Lactate Acid Inhibition of Salmonella Typhimurium in Yogurt

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Abstract
How lactic acid inhibits growth of S. typhimurium in yogurt was determined. This inhibition was demonstrated by microscopic examination not to be due to bacteriolysis. Neither growth nor metabolic activity could be initiated after cells were washed in phosphate buffer and exposed to 1.5% lactic acid for 1 h at 37.degree. C, indicating that lactic acid inhibition is irreversible. The growth rate of S. typhimurium at 37.degree. C was computed at various combinations of pH and lactic acid concentrations, and the intracellular conditions (pH and lactic acid concentration) at bacteriostasis were subsequently extrapolated. Cellular death resulted when these intracellular bacteriostatic conditions were surpassed. Growing cells could be used indirectly to determine intracellular conditions at the time of death. Intracellular pH (pHi) and inhibition of the growth rate were unrelated. Bacteriostasis was observed when HCl was used to lower the pHi of Salmonella to 5.5, whereas a bactericidal effect was observed when the pHi was lowered to 5.5 with lactic acid. The lactate anion, rather than H, exerted the inhibitory effect against S. typhimurium. When the pHi became < 5.3, inhibition was from the H+ concentration. Lactic acid inhibition was a complex and variable mechanism in relationship with pHi. Lactic acid entered the cell in the undissociated state. Once inside the cell, it dissociated because the pHi was higher than the external pH. The dissociated moiety accumulated because it could not leave the cell in this form, consequently lowering the pHi. Inhibition of S. typhimurium in yogurt is from the intracellular dissociated moiety of lactic acid.