Effect of heat shock and recovery temperature on variability of single cell lag time of Cronobacter turicensis.

The effect of heat stress and subsequent recovery temperature on the individual cellular lag of Cronobacter turicensis was analysed using optical density measurements. Low numbers of cells were obtained through serial dilution and the time to reach an optical density of 0.035 was determined. 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 of untreated and sublethally heat treated 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 with recovery temperature. To test the validity of the model in food, growth of low numbers of untreated and heat treated Cr. turicensis in artificially contaminated infant first milk was measured experimentally and compared with predictions obtained by Monte Carlo simulations. Although the model for untreated cells slightly underestimated the actual growth in first milk at low temperatures, the model for heat treated cells was 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.

Genotypic characterization of Brochothrix spp. isolated from meat, poultry and fish.

AIM: To study genotypic diversity of isolates of Brochothrix thermosphacta recovered from meat, poultry and fish. METHODS AND RESULTS: A total of 27 bacteria isolated from 19 samples of meat, poultry and fish were identified phenotypically and genotypically using PCR amplification of 16S-23S rDNA intergenic transcribed spacer (ITS-PCR), repetitive sequence-based PCR (rep-PCR) and 16S rDNA sequencing. Using ITS-PCR, all bacteria showed the same DNA profile as the reference strains of Br. thermosphacta, allowing typing of the isolates at species level. Using 16S rDNA sequencing, all isolates were identified, at genus and species level, as Br. thermosphacta. Identification as Br. campestris was observed with a lower, but very close, level of similarity. Rep-PCR was more discriminatory than ITS-PCR and allowed differentiation of four subgroups among the isolates. CONCLUSION: Minor genotypic differences among Br. thermosphacta strains from meat, poultry and fish were observed. SIGNIFICANCE AND IMPACT OF THE STUDY: A rudimentary exploration of genotypic differences of Br. thermosphacta from meat, poultry and fish resulted in preliminary confirmation of the suitability of ITS-PCR for typing Br. thermosphacta and confirmed the value of rep-PCR fingerprinting to discriminate between Br. thermosphacta strains.