Abstract
Viable but non-culturable (VBNC) cells are alive bacteria cells, but lose their culturability in conventional culture media, usually escape detection by the plate count method and pose a serious threat to food safety and public health. Therefore, it is urgent to study the VBNC status, and to provide theoretical basis and scientific basis for food processing and safety control caused by pathogenic microorganisms. In this study, Escherichia coli O157:H7 was induced to the VBNC state at two different temperatures (-20°C and 4°C) and its resuscitation and morphological changes under different nutritional conditions were studied. The initial inoculum of 2.1 × 107 CFU/mL E. coli O157:H7 cells were induced into the VBNC state in normal saline, distilled water, LB broth at -20 °C after 176, 160, 80 days, respectively. The results showed that E. coli O157:H7 reserved at -20°C, and LB culture medium were easier to enter VBNC state than others conditions, the cells still had metabolic activity and the cell morphology changed from the typical rod shape to short rod and the cell size decreased. The resuscitate ways including the direct warming resuscitation, gradual warming resuscitation, adding chemical substance resuscitation, and adding nutrients resuscitation were studied. The results showed that the optimal conditions of 5% Tween 80 and 3% Tween 80 acculated the resuscitation of E. coli O157:H7 VBNC state cells induced by low temperature LB medium and low temperature saline. E. coli O157:H7 VBNC state failed from resuscitation when incubating in LB broth, respectively using direct warming and adding nutrients substance. This study provides new insights into induction and resuscitation of VBNC E. coli O157:H7 and offers an approach for investigating the formation mechanism of VBNC foodborne pathogens in food safety.
Highlights
Foodborne pathogens are tremendous threats to food safety and global health issues, which can cause severe disease in humans via contaminated water or food
When resuscitation of the viable but non-culturable (VBNC) cells was performed by increasing temperature from 15◦C to 37◦C, the results showed that the method was unable to induce resuscitation of E. coli O157:H7 and none colonies appeared on the plate (Table 1)
The initial inoculum of 2.1 × 107 CFU/mL E. coli O157:H7 cells were induced into the VBNC state in normal saline, distilled water, Luria Bertani broth (LB) broth at −20◦C after 176, 160, 80 days, respectively
Summary
Foodborne pathogens are tremendous threats to food safety and global health issues, which can cause severe disease in humans via contaminated water or food. When conditions are unsuitable for sustaining normal growth, this state is a survival strategy by many bacteria In this physiological condition, the cells that could not form colony on enriched agar media, but exhibit detectable metabolic function. In Japan, the VBNC state of E. coli O157:H7 cells was found to be widespread in a natural freshwater environment and some caused gastroenteritis outbreak in salted salmon roe (Ohtomo and Saito, 2001; Zhao et al, 2017, 2018). This suggests that the VBNC pathogens could bring serious risks to food safety and public health
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