Effect of temperature on the growth of Staphylococcus aureus in ready-to-eat cooked rice with pork floss.

Cooked rice with pork floss (CRPF) wrapped in dried seaweed is one of the most popular ready-to-eat (RTE) foods in many Asian countries, particularly in Taiwan. The products are susceptible to Staphylococcus aureus contamination and temperature abuse during manufacturing, distribution, and storage. The objective of this study was to examine the effect of temperature on its growth in RTE CRPF for use in risk assessment and prevention of staphylococcal food poisoning (SFP). Inoculated CRPF samples were stored at 4, 12, 18, 25, and 35°C, and the change in the populations of S. aureus during storage were analyzed using three primary models to determine specific growth rate (µmax), lag-phase duration (λ), and maximum population density (ymax). The Ratkowsky square-root and Huang square-root (HSR) models were used as the secondary models to describe the effect of temperature on µmax, and a linear and an exponential regression models were used to describe the effect of temperature on λ and ymax, respectively. The model performance was evaluated by the root mean square error (RMSE), bias factor (Bf), and accuracy factor (Af) when appropriate. Results showed that three primary models were suitable for describing the growth curves, with RMSE ≤ 0.3 (log MPN/g). Using µmax obtained from the Huang model, the minimum growth temperature (Tmin) estimated by the HSR model was 7.0°C, well in agreement with the reported Tmin. The combination of primary and secondary models for predicting S. aureus growth was validated by additional growth curves at 30°C, which showed that the RMSE was 0.6 (log MPN/g). Therefore, the developed models were acceptable for predicting the growth of S. aureus in CRPF under likely temperature abuse conditions and can be applied to assess the risk of S. aureus in CRPF and design temperature controls to reduce the risk of SFP.

Establishment of a novel multiplex PCR assay and detection of toxigenic strains of the species in the Bacillus cereus group.

Five different enterotoxins and one emetic toxin of Bacillus cereus have been characterized. To amplify all of the enterotoxin and emetic-specific sequences of the species in the B. cereus group, a multiplex PCR with 12 primer pairs was established. In developing the assay method, a common terminal sequence at the 3' ends of all primers was chosen and a hot start Taq polymerase was used to overcome primer dimer formation. The assay was successfully applied to analyze the toxigenic potential of 162 food-poisoning and food-related strains. Results showed that there were 10 toxigenic patterns for all the test strains. All of the B. cereus strains carried at least one toxin gene. More than 70% of Bacillus mycoides strains carried no known toxin genes. The toxin profiles and toxin genes of B. mycoides strains were significantly different from B. cereus strains (P < 0.05), although the two species were closely related. The results suggest that many B. mycoides strains might be less prone to cause food poisoning. They also indicate the importance of detecting the toxin genes together with the detection of the species in the B. cereus group.