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Xu, Yizhi (2014) Aspects of modelling variability of single cell lag time for Cronobacter spp. after exposure to sublethal heat treatment in normal and stressful environments. Doctoral thesis, London Metropolitan University.
The growth profiles of five strains of Cronobacter spp. at low temperature (4 oC to 8 oC) and their thermal resistance to mild heat treatment (48 oC to 50 oC) were investigated with a view to developing a model to predict lag times of individual cells, using a strain with a relatively high thermotolerance and able to grow well at refrigeration temperature (7 oC).
The effect of heat stress (49 oC for 7 min) and subsequent recovery temperatures (7 oC to 22 oC) on the individual cellular lag times of one strain of Cronobacter turicensis were analysed using optical density measurements. It was found that the distribution of individual lag times of Cr. turicensis shifted right and became more spread when the recovery temperature decreased, and the distribution was more skewed after heat stress. Assuming the lag of a single cell follows a shifted Gamma distribution with a fixed shape parameter, the effect of recovery temperature on the individual lag times of untreated and sublethally heat stressed cells of Cr. turicensis were modelled. It was found that the shift parameter (Tshift) increased asymptotically as the temperature decreased, while the logarithm of the scale parameter (θ) decreased linearly as recovery temperature increased.
To test the validity of the model, growth of low numbers of untreated and heat stressed Cr. turicensis in tryptone soy broth (TSB) and infant first milk was measured experimentally between 7 oC and 22 oC and compared with predictions obtained by Monte Carlo simulations. It was found that in TSB, in most cases, the simulations from both models underestimated the actual growth of individual cells of Cr. turicensis from challenge tests; while in first milk, the untreated model slightly underestimated the actual growth at low temperatures (7 oC and 12 oC). The heat stressed model in first milk was generally in agreement with the data derived from the challenge tests and provides a basis for reliable quantitative microbiological risk assessments for Cronobacter spp. in infant milk.
The study has made a contribution to understanding and modelling the responses of untreated and sublethally heat stressed Cr. turicensis at 7 oC, 12 oC and 22 oC at single cell level.
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