The effect of colony formation on the heat inactivation dynamics of Escherichia coli K12 and Salmonella typhimurium

The effect of a gelified matrix on the heat inactivation of Escherichia coli K12 and Salmonella typhimurium was investigated. Compared to liquid systems, microbial populations grow as colonies due to the gelified character of the solid environment. In addition, the effect of colony density as well as the effect of the concentration of the gelling agent on the microbial inactivation kinetics were studied. More specifically, E. coli K12 MG1655 or S. typhimurium cells were grown in a gelified medium (Brain Heart Infusion or Tryptic Soy broth supplemented with 1.5% or 2.5% (w/v) xanthan gum) at 37 °C until they reached the stationary phase. Especially for E. coli, different colony densities were obtained by starting growth at 37 °C from different initial cell counts, i.e., a lower inoculum level results in bigger stationary phase colonies. Hereafter, these colonies were inactivated at 54 °C. Cells inactivated as colonies in the gelified environment are protected from heat stress, leading to an increased survival compared to the liquid system for both studied microorganisms. Results indicated that the density of the colonies has no significant influence on the heat resistance in the range 103-106 CFU/mL, but when increasing the initial cell count to the level of 108 CFU/mL less thermotolerance was observed.