An interaction of the commensalism type between P. vulgais and S. cerevisiae was discovered and verified. A continuous flow apparatus for investigation of this interaction was described. A Coulter Counter was used for detecting the number of the microorganisms grown in mixed cultures. Size distribution curves were prepared and used as a means for determining the Coulter Counter settings for counting each organism. Coulter Counter counts were compared with Petroff-Hausser counts; the results showed approximately 20 per cent discrepancy which is reasonable since the Petroff-Hausser method is quite crude.

P. vulgaris in mixed culture and in the media of Table I was shown to be completely dependent upon S. cerevisiae. S. cerevisiae was found to be unaffected by P. vulgaris, thus, the definition of commensalism is fully satisfied. The search for the biochemical causing this interaction led to the trial of the following: 1 per cent yeast extract, 0.01 per cent DPN, 1 per cent Nicotinamide, and 0.01 per cent Niacin; all sterilized by filtration. The slug addition of these biochemicals to the culture vessels caused the P. vulgaris to be independent of the S. cerevisiae. As a result, Niacin or near biochemical relatives are believed to be the essential compounds causing the interaction between P. vulgaris and S. cerevisiae. The steady state population of the P. vulgaris was found to increase with Niacin concentration up to a certain limit above which further Niacin has no effect. The concentration of Niacin-like activity formed per yeast cell was also calculated; this concentration per yeast cell was found to vary with the flow rate. Concentration of the media and aeration were also found to have an effect on the steady state populations.

Inoculating cultures of S. cerevisiae with a faster growing bacterium, E. coli, showed E. coli increasing in number until it reached a steady state while S. cerevisiae decreased in number to a null value. This result was in agreement with that predicted by published theories of contamination, Golle (1953).