Reaction to Gram's stain by certain spore-forming bacteria.

It has been customary to consider that the greater number of the spore-forming bacteria are positive in their reaction to Gram's stain. The positive correlation of these two properties has been considered very nearly perfect with the mesophilic bacteria. It might be expected that the same correlation would be obtained with the thermophilic bacteria. Various investigators working with the spore-forming thermophiles have reported some of these organisms to be Gram negative. The stain was made usually upon cultures at the end of 24 hours', and in some cases 48 hours', incubation. These statements would give further exceptions to the Gram-positive/spore-formation correlation of the bacteria, when their temperature requirements are disregarded.

It has been the experience of the author that some of the spore-forming thermophilic bacteria are Gram negative at the end of 24 hours' incubation. These organisms, however, were positive in younger cultures. Many strains began to lose their power of retaining the violet stain at the end of 8 hours' incubation. Opinion might differ as to whether this would destroy the Gram-positive/spore-formation correlation. The fact, however, that the organisms were positive in young cultures should be sufficient evidence to prove that the correlation is confirmed. It is probable that the change in the staining reaction has a metabolic significance, which might be utilized in systematic bacteriology. It is believed that this characteristic should be investigated and the results recorded with the description of the organism.