Combined Effects of Water Activity, Solute, and Temperature on the Growth of Vibrio parahaemolyticus

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Vibrio parahaemolyticus was grown at 36 C in tryptic soy broth (pH 7.8) containing added levels of NaCl ranging from 0.5 to 7.9% (wt/wt). The fastest generation time was 16.4 min in tryptic soy broth containing 2.9% NaCl (TSBS) which corresponded to a water activity (aw) of 0.992 (±0.005). Tryptic soy broth containing lower or higher levels of NaCl resulted in higher or lower aw, respectively, and slower generation times. Growth was measured turbidimetrically at 36 C in TSBS containing added amounts of NaCl, KCl, glucose, sucrose, glycerol, or propylene glycol. The solutes used to reduce aw to comparable levels resulted in extended lag times of varied magnitude, dissimilar growth rates, and different cell numbers. Reduction of aw with glycerol was less inhibitory to growth than similar aw reductions with NaCl and KCl. Sucrose, glucose, and propylene glycol generally had the greatest effect on extending the lag times of V. parahaemolyticus when the addition of these solutes was made to establish similar aw levels lower than 0.992. Minimal aw for growth at 15, 21, 29, and 36 ± 0.2 C for each of four strains of V. parahaemolyticus was tested in TSBS containing added solutes. Reduced aw was generally most tolerable at 29 C, whereas higher minimal aw for growth was required at 15 C. Solutes added to TSBS to achieve reduction in aw, minimal aw for growth after 20 days, and incubation temperatures were as follows: glycerol, 0.937, 29 C; KCl, 0.945, 29 C; NaCl, 0.948, 29 C; sucrose, 0.957, 29 and 36 C; glucose, 0.983, 21 C; and propylene glycol, 0.986, 29 C. Each of the four strains tested responded similarly to investigative conditions. It appears that minimal aw for growth of V. parahaemolyticus depends upon the solute used to control aw.

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