SUMMARY: Lactobacilli, streptococci, leuconostocs, pediococci and aerococci were examined in a soft agar medium which provided aerobic and anaerobic conditions in the one culture. The relation to oxygen varied in some instances with the energy source and the method by which it was sterilized. Other variations were determined by the temperature of incubation, the pH value of the medium and the salt concentration. Only a proportion of the organisms showed preferences or requirements for diminished aeration or anaerobic conditions. In some cases an aerobic environment was optimal or essential for the utilization of a fermentable substrate. The soft agar medium showed that the fermentation of sugars and other substances might occur as a sequel to mutation. Liquid media were compared with soft agar media; they provided little information about oxygen relationships. This comparison suggested several possible reasons for the variability of lactic acid bacteria reported by investigators who used liquid media in tests for fermentation.


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