable cell elements, mainly granules and coccobacilli, were released and proved able to grow into large bodies or multiply by fission or budding. Derivation of viable filterable forms from L-form cultures and parental strain and their identification as Mycobacterium bovis based on specific IS6110 PCR was noteworthy. We suggest that formation of giant filaments and thick common membranous envelopes, observed under stress conditions, may serve a twofold purpose - protection against an unfavourable environment, and a role in reproduction of Mycobacterium bovis L-forms. The observed L-form conversion phenomenon in Mycobacterium bovis seems to be associated with an adaptive strategy of this pathogen for survival and reproduction in an unfavorable environment.