The effect of transient temperatures on the growth of Salmonella typhimurium LT2 in gelatin gel

The growth of colonies of Salmonella typhimurium derived from single immobilised cells was studied while subjected to constant and sinusoidally-varying temperatures. The bacteria grew in microbiological culture media adjusted to different pH and sodium chloride (NaCl) concentration and solidified with gelatin that was contained within a cassette formed between sheets of PVC film that allowed gaseous exchange. At pH 7.0 and 0.5% (wv) NaCl and either 12 °C or 20 °C, S. typhimurium grew at a rate similar to that in liquid medium. The decrease in growth rate at 20 °C at a lower pH or higher NaCl concentration was greater in the case of immobilised cells than for cells in liquid medium. The change in the numbers of viable bacteria was measured with time under sinusoidally-varying temperatures between 4 and 22 °C and between 12 and 22 °C of period in the range 12 to 480 min. The experimental growth curves were compared with predictions based on isothermal growth in liquid medium. The discrepancies between experiment and prediction were greater for gels stressed by NaCl or pH than for gels at pH 7.0 and containing 0.5% (wv) NaCl, consistent with the isothermal observations.