Effect of Shift in Growth Temperature on Tolerance of Psychrotrophic and Mesophilic Strains of Bacillus cereus to Heat and Sodium Chloride

Abstract

A shift in growth temperature of a psychrotrophic (F3802A/84) strain and a mesophilic strain (B4ac-1) of Bacillus cereus grown at 30°C for 10 h, then at 37°C or 40°C for 14 h, enhanced thermotolerance. Sodium chloride, at concentrations of 2.0 and 4.0% in brain heart infusion (BHI) broth, had no effect on thermotolerance of strain B4ac-1 heated at 50°C, whereas the same concentrations of NaCl caused a decrease in thermotolerance of strain F3802A/84 heated at 48°C. A downshift in growth temperature from 30°C to 10°C followed by incubation for 3 to 9 days increased thermotolerance of strain F3802A/84 but not strain B4ac-1 heated in BHI broth containing 2.0 or 4.0% NaCl compared to thermotolerance in BHI broth containing 0.5% NaCl. Protein analysis using one-dimensional gel electrophoresis revealed an increase in proteins with molecular weights of 54, 50, 44, and 42 kDa in cells of strain F3802A/84 and 83 and 69 kDa in cells of strain B4ac-1 subjected to an upshift in growth temperature from 30°C to 37°C or 40°C, respectively. A downshift in growth temperature from 30°C to 10°C resulted in substantial amounts of proteins with molecular weights of 63, 40, and 29 kDa produced by strain F3802A/84 and 63 kDa to be produced by strain B4ac-1. Proteins produced in response to upshift or downshift in growth temperature are suspected to play an important role in heat resistance of the psychrotrophic and mesophilic strains of B. cereus examined in this study. Changes in resistance to heat or refrigeration temperatures, as well as tolerance to NaCl, as affected by previous exposure of cells to temperature shifts may influence the ability of B. cereus to grow in minimally processed foods during distribution and storage.