Lactate Acid Inhibition of Salmonella Typhimurium in Yogurt

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 degrees C, indicating that lactic acid inhibition is irreversible. The growth rate of S. typhimurium at 37 degrees 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 (pHi) and inhibition of the growth rate were unrelated. Also, bacteriostasis was observed when hydrochloric acid 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 the hydrogen ion, exerted the inhibitory effect against S. typhimurium. When the pHi 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 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. Thus, inhibition of S. typhimurium in yogurt is from the intracellular dissociated moiety of lactic acid.

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Modeling the effect of temperature on survival rate of Salmonella Enteritidis in yogurt

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“…An inactivation of pathogenic bacteria in fermented dairy products, including yogurt, has been observed in many studies (Rubin and Vaughan 1979, Rubin et al 1982, Alm 1983, Kotz et al 1990, Issa and Ryser 2000, Hlvarez-Ordóñ ez et al 2013, Cirone et al 2013. The main mechanism of bactericidal effect of yogurt on foodborne pathogens seems to be the decline in pH due to lactose fermentation by Lactobacillus delbrueckii ssp.…”

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