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Lactate Acid Inhibition of Salmonella Typhimurium in Yogurt

Department of Environmental and Industrial Health, School of Public Health, The University of Michigan, Ann Arbor 48109

ABSTRACT

We determined how lactic acid inhibits growth of Salmonella typhimurium in yogurt. 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°C, indicating that lactic acid inhibition is irreversible. The growth rate of S. typhimurium at 37°C was computed at various combinations of pH and lactic acid concentrations, and the intracellular conditions (pH and lactic acid concentration) at bacteriostasis subsequently were extrapolated. Cellular death resulted when these intracellular bacteriostatic conditions were surpassed. Thus, growing cells could be used indirectly to determine intracellular conditions at the time of death. Intracellular pH (pH_i) and inhibition of the growth rate were unrelated. Also, bacteriostasis was observed when hydrochloric acid was used to lower the pH_i of Salmonella to 5.5 whereas a bactericidal effect was observed when the pH_i was lowered to 5.5 with lactic acid. The lactate anion, rather than the hydrogen ion, exerted the inhibitory effect against S. typhimurium. When the pH_i became less than 5.3, inhibition was from the hydrogen ion concentration. Thus, lactic acid inhibition was a complex and variable mechanism in relationship with pH_i. Lactic acid entered the cell in the undissociated state. Once inside the cell, it dissociated because the pH_i was higher than the external pH. The dissociated moiety accumulated because it could not leave the cell in this form, consequently lowering the pH_i. Thus, inhibition of S. typhimurium in yogurt is from the intracellular dissociated moiety of lactic acid.

¹ Laboratory of Soil Microbiology, Department of Agronomy, Cornell University, Ithaca, NY 14853.

² American Type Culture Collection, 12301 Park-lawn Drive, Rockville, MD 20852.