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Research Project: Intervention Strategies for Controlling Human Pathogens Associated with Fermented and Acidified Vegetables

Location: Food Science and Market Quality and Handling Research Unit

Title: Escherichia coli O157:H7 stationary phase acid resistance and assessment of survival in a model vegetable fermentation system

item JONES, CLARA - North Carolina State University
item Price, Robert
item Breidt, Frederick

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2019
Publication Date: 5/1/2020
Citation: Jones, C.M., Price, R.E., Breidt, F. 2020. Escherichia coli O157:H7 stationary phase acid resistance and assessment of survival in a model vegetable fermentation system. Journal of Food Protection. 83(5):745-753.

Interpretive Summary: The microbial ecology of vegetable fermentations results in the predominance of lactic acid bacteria that effectively kill disease causing bacteria. While it is well known that the fermentation environment is lethal to most bacterial pathogens, some important details about the time and conditions needed for safety remain to be determined. To help fill this void, we have investigated how the acid resistance of disease-causing Escherichia coli (E. coli) strains influenced survival. Surprisingly, we found that an acid resistant E. coli strain did not survive as well as an acid sensitive strain in some laboratory fermentations. The results are important because they indicate that there are further details about how cultures survive or die off in vegetable fermentations that need to be investigated and understood. The data will help guide future research and may be useful for defining the conditions leading to the manufacture of safe fermented vegetable products.

Technical Abstract: Escherichia coli O157:H7 (STEC) acid resistance may aid the pathogen’s ability to cross the human gastric barrier, which makes it an organism of concern in acidic foods. Our objective was to determine how STEC acid resistance may correlate with survival during vegetable fermentations. Seven E. coli O157:H7 strains were screened to assess acid resistance in simulated stomach acid at pH 2. The strains were separated into two groups that differed in acid resistance (P < 0.05), with three being acid sensitive and four acid resistant. The growth rates of these strains were measured in a Luria broth at pH values from 4.2 to 6.8. Two strains having similar growth kinetics, B201 (acid sensitive) and B241 (acid resistant), were selected for further analysis. B201 was found to be missing (compared with B241) two glutamic acid decarboxylase regulatory genes required for acid resistance, gadE and gadX. These strains were challenged in lactic acid (100 mM) solutions, including cucumber juice (CJ) media at pH 3.3. As expected, B201 was more acid sensitive than B241, and a filtered fermented CJ was more inhibitory than similarly acidified CJ. In competitive growth studies with Lactobacillus plantarum LA445 in CJ, B201 or B241 grew from approximately 10^4 to 10^8 CFU/mL within 24 h, but the STEC strains were below the limit of detection by 48 h. In all fermentations, L. plantarum reached 10^8 CFU/mL by 48 h. However, in three of four independent fermentation experiments, strain B201 survived longer than B241. This was possibly due to buffering in B241-LA445 fermentation brines that had increased lactic acid for a given pH compared with B201-LA445. These data indicate that stationary-phase acid resistance may not accurately predict STEC survival during vegetable fermentations.