This communication describes a rapid, simple and reliable method for the determination of the cellular RNA content. In the study of microbial growth and product formation the cellular RNA content is a good measure of the activity of the biomass. Results from applying the method to monitor transient experiments in a chemostat with Lactococcus cremoris are presented.INTRODUCTION.
s. B E N T H I N A N D J. VILLADSEN. i~i .Inhibition of Lactobacillus bulgaricus by lactic acids at constant p H is governed by two factors : the total concentration of lactic acids and the D/L ratio of lactic acid stereoisomers. L-Lactic acid is more inhibitory than the D-isomer, the latter being the only isomer produced by Lact. bulgaricus. T h e major inhibitory effect is an extremely prolonged lag phase (2-39 h for the range CL25 g 1-' added lactic acids). Added lactic acids reduce the maximum cell concentration obtained in pH controlled fermentations, but have virtually no effect on maximum specific growth rate and on the yield of biomass on produced lactic acid. The added lactic acids have no effect on the growth-related ATP consumption nor on the non-growth related ATP consumption. T h e culture is heterogeneous since a part of the culture is unable to grow in the presence of L-lactic acid. A different inhibitory effect of D-and L-lactic acid has implications for yoghurt manufacture and potential for improved food preservation.
Quantitative analyses of the utilization of amino acids by Lactococcus luctis subsp. crernoris FD1 in yeast extract medium (YE) and in casein peptone medium (CP) have been performed. Both free and peptide-bound amino acids were measured. In the CP most amino acids are peptide-bound and some amino acids are virtually only present in peptides. Thirty-six per cent of all peptide bonds in CP are hydrolysed during fermentation (6.3 mmol peptide bonds per gram biomass formed) and there is a transition of the growth rate related ATP consumption Yx.4TP (mmol ATP g biomass-') from 25 mmol g -' to 71 mmol g-' coincident with a decrease of the peptide consumption. In YE most of the amino acids are o n the free form and only 26% of the peptide bonds are hydrolysed during fermentation (1.5 mmol peptide bonds per gram biomass formed). A constant Y,.\.,.,. = 38 mmol g-' prevails throughout the fermentation in YE.
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