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The yeasts Saccharomyces cerevisiae, Candida utilis, and Candida lipolytica were used to investigate the action of different concentrations of fatty acids (from acetic to myristic acid) on cell growth, division, uptake of inorganic phosphate, and substrate oxidation. The former two yeasts were found to undergo an inhibition of growth, cell division, and phosphate uptake at lower acid concentrations and to experience the inhibition of substrate oxidation at higher acid concentrations. The concentration dependence of the action of fatty acids can be classified into four categories: 1) subthreshold concentrations which do not inhibit growth and have either no effect on, or stimulate, oxygen consumption; 2) threshold concentrations which lower the rate of growth, cell division, and phosphate uptake but do not inhibit the oxidation of carbon substrate; 3) above-threshold concentrations which inhibit partially even substrate oxidation, and 4) microbicide concentrations. Candida lipolytica displays the same sensitivity toward the action of fatty acids as the above yeast species; however, the threshold concentrations are higher and can be quickly lowered owing to oxidation by the yeast. The concentrations of fatty acids found in the medium after cultivations of yeast with n-alkanes are of the same order as limiting concentrations; the formation of acids with twelve and less carbons in the molecule can thus be assumed to be one of the basic reasons for lowering of biomass yields during cultivations on these hydrocarbons.

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The production of bacterial enzymes in batch fermentations is compared with results obtained in continuous culture. When studying the production of alpha-amylase in Bacillus subtilis it was found that instability of the enzyme synthesis was due to nonhomogeneity of the population rather than to "the culture's history" (i.e. succession of several physiological states necessary for the enzyme production). The plasmid contained in the production clone was found to be the factor responsible for the alpha-amylase production. Predominance of the production clone or of the nonproduction one depends on the cultivation conditions used. As compared with batch cultivation the continuous production yields higher enzyme concentrations under optimal conditions and the fermentor productivity may be four to five times higher.