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ARS Home » Research » Publications at this Location » Publication #106043


item Zaika, Laura

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Shigella bacteria cause gastrointestinal illness when consumed in foods. To control this bacterium in food, quantitative data are needed to determine the influence of food ingredients, processing and environmental conditions on its ability to survive. However, such information is not readily available. A study was conducted to determine the survival characteristics of Shigella flexneri as affected by factors (temperature, acidity) that have a major impact on bacterial survival. Studies were conducted using broth cultures adjusted to pH 2 to 5 and incubation temperatures of 4 to 37 deg C. Survival of the bacteria increased as temperature decreased and acidity decreased. Survival characteristics were described in terms of D-value and time to 99.99 percent inactivation. The data obtained will be used to develop a non-thermal inactivation model, which could be used by the food producer, food distributor, regulatory agency or the consumer to estimate the potential for survival of Shigella if present as a contaminant in food.

Technical Abstract: The survival characteristics of Shigella flexneri were determined in brain- heart infusion broth as a function of pH (2-5) and temperature (4-37 deg C). Stationary phase cells were inoculated into sterile media to give initial populations of 6-7 log10 CFU/ml. Bacterial populations were determined periodically by aerobic plate counts. Survivor curves were fitted from plate count data using a two-phase linear model to derive lag times and slopes of the curves, from which D-values and times to a 4-D (99.99 percent) inactivation (T4D) were calculated. Survival increased as temperature decreased and as pH increased. Bacterial populations reached undetectable levels (less than 1.3 log10 CFU/ml) at 37, 28, 19, 12 and 4 deg C after 5, 15, 23, 85 and 85 days, respectively, in media adjusted to pH 4, and after 1, 7, 9, 16 and 29 days, respectively, in media adjusted to pH 3. In media adjusted to pH 2, bacterial populations were stable for 2- 12 h at temperatures of 19 deg C or lower and reached undetectable levels after 1-3 days, while at 28 and 37 deg C the bacteria became undetectable after 8 and 2 h, respectively. In media adjusted to pH 5, bacterial levels decreased only 0.5-1.5 log10 CFU/ml after 75 days at 4 deg C and to undetectable levels after 135 days at 12 deg C, while growth occurred at the higher temperatures. These results indicate that S. flexneri is acid resistant and suggest that acidic foods may also serve as vehicles for infection with this bacterium.