Biological Studies of Saprophytic Acid-Fast Organisms: I. Dissociation of Mycobacterium phlei

The group of saprophytic acid-fast organisms represent a large number of organisms isolated from various sources and labeled as different species. The degree of chromogenicity, the rapidity of growth, the source of isolation and other minor physical and biological characteristics have been used for differentiation. An attempt by a number of workers to classify them has proved futile. From our study of M. phlei, where some eight or perhaps ten different variants were isolated and studied, we are inclined to believe that in all probability most saprophytic acid-fast cultures mentioned above are not different species, but variants from what appears to be one single strain. It seems that at present we should abandon any attempt to devise a classification of the group until we acquaint ourselves with the utmost limits of variability which exist freely among the saprophytic acid-fast organisms. We have made no attempt yet to label these variants, as they are too numerous and many of them are unstable. Dissociation of M. phlei is not a simple phenomenon of splitting "S" from "R" types, etc. We have observed the reaction going on in plate cultures and superficially we considered it as a simple process, but as we became more deeply involved, we realized that there are many transitional stages which could have escaped detection. These transitional colonies are so unstable that topographical appearance may be modified within a few days. The chromogenicity of the variants is also an unstable characteristic. The medium, light, temperature, carbon-dioxide and oxygen supplies in the cultures play important parts in the development of this characteristic. We cannot recommend too strongly uniform medium constituents and proper handling of the cultures. Our most uniform results were obtained when we used the special flat culture flask for the solid medium. Not a single yellow or white variant, when inoculated in guinea pigs, produced progressive disease. Most of the animals developed similar lesions to those described in the first part of this paper. However, we have seen some progressive disease in guinea pigs inoculated with modified M. phlei, and using a special method of inoculation, but the results were so inconstant and variable that at present we defer commitment until we have eliminated the disturbing factors. This failure, however, does not eliminate the possibility of the enhancement of virulence. We have not yet used all the variants and the transitional colonies