Incidence of enterotoxigenic Staphylococcus aureus in milk and Egyptian artisanal dairy products

Abstract

Staphylococcus aureus (S. aureus) is one of the most concerning foodborne pathogens due to its ability to produce wide varieties of heat-stable enterotoxins. Hence, this study was designed to investigate the prevalence of the enterotoxigenic S. aureus strains and their enterotoxins in milk and in some representative artisanal Egyptian dairy products. Additionally, the prevalence of Methicillin-Resistant S. aureus (MRSA; antibiotic resistant strain) was considered throughout the study. Three-hundred and fifty samples (n = 350) were randomly collected from Assuit city in upper Egypt, including market cow's milk, yoghurt, Domiati cheese, Kareish cheese, Ras cheese, cooking butter and small-scale ice cream. The identification of the S. aureus strains was done using conventional biochemical method (catalase test, egg yolk reaction test, anaerobic utilization of mannitol and coagulase test) and PCR. Furthermore, Staphylococcal enterotoxins genes (SEA and SED) were detected by PCR while the classical enterotoxins were examined in 30 samples of Ras cheese and small-scale ice cream using ELISA technique. S. aureus was isolated from 142 out of 350 (41%) [95% CI: 35.4–45.7%] examined samples at levels of up to 5.79 log10 cfu/ml or g. The prevalence of S. aureus was significantly (P < 0.05) higher in small scale ice cream (74%) than in other food categories with a mean count of 3.74 ± 1.03 log10 cfu/g. MRSA strains were recovered from 54 (15%) [95% CI: 11.6–19.2%] samples with a log mean value of 3.54 ± 0.523 log10 cfu/ml or g. In addition, seventeen (30%) of the S. aureus isolates tested by PCR were found to be positive for enterotoxin genes. Out of these, fifteen strains carried one gene and only two strains harbored both A and D genes. Additionally, 4 (7.4%) of MRSA isolates from the strains recovered from Ras cheese were enterotoxigenic. Finally, the enterotoxin E (SEE) was determined in 8 out of 30 (26.67%) examined samples that were contaminated with high level of enterotoxigenic S. aureus. It is concluded that additional preventive measures must be applied in the artisanal dairy processing chain in Egypt to avoid milk contamination and consumers' exposure to enterotoxigenic strains of S. aureus.