Survival of acid-adapted Salmonella typhimurium in fermented millet and acidified broth at different storage temperatures

Abstract

Salmonella typhimurium KSN 533 was adapted to acid by exposure to lactic acid at pH 5.0 for 18 h. Subsequently, acid-adapted and non-adapted cells were challenged with lactic acid (pH 3.8) in Brain-heart infusion (BHI) at 4°C and 30°C. Acid-adapted and non-adapted cells were also inoculated into already fermented millet broth (pH 3.8) and at beginning of millet fermentation with Lactobacillus fermentum starter culture. Survival curves of acid-adapted and non-adapted cells at pH 3.8 were fitted with the Weibull distribution model. Acid-adapted cells were generally resistant to subsequent acid stress than non-adapted cells. Regardless of adaptation, cells were more sensitive to acid shock (pH 3.8) at 30°C than at 4°C storage. Whereas both acid-adapted and non-adapted S. typhimurium cells survived in appreciable numbers of 5.5 and 3.5 log cfu/ml respectively after 72 h storage at 4°C, no viable cells were detected for both acid-adapted and non-adapted cells after 12 an 9 h respectively at 30°C. Acid-adaptation induced protection against lethal acid and cross protection against cold stresses. Regardless of adaptation, viable population of Salmonella showed a slight increase during the early stages of millet fermentation. Similar inactivation rates were observed for both acid-adapted and non-adapted cells when inoculated at the beginning of fermentation. Thus acid adaptation does not appear to influence survival of S. typhimurium when inoculated at the beginning of the fermentation although acid-adapted cells survived better in already fermented millet.

Keywords: acid-adaptation, millet fermentation, Salmonella, lethal acid challenge