The change in heat inactivation of Escherichia coli O۱۵۷:H۷ after entering into the viable but non-culturable state in salted fish, Hypophthalmichthys molitrix

Many species of non-sporulating bacteria including Escherichia coli can enter into a viable but non-culturable (VBNC) state under stress conditions. In this study, the change in thermal resistance of E. coli O۱۵۷:H۷ after entering into the VBNC state in salted silver carp, Hypophthalmichthys molitrix, was investigated. E. coli O۱۵۷:H۷ was inoculated on the fish control group (TF) and on those fish with ۳۰% NaCl (TF + ۳۰% NaCl) at room temperature. Culturability of bacteria was determined using routine culture and colony counting on Sorbitol MacConkey agar. When bacteria were non-cultivable, the RT-PCR of ۱۶S rRNA gene (including direct extraction and purification of RNA, DNase I treatment for removing DNA contamination, cDNA synthesis and electrophoresis of PCR products of cDNA) was used to detect VBNC E. coli O۱۵۷:H۷. Also, cultivable and VBNC E. coli O۱۵۷:H۷ were individually heat-treated at ۵۵, ۶۲ and ۷۰ °C for ۵ min. The samples were cooled and after ۲۴ h, the thermal resistance of bacteria was determined through viability detection using RT-PCR of ۱۶S rRNA gene. The culturability of bacteria was kept in fish treatment but they were non-cultivable in fish under ۳۰% NaCl after ۵ days. The positive expression of ۱۶S rRNA in all studied treatments indicated the entering of E. coli O۱۵۷:H۷ into the VBNC state in fish treatment under ۳۰% NaCl. Moreover, the RT-PCR of ۱۶S rRNA gene showed that only VBNC forms of E. coli O۱۵۷:H۷ showed viability at ۶۲ °C for ۵ min which indicated the increased resistant of VBNC bacteria to the thermal inactivation. The ability of E. coli O۱۵۷:H۷ to enter into VBNC state in salted fish and also the increase of its thermal resistance suggest that VBNC E. coli O۱۵۷: H۷ can be considered as critical threat to public health and food safety.

کلیدواژه ها:

Escherichia coli O۱۵۷:H۷ ، VBNC state ، Thermal resistance