Survival, growth, and inactivation of acid-stressed Shigella flexneri as affected by pH and temperature

A study was done to determine the survival, growth, and inactivation characteristics of unadapted, acid-adapted, and acid-shocked Shigella flexneri 2a cells as affected by pH and temperature. The pathogen was grown at 37°C for 18 h in tryptic soy broth containing no glucose (TSBNG) (unadapted cells) and TSBNG supplemented with 1% glucose (TSBG) (acid-adapted cells). Cells grown in TSBNG were acid-shocked by adjusting 18-h cultures to pH 4.5±0.05 with lactic acid. All three cell types were separately inoculated into tryptic soy broth (6.6-7.0 log₁₀ cfu/ml) containing 0.25% glucose (TSB) acidified to pH 3.5-5.5 with lactic acid and incubated at 4, 12, 21, 30, and 48°C for up to 144 h. Overall, inactivation of S. flexneri cells at low pH was enhanced with an increase in incubation temperature. All three types of cells survived for 144 h at 4°C in TSB acidified to pH 3.5, compared to <24 h at 30°C and 2 h at 48°C. The population of all three cell types increased significantly (α=0.05) within 24 h when cells were incubated at 12, 21, or 30°C in TSB at pH 5.0, 5.5, or 7.3. Prior exposure of the S. flexneri to an acidic environment (acid-adapted or acid-shocked cells) resulted in increased resistance to extreme acid and temperature conditions. Acid-adapted cells decreased by approximately 2.5 log₁₀ cfu/ml when incubated at 4°C for 144 h, compared to a 6-log₁₀ reduction in control (unadapted) cells. When cells were exposed to low pH (3.5-4.5) and high temperature (48°C), significantly higher (α=0.05) populations were recovered on tryptic soy agar (TSA) than on TSA supplemented with 4% NaCl (TSAS), indicating that a portion of S. flexneri cells were injured. Results show that the ability of S. flexneri to survive and grow at a given pH is influenced by previous exposure to acidic environments and by incubation temperature.