(A) chemostatic study on graowth responses of lactobacillus bulgaricus NLS-4 to culture conditions

Abstract

The influence of culture conditions on the growth of $\mbox{\underline{Lactobacillus}}$ $\mbox{\underline{bulgaricus}}$ NLS-4 was studied. The organism was cultured continuously in a chemostat with media having limited amount of glucose by which the growth was regulated. When each culture reached steady state, the culture broth was sampled and analyzed for biomass, metabolites and residual substrate as well as microscopical observation of the cell morphology. These continuous cultures were originally designed using growth parameters obtained simply through batch cultures. The specific growth rate which was represented by the dilution rate in the steady-state continuous culture was found to have a close relationship with the fermentation balance of this organism. Under the glucose-limited continuous culture, the L. bulgaricus cells which have long been known as homofermentative lactic acid bacteria showed their typical character only when they grew at the growth rates higher than 0.45 $hr^{-1}$. When they grew slower than this, considerable amounts of formate, acetate and ethanol appeared in the culture solution besides the lactate indicating a heterofermentative nature. The values of $Y_{ATP}$, which is the growth yield per ATP molecule, also varied within the range of 13.3 to 23.5 g cell/mole ATP depending upon the specific growth rate. These values are significantly higher than the average $Y_{ATP}$ value of 10.5 which has been widely recognized since the discovery by Bauchop \& Elsden. The difference may be due to the less need of maintenance energy under the present culture conditions. The level of dissolved oxygen in the culture medium affected on the fermenatation balance, maintenance energy, yield coefficient based on glucose ($Y_{glc}$) and the internal enzyme system. The homofermentative character of $\mbox{\underline{L.}}$ $\mbox{\underline{bulgaricus}}$ NLS-4 was maintained when the dissolved oxygen tension was as low as 15.6 mmHG. Above this level, ...