Survival of Acid-adapted Escherichia coli O157:H7 and non-O157:H7 Shiga Toxin-Producing E. coli (STEC) During Processing of Dry, Fermented Sausages
Open Access
- Author:
- Rivera, Minerva
- Graduate Program:
- Food Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 27, 2017
- Committee Members:
- Catherine Nettles Cutter, Dissertation Advisor/Co-Advisor
Catherine Nettles Cutter, Committee Chair/Co-Chair
Edward G Dudley, Committee Member
Sara Milillo, Committee Member
Jonathan Alexander Campbell, Outside Member - Keywords:
- fermented sausages
E. coli O157:H7
non-O157 STEC
acid adaptation - Abstract:
- Escherichia coli O157:H7 was first recognized as a human pathogen following outbreaks of hemorrhagic colitis in the early 1980s. Due to the infectious dose of E. coli O157:H7, the severity of the disease, and the possibility of lifelong sequalae, the U. S. Department of Agriculture-Food Safety and Inspection Service (USDA-FSIS) declared it as an adulterant in raw ground beef in 1994. In 2012, non-O157:H7 Shiga toxin-producing E. coli (STEC) serogroups (O26, O45, O103, O111, O121, and O145) also were declared adulterants in raw beef products by USDA-FSIS. Until recently, the majority of the research done on interventions used to control STEC has been done on E. coli O157:H7. However, recent research suggests that some non-O157:H7 STEC may be more resistant to some interventions than E. coli O157:H7. The purpose of this study was to assess the differences in survival between E. coli O157:H7 and non-O157:H7 STEC when exposed to acid adaptation, followed by various stressors typically encountered during processing of dry, fermented sausages (DFS), including fermentation, lowered water activity (aw), and long-term, vacuum packaged storage (VPS). Experiments were conducted in vitro and in situ by exposing STEC to low pH, low water activity, and/or desiccation. In a series of in vitro experiments, control (non-acid adapted) and acid-adapted STEC cocktails were exposed to modified TSB (lowered pH and lowered aw; 24°C for 4 days), subjected to desiccation on paper disks, and evaluated for survival. In situ experiments were carried out in modified ground beef slurries (with lowered pH and lowered aw; 24°C for 4 days). Finally, acid-adapted E. coli O157:H7 and non-O157:H7 STEC were evaluated for survival throughout processing of DFS, including fermentation, drying, and long-term VPS. Results from the first in vitro experiments showed that populations of the control non-O157:H7 STEC serogroups O45, O103, O111 and O121 were significantly different from populations of E. coli O157:H7,and non-O157:H7 STEC serogroups O26 and O145. In contrast, when STEC serogroups were acid-adapted, all populations of non-O157 STEC were significantly different from populations of E. coli O157:H7. There were no significant differences between control (non-adapted) and acid-adapted cells between any individual serogroup. Reduction rates for all serogroups demonstrate that they not only survived well in acidified TSB, but also were able to grow over time, albeit minimally. Results from the second in vitro experiment, TSB with modified water activity (aw=0.88 or 0.78), demonstrate that there was no significant difference between E. coli O157:H7 and any of the non-O157:H7 STEC, regardless of acid-adaptation. In a third in vitro experiment, exposure to desiccation on dry discs, there was no significant difference in survival between E. coli O157:H7 and the non-O157:H7 STEC, regardless of acid-adaptation. Results from an in situ experiment, acidified ground beef slurries, indicated that all non-O157:H7 STEC behaved similarly to E. coli O157:H7, with the exception of O145 STEC which had a higher reduction than O157:H7. There were no significant differences in ground beef slurries with modified water activity. These results suggest than non-O157:H7 STEC behave similarly to E. coli O157:H7 when exposed to laboratory stress conditions of low pH (pH 4.5) and low water activity (0.88 or 0.78). These results are in agreement with other researchers; interventions that work on O157:H7 are likely to work on non-O157:H7 STEC. Experiments conducted with DFS processing demonstrated that the highest overall reductions in non-O157:H7 STEC was observed for E. coli O26 (2.66 log10 CFU/g), while the lowest were observed for E. coli O111 (2.29 log10 CFU/g) following fermentation, drying, and long-term VPS. All serogroups demonstrated total reductions >2.0 log10 CFU/g and there was no significant difference between the log reduction rates of non-O157:H7 STEC, when compared to E. coli O157:H7. These results suggest that non-O157:H7 STEC survival during the production of a DFS was comparable to E. coli O157:H7 and that all tested STEC serogroups are able to survive DFS processing. Overall, results also suggest that acid-adaptation had minimal or no significant effect on the survival of any of the tested serogroups. Moreover, results suggest that interventions that work for E. coli O157:H7 will be equally effective against non-O157 STEC. These results could be useful to regulatory agencies by providing the science needed to make recommendations and/or regulations to mitigate the risk of non-O157:H7 STEC in the food industry. Additionally, these results may be of use to researchers and/or quality control personnel seeking to validate interventions and processes to ensure the safety of dry fermented sausage products for the consuming public.