Modelling the growth boundaries of Staphylococcus aureus: Effect of temperature, pH and water activity

Department of Food Science and Technology, University of Cordoba, Spain.
International journal of food microbiology (Impact Factor: 3.08). 06/2009; 133(1-2):186-94. DOI: 10.1016/j.ijfoodmicro.2009.05.023
Source: PubMed


The microbial behaviour of five enterotoxigenic strains of Staphylococcus aureus was studied in the growth/no growth domain. A polynomial logistic regression equation was fitted using a stepwise method to study the interaction of temperature (8, 10, 13, 16 and 19 degrees C), pH (4.5; 5.0; 5.5; 6.0; 6.5 7.0 and 7.5) and water activity (A(w)) (19 levels ranging from 0.867 to 0.999) on the probability of growth. Out of the 284 conditions tested, 146 were chosen for model data and 138 intermediate conditions for validation data. A growth/no growth transition was obtained by increasing the number of replicates per condition (n=30) in comparison to other published studies. The logistic regression model showed a good performance since 96.6% (141 out of 146 conditions) of the conditions for model data and 92.0% (127 out of 138 conditions) for validation data were correctly classified. The predictions indicated an abrupt growth/no growth interfaces occurred at low levels of temperature, pH and A(w). At 8 degrees C, S. aureus grew only at optimum levels of pH and A(w) while at temperatures above 13 degrees C, growth of S. aureus was observed at pH=4.5 and A(w)=0.96 (13 degrees C), 0.941 (16 degrees C) and 0.915 (19 degrees C). The optimal pH at which growth of S. aureus was detected earlier was 6.5. However, a slight decrease of the probability of growth was noticed in the pH interval of 7.0-7.5 at more stringent conditions. The ability of S. aureus to grow at low A(w) was shown since growth was detected at A(w)=0.867 (T=19 degrees C; pH=7.0). Finally, a comparison of model predictions with literature data on growth/no growth responses of S. aureus in culture media and cooked meat was made. Model predictions agreed with published data in 94% of growth cases and in 62% of no growth cases. The latter discordance is highly associated to other environmental factors (such as other preservatives, strains etc.) included in published models that did not match the ones included in our study. This study can help manufacturers in making decision on the most appropriate formulations for food products in order to prevent S. aureus growth and enterotoxin production along their shelf-life.

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Available from: Antonio Valero, Jan 13, 2014
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    • "C), but showed restrained growth at less than 10 o C. Valero et al. (2009) reported that S. aureus growth is restrained at temperatures less than 8 o C under optimal a w and pH conditions. Comparison of the primary models. "
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    • "For food safety purposes, this value should be as lowest as possible, what implies that variability associated to the predicted microbial response would be reduced. At T 10 °C, pH 5.00 and a w 0.967, the information given by the SMRBF model states that p G =0.10, p GT =0.40 and p NG =0.50 (P = 1 in 10% of cases; 0 b P b 1 in Table 4 Observed conditions from the study of (Valero et al., 2009) in which the replicates tested in generalization data showed simultaneously growth and no growth and comparison to the predictions given by the SMRBF model. "

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    • "The results obtained in BHI broth demonstrated that incubation temperature had a strong effect on S. aureus growth in BHI broth, as widely reported in other studies (Lanciotti, Sinigaglia, Gardini, Vannini, & Guerzoni, 2001). A longer lag phase values and slower growth was obtained at 10 °C, as shown by Valero et al. (2009), who found that S. aureus could growth slowly at low temperatures (b10 °C) only at optimum levels of pH (>6.0) and a w (0.990– 0.997). Sutherland et al. (1994) reported slightly higher μ max and similar lag phase values at 10 °C/pH 6.59/NaCl 2.5% (0.029 h − 1 and 63.85 h respectively). "
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