Modeling the Effect of Thermal Stress on the Lag Phase of Bacillus cereus Strains in Reconstituted Infant Formulae

Introduction: When cells are inoculated in a new environment, the specific growth rate is reached only after an adjustment period, called lag phase. It can be short or long depending on the pre-inoculation history of the cells. After going through a stressful condition, the adaptation period is longer than what they would need without pre-inoculation stress.

Purpose: In this talk we investigate the validity of the above conclusion, using Bacillus cereusas a model organism and heat treatment as a stress condition.

Methods: Three strains of Bacillus cereus were grown in RIF (Reconstituted Infant Formulae) at temperatures ranging from 12 to 25°C, after exposing them to thermal stress (72°C for 25 seconds). At 22°C also non-stressed cells were grown for control. Their lag times and specific growth rates were estimated using viable count measurements. The method of Analysis of Variance was used to study the variation of the parameter h₀, the product of the maximum specific growth rate and the lag.

Results: The heat treatment didn’t affect the specific growth rates for none of the strains. For the strains B596 and F4810/72 grown in RIF, there was no significant difference between the h₀ values (P>0.1), but the strain B594 differs from the other two strains in this respect, too, just like regarding the growth rate (P<0.01). For the final predictive models, the h₀ value was taken as constant for each strain, estimated by the multiplicative average of its observed values (6.2, 4.1 and 4.9 for the strains B594, B596 and F4810/72, respectively). Finally, not surprisingly, the h₀value is low (ca 1) for the unheated cells, where the growth curves sometimes had hardly any lag.

Significance: Modelling the h₀ parameter leads to a simple way to model the effect of pre-inoculation history on the lag phase.