Modelling of the Staphylococcus aureus growth and enterotoxin a production based on different conditions

• Ano de defesa: 2023
• Autor(a) principal: Silva, Mírian Pereira da
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Viçosa; Ciência e Tecnologia de Alimentos
• Palavras-chave em Português: Testes microbiológicos - Modelos matemáticos; Staphylococcus aureus; Queijo-de-Minas - Contaminação; Enterotoxinas; Microbiologia de Alimentos
• Link de acesso: https://locus.ufv.br//handle/123456789/32203; https://doi.org/10.47328/ufvbbt.2024.060

Staphylococcus aureus is one of the pathogens most involved in food outbreaks. Enterotoxigenic S. aureus produce enterotoxins stable at high temperatures. This is a concern for dairy products, like Minas frescal cheese, since enterotoxins can be produced in early stages of production (raw milk) and remain stable after thermic treatments, like pasteurization. Therefore, it is necessary to adopt appropriate measures to prevent contamination and the eventual toxins production. Predictive microbiology is a powerful tool for food safety and quality. Using mathematical models, it is possible to predict the behavior of microorganisms under specific conditions. Given that Minas frescal cheese is strongly associated with S. aureus, mathematical models are relevant to demonstrate how bacterial growth and enterotoxins production are influenced by production and storage conditions. Thus, this study aimed to characterize the physicochemical aspects and the presence of S. aureus in Minas frescal cheese and, based on this data, create predictive models of growth and the production of enterotoxin A by S. aureus. Samples of Minas frescal cheese (n = 50) were obtained and subjected to pH, sodium chloride concentration, and temperature analysis. The presence and characterization of Staphylococcus spp. were evaluated, and coagulase-positive isolates were investigated for the presence of the nuc gene for S. aureus identification and enterotoxin genes (sea, seb, sec, sed, see). Based on this data, growth models (Combase and response surface model) and enterotoxin production models (probabilistic logistic regression model) were developed and statistically and experimentally validated. All coagulase-positive isolates tested were confirmed as S. aureus, and none presented genes related to enterotoxins. The pH, temperature, and salt concentration varied from 5.80 to 6.62, from 5 °C to 12 °C, and from 0.85% to 1.70%, respectively. The model generated by Combase assessed the influence of temperature, pH, and salt concentration on the growth kinetics (maximum growth rate and lag time) of S. aureus and found values ranging from 0.012 to 0.419 log CFU/mL/h of growth rate and adaptation time from 4.60 to 159.24 hours. Temperature was the factor that most influenced the growth kinetics, and salt concentration did not significantly influence the responses. Subsequently, response surface models and logistic regression models were developed to assess the growth responses of S. aureus and the probability of enterotoxin A (SEA) production, respectively. In addition to the factors assessed in the growth kinetics, different levels of initial contamination of S. aureus and time were incorporated into the models. With the exception of sodium chloride concentration, all factors significantly affected the growth and production of enterotoxin A by S. aureus. The maximum population obtained ranged from 8.27 to 9.36 log CFU/mL at 25 °C and from 3.90 to 8.27 log CFU/mL at 15 °C. At 10 °C, at the lowest contamination level evaluated, S. aureus (100 CFU/mL) remained below the detection limit (<1.0 CFU/mL) throughout incubation, and no SEA production occurred, while for the other concentrations (103 CFU/mL and 105 CFU/mL), the maximum population was 7.79 CFU/mL, highlighting the importance of maintaining Minas frescal cheese at refrigeration temperatures and controlling the initial contamination level. SEA production occurred for all conditions evaluated at 15 °C and 25 °C, but at different times. The models showed a good fit to the experimental data, with satisfactory values for R² (0.90), accuracy factor (1.09), and bias factor (0.99) for the growth model; a high agreement percentage (94.4%), Nagelkerke's R² (0.92), and the Hosmer and Lemeshow test (p > 0.05) for the SEA production model. Additionally, experimental validation in culture media, milk, and cheese confirmed that the models are suitable for predicting the growth and the probability of SEA production by S. aureus in Minas frescal cheese. Keywords: Staphylococcus aureus. Enterotoxin A. Minas frescal cheese. Predictive microbiology. Mathematical models.