Growth of Shigella flexneri in foods: comparison of observed and predicted growth kinetics parameters

Shigella causes foodborne gastrointestinal illness; however, little information is available on its ability to grow in foods. Commercially available sterile foods (UHT milk, beef broth, chicken broth, vegetable broth, meats, vegetables) were inoculated with S. flexneri 5348 and incubated at 12, 15, 19, 28 or 37 °C. Growth curves were fitted from plate count data by the Gompertz equation and exponential growth rates, generation times, lag times and maximum population densities were derived. The observed kinetics values, expressed as T1000 (time, h, required for a 3 log increase in bacterial population), were compared with values calculated using published growth models. Observed and calculated values compared favorably for growth at 19–37 °C. S. flexneri grew well in milk at 15–37 °C but growth at 12 °C was variable. The bacteria readily grew in most foods, even at 12 °C; but died off in carrots at 19 and 28 °C. Factors other than those used in the growth model may influence bacterial growth in specific foods.