Behaviour of an Escherichia coli strain deleted in the salt stress response gene kdpA during production and ripening of a semi-hard cheese

In order to survive cheese production and cheese ripening Escherichia coli needs to adapt to the encountered stress such as the raised salt concentrations. Such adaptation typically involves the induction of stress response genes. Therefore, impaired or lacking activity in such genes may considerably affect survival of E. coli upon stress exposure such as encountered during cheese production. In previous in vitro studies, an association between E. coli stress response gene expression and survival observed in raw milk cheese was found, indicating that reduced or absent induction of the salt stress response gene kdpA might have an impact on survival of an E. coli strain during raw milk cheese production. In this study, therefore, the survival of an E. coli strain deleted in the kdpA gene was investigated in comparison to its wildtype strain during production and ripening of semi-hard cheese. After an initial increase in E. coli counts a slow decrease of 1.80 and 1.38 log(10) cfu/g until the end of the 16 week ripening period was observed for the kdpA-deletion and the wild-type strain, respectively, without significant differences between these strains. This indicates that lack of the kdpA gene did not considerably affect E. coli survival in cheese and therefore other stress response mechanisms are probably of higher importance for survival of E. coli in raw milk cheese.