Effect of growth rate and glucose concentration on the biochemical properties of Streptococcus mutans Ingbritt in continuous culture

Abstract

A comparison was made of the properties of Streptococcus mutans Ingbritt grown in continuous culture under conditions of excess glucose (nitrogen limitation) and limiting glucose at mean generation times of 1.7 to 14 h. Only low levels of glucoamylase-specific glycogen were formed in cells from either culture, and the total carbohydrate content of the cells under excess glucose was only at most 1.6-fold higher than in the glucose-limited culture. A negligible amount of cell-free polysaccharide was formed in either culture, although a significant level of glucosyltransferase activity was observed in both, with the highest activity at D = 0.2 and 0.4 h(-1) with a glucose limitation. Other differences were observed. (i) Lactate was the main end product of the glucose-excess culture, whereas acetate, formate, and ethanol were the main products of the glucose-limited culture except at a mean generation time of 1.5, when lactate represented 30% of the products. (ii) The yield (in grams per mole of glucose) of the latter culture was 2.6- to 4.0- fold higher than the yield of the glucose-excess culture. (iii) Washed cells from the glucose-limited culture were much more acidogenic (1.7- to 6.2-fold) than the glucose-excess cells when incubated with glucose, sucrose, and fructose. Endogenous glycolytic activity by the latter cells was significant, being 31 to 92% of the exogenous glucose rate at the four dilution rates. (iv) Cells from the glucose-excess culture were more insensitive to fluoride than cells from the glucose-limited culture. The NaF 50% inhibition dose values for the effect of fluoride on the metabolism of glucose, sucrose, and fructose were calculated for the four dilution rates at four pH values. This analysis indicated that rapidly metabolizing cells were more sensitive to fluoride than cells that metabolized the sugars more slowly.