In a recent publication, the Head of Microbiology at research organization Campden BRI, Dr. Roy Betts, collaborated with Thermo Fisher Scientific to provide an overview of the food-borne pathogen Staphylococcus aureus group (including S. aureus, S. hyicus, and S. intermedius) as it pertains to food microbiology and applications involving food safety and quality.1
Because most incidences of food-borne illness resulting from Staphylococcus exposure are relatively minor experiences of gastrointestinal distress (onset one to six hours after eating contaminated food; duration of illness approximately 24 hours), many cases of food-borne Staphylococcus infection go unreported. Even so, a minority of individuals suffer severe symptoms, and the Centers for Disease Control (CDC) estimates 241,188 annual cases of illness in the United States alone. Of these, approximately 1,064 cases require hospitalization, and six prove fatal every year. Regulations for the control of Staphylococcus in food products vary by country.
Dr. Betts describes the microbe, which spans over 30 species, as non-motile, facultatively anaerobic, Gram positive, catalase positive spheres that tend to cluster. He indicates that the microbe itself routinely colonizes on host (including human) mucus membranes and skin. The food-borne illness associated with the organism arises from the products of an enterotoxin-producing enzyme-coagulase that illness-causing Staphylococcus species produce as they grow on contaminated food products. This toxin is hearty enough to persist after boiling and/or maintaining heat at 121ºC for several minutes, rendering it almost impossible to eradicate. Furthermore, a food product can test negative (or microbiologically acceptable) for Staphylococcus after cooking or processing but still cause food poisoning since the enterotoxin remains active. While contamination sufficient to produce the toxin requires relatively high numbers of Staphylococcus (105 or 106 per g or ml of food), exposure to a mere 1 µg of the toxin itself can produce sickness.
As with most organisms, the most likely vector for food-borne illness resulting from Staphylococcus is poor food handling protocols. Dr. Betts offers the following foods as those most commonly associated with this particular food-borne illness: meats, poultry, egg products, deli-type salads, bakery products (especially cream-filled), prepared sandwich fillings, and dairy products. Generally, these food products require significant handling followed by potential storage at slightly elevated temperatures, making them prime targets for contamination and growth at sufficient levels to promote toxin production. The author notes the following key areas for minimizing the risk of contamination and/or recontamination during food production: water activity (Aw 0.83 to stop growth; possibly higher to prevent toxin formation), pH (4 to stop growth; 5 to prevent toxin formation), and temperature (7ºC to inhibit growth; 9ºC to 10ºC to prevent toxin formation).
Of course detection and enumeration methods are valuable resources for food industry professionals entrusted with consumer safety and food quality. Dr. Betts offers selective differential media- Baird-Parker Agar and Rabbit Plasma Fibrinogen– as those most commonly used for this purpose. Confirmation generally relies on coagulase testing or Latex Agglutination Test protocols to verify the presence of coagulase positive organisms grown on BPA. Other alternatives include automated testing, immunoassays, and molecular PCR-based testing. Immunoassays specific to the enterotoxin (rather than the microbe) are particularly useful given the specific features of the Staphylococcus group previously described. Together, these measures can assist industry professionals in identifying and/or preventing outbreaks of foodborne illness.
1 Betts, R. ‘Microbial Update: Staphylococci.’ International Food Hygiene 24 (5): 9-11.