DNA analysis demonstrates that mycococcus forms are not mycobacteria.

Previous experimental evidence has suggested that the mycobacteria may exist in morphological forms other than the well characterised acid-fast bacillus. However none of these studies have been able to show conclusively that these variant forms are derived from the mycobacteria and therefore much debate has centered around the exact nature of these organisms. In this study we have examined stored cultures of the mycococcus form of Mycobacterium bovis BCG and M. phlei which were prepared by Csillag in 1972 and 1969. Restriction fragment patterns of the DNA of the variant forms and the parent mycobacteria were not similar and chromosomal DNA from the parent mycobacteria did not hybridise with the DNA of the variant forms. Furthermore biochemical studies indicate that the variant forms are environmental contaminants. Although this study shows conclusively that the mycococcus is not derived from the mycobacteria, we believe that the search for variant morphological or metabolic forms of the mycobacteria should remain an active area of investigation, but that no study should be considered complete without the full use of newer molecular technology.