INFLUENCE OF PROPIONATE ON GROWTH AND FERMENTATIVE ACTIVITY OF LACTOBACILLI

Propionic acid was added to cultures of Lactobacillus helveticus and L act. casei growing in a complex medium. The effect of different concen trations of this fatty acid on the growth kinetic, substrate consumpti on rate and coefficient and product formation rate was examined. Low c oncentrations of propionic acid produced a low increase or no modifica tion of the growth rate of lactobacilli. By contrast, higher concentra tions reduced the growth rate and substrate yields in all the strains. The fermentative activity of lactobacilli increased in all the cultur es with propionate. When the concentration of glucose in the medium wa s low, the population density of all the strains at the stationary gro wth phase was lower in the presence of propionate than in control cult ures. An increase in the concentration of glucose in the medium counte racted the inhibitory effect of propionate. The addition of lithium la ctate to the medium gave greater inhibition even at low propionate con centrations. The inhibition results were explained by assuming that pr opionic 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 activi ty of the cells. When metabolism cannot supply the ATP required for pr oton extrusion, growth rate and biomass production decrease.