INFLUENCE OF PROPIONATE ON GROWTH AND FERMENTATIVE ACTIVITY OF LACTOBACILLI

Propionic acid was added to cultures of Lactobacillus helveticus and Lact. casei growing in a complex medium. The effect of different concentrations of this fatty acid on the growth kinetic, substrate consumption rate and coefficient and product formation rate was examined. Low concentrations of propionic acid produced a low increase or no modification of the growth rate of lactobacilli. By contrast, higher concentrations reduced the growth rate and substrate yields in all the strains. The fermentative activity of lactobacilli increased in all the cultures with propionate. When the concentration of glucose in the medium was low, the population density of all the strains at the stationary growth phase was lower in the presence of propionate than in control cultures. An increase in the concentration of glucose in the medium counteracted the inhibitory effect of propionate. The addition of lithium lactate to the medium gave greater inhibition even at low propionate concentrations. The inhibition results were explained by assuming that propionic acid could diffuse through cell membranes in the undissociated form, increasing the inward leak of H+ in the cells. The extrusion of H+ by the H+-ATPase is possible by increasing the fermentative activity of the cells. When metabolism cannot supply the ATP required for proton extrusion, growth rate and biomass production decrease.