Pure cultures o! the yeast S. cerevisiae do not follow the classical continuous culture theory when tested in a variety or chemostats. Using a Coulter Counter, yeast populations were sized and enumerated electronically over a range or dilution rates. Total dry weight and Kjeldahl nitrogen per cell were determined for a variety or dilution rates. From these data it was shown that the yeast definitely increased in size as the dilution rate increased. Also, it appeared that the ratio of rate of synthesis of cell wall material to the synthesis of proteins decreased as the dilution rate increased.

Step changes in various environmental factors (dilution rate, medium concentration, glucose concentration, ammonium sulfate, and temperature) indicated that yeast populations may not show a smooth transition from the initial to the final steady states. Decaying oscillations of the yeast population were observed in response to step changes in dilution rate and glucose concentrations. Step changes in ammonium sulfate or in total medium concentrations resulted in smooth responses to a new population level.

To investigate the complex responses to dilution rate and glucose a frequency response analysis was employed. The growth of S. cerevisiae varied in a sinusoidal fashion in response to sinusoidal forcing of dilution rate and medium concentration. Bode diagrams were prepared for yeast response to dilution rate and medium concentration. Time constants were determined from the Bode diagrams. The time constants were in the order of several minutes and possible relations to growth control mechanisms can be postulated. The time constants probably pertain to accumulation of intracellular materials.

A two-stage continuous culture system was built to investigate the interaction of E. coli and S. cerevisiae. By feeding a continuous pure yeast culture from the first stage into the mixed culture second stage it was found that the E. coli apparently produces a substance that inhibited the growth of yeast.