Effect of sodium chloride on the survival of Shigella flexneri in acidified laboratory media and cucumber puree.

Abstract

This study investigated the effects of sodium chloride (NaCl) and various acids, alone or in combination, on Shigella flexneri growth in laboratory medium and cucumber puree. Shigella flexneri was treated with various acids (acetic, citric, malic, tartaric, propionic, lactic and phosphoric acid) alone or with 3, 6 or 9% NaCl. Pronounced antagonistic effects were observed in Sh.flexneri treated with acetic or lactic acid in combination with 3% NaCl. Next, Sh.flexneri was pre-exposed to 3% NaCl and then treated with various acids; acid-stressed cells were then inoculated onto agar plates containing 3% NaCl. There was no significant difference in the reduction of Sh.flexneri, regardless of treatment (P>0·05). Finally, Sh.flexneri was inoculated into cucumber puree to which various concentrations of acetic acid had been added with and without 3% NaCl. Antagonistic effects were observed with a treatment of either 0·5 or 1% acetic acid combined with 3% NaCl. Antagonistic effects were observed when Sh.flexneri was exposed to acetic or lactic acid with NaCl. This study shows that depending on the type of acid, the addition of NaCl can increase the resistance of Sh.flexneri to acid treatments. This may provide useful information for developing methods of preserving acidified foods.