Author
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Zaika, Laura |
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Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/5/2002 Publication Date: N/A Citation: N/A Interpretive Summary: Shigella may cause gastrointestinal illness when consumed in foods. To control this bacterium in food, quantitative data areeeded to determine the influence of food ingredients, as well as processing and environmental conditions, on its ability to survive. However, such information is not readily available. A study was conducted to determine the effect on Shigella flexneri of organic acids (acetic, citric, lactic, malic, tartaric). These acids are naturally present in foods, and are used as food additives and employed in intervention strategies to reduce the levels of pathogenic bacteria on animal carcasses. Studies were conducted using broth cultures supplemented with 0.04 M (less than 1 percent) acetic, lactic, citric, malic or tartaric acid and adjusted to pH 4, an acidity level that does not promote growth of Shigella. Broth adjusted to pH 4 with HCl was used as a control. Survival of Shigella in the broth media containing the acids was studied at incubation temperatures of 4 to 37 deg C. Survival of the bacteria increased as temperature decreased. Survival characteristics were described in terms lag time, D-value and time to 99.99 percent inactivation. Shigella inactivation in media containing organic acids was greater than in control media. The overall inhibitory effect of the acids was: acetic, lactic (greater than) citric, malic, tartaric (greater than) HCl. The results indicate that foods containing low levels (less than 1 percent) of acids, if contaminated with Shigella, can cause illness. The data obtained will 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 of bacterial pathogens in acidified foods depends not only on the hydrogen ion concentration, but also on the type of acid and the storage temperature. Shigella flexneri is a food borne pathogen which is acid tolerant. The survival of Shigella flexneri strain 5348 was studied in brain heart infusion broth (BHI) supplemented with 0.04 M (less than 1 percent) acetic, citric, lactic, malic, or tartaric acid and adjusted to p 4 with HCl or NaOH. The control medium was BHI broth adjusted to pH 4 with HCl. Stationary phase cells were inoculated into media to give initial populations of 6 to 7 log10 CFU/ml and incubated at 4, 19, 28, and 37 degrees C. A two-phase linear inactivation model was applied to plate count data to derive lag times (t-L) and slopes of the curves, from which D-values and times for a 4-log10 decrease in population (T-4D) were calculated. In all cases, survival increased with decreasing temperature. For each acid, t-L , D-value and T-4D increased with decreasing temperature. All acids inhibited S. flexneri to some extent but to differing degrees as follows: lactic, acetic (greater than) citric, malic, tartaric acid (greater than) HCl. The T-4D values for the control medium and media containing acetic, citric, lactic, malic, and tartaric acids were 64, 47, 50, 34, 58, and 52 h, respectively, at 37 degrees C, and 2607, 1498, 1905, 1346, 1726, and 2134 h, respectively, at 4 degrees C. Results indicate that organic acids may aid in inactivation of Shigella. However, these data also suggest that foods containing low levels (less than 1 percent) of acids, stored at or below room temperature, if contaminated with Shigella, could cause illness. |
