关键词: 大肠杆菌O26；热休克；乳酸应激；存活；应激基因

Abstract: The aim of this study was to investigate the effects of heat shock-lactic acid stress on the survival of and the expression of related genes in Escherichia coli O26. First, 22 bovine-borne O26 strains were collected and their tolerance to hydrochloric acid and lactic acid were compared. Then, strains with different tolerance to lactic acid were selected to evaluate their survival under heat shock-acid stress conditions. Finally, one representative strain was further selected to examine the differential expression of the general stress response gene rpoS, acid stress response genes (asr, ycfR, gadA), and heat stress response genes (rpoH, dnaK, clpB, and groEL) at 2 and 4 h of acid stress treatment using real-time polymerase chain reaction (real-time PCR). Results showed that the survival of all 22 strains significantly decreased (P < 0.05) after treatment with lactic acid or hydrochloric acid for 2 h. Nevertheless, acid tolerance varied among strains and the tolerance of each strain to lactic and hydrochloric acid was also different. Compared with the control group cultured in normal?tryptic soy broth (TSB), the survival rate of a mixed culture of five O26 strains gradually decreased in the acid stress and heat shock-acid stress groups, being higher in the latter than in the former, indicating that heat shock exerted a cross-protective effect and enhanced the acid resistance of O26. Real-time PCR detection showed that acid stress induced the down-regulation of stress response gene expression, whereas the expression of these genes increased significantly in heat shock-acid stress group compared with the acid stress group, indicating that the cross-protection of heat shock was related to the increased expression of the genes. It is suggested that when the combination of non-lethal heat treatment and acidification is applied in food processing, more attention should be paid to the potential of cross-protection to cause increased food safety risk.

Key words: Escherichia coli O26; heat shock; lactic acid stress; survival; stress response gene