Staphylococcal food poisoning is one of the most common causes of reported foodborne diseases in the U.S. It is caused by eating foods contaminated with toxins produced by Staphylococcus aureus (S. aureus)—a gram-positive coccal bacterium commonly found in air, dust, water, milk, and on the skin and in the nose of up to 25% of healthy people and animals. Usually it causes no illness unless it is transmitted through food. Staphylococci do not form spores but can contaminare food during preparation, processing, and handling.
Sick employees and poor hygienic practices are major causes of staphylococcus foodborne disease outbreaks. Contaminated equipment and environmental surfaces also can lead to S. aureus infections. Products that are frequently implicated in S. aureus food poisoning include poultry, meat, dairy products, potatoes, tofu, eggs, tuna, deli salads, cream-filled pastries, cream pies, sandwich fillings, etc. In general, a food product that has not been kept hot enough (>140°F), cold enough (<40°F), and/or at room temperatures after preparation are vulnerable to staphylococcus food poisoning.
S. aureus is capable of growing in food products and produce staphylococcal enterotoxins (the causative agent of staphylococcal food poisoning) in a wide range of temperatures (45-118ºF; optimum 85-99ºF), pH (4.2-9.3; optimum 7-7.5), and sodium chloride concentration (up to 15% NaCl). S. aureus also is a desiccation-tolerant organism capable of surviving in potentially dry and stressful environments such as clothing and surfaces. All these characteristics favor the growth of S. aureus in a wide range of foods and environmental conditions.
Apart from putting consumers at risk, staphylococcal foodborne illness imposes grave economic losses to food manufacturers due to recalls and other legal expenses. In recent years, livestock-associated methicillin-resistant Staphylococcus aureus (MRSA) has become a rapidly emerging, devastating human pathogen associated with high mortality worldwide. Studies have shown commercially distributed meat products as a potential vehicle for MRSA transmission in a farm-to-fork model.
Staphylococcal enterotoxins are produced as a byproduct during the growth of certain strains of S. aureus. The toxin production requires considerable growth by the microbe and is normally not present until the bacterial population reaches 100,000 per gram of food. S. aureus can produce nine types of toxins that are frequently responsible for foodborne illness outbreaks worldwide. Staphylococcus enterotoxins are highly stable, heat resistant, and resistant to environmental conditions such as freezing, drying, and low pH. They cannot be destroyed by cooking. An estimated 0.1 µg of staphylococcus enterotoxins can cause food poisoning in humans. The microorganism is readily destroyed by heat, but the toxin is heat stable, and still capable of causing staphylococcal foodborne disease outbreaks.
Source of contamination.
Poor sanitation and improper food-handling practices in the food industry is the number one cause of staphylococcus foodborne disease outbreaks. Insufficient cleaning of the processing equipment or utensils, storage in unsanitary environments, prolonged exposure of prepared food at room temperature, inadequate cold-holding temperature, and slow cooling of the prepared food are other contributing factors responsible for S. aureus foodborne disease outbreaks.
Studies have clearly shown that food handling was responsible for 42% of staphylococcus foodborne disease outbreaks reported in the U.S. from 1975-1998. Studies also have shown that S. aureus can adhere to the surface of gloves worn by food handlers and be a source of contamination if not changed frequently.
The conclusive diagnostic criteria of S. aureus foodborne disease are mainly based upon detection of the enterotoxins in food products (using bioassays and immunological techniques), and by enumerating at least 105 (100, 000) colonies S. aureus per gram.
The following best practices can help prevent staphylococcal foodborne disease outbreaks.
- Cook food thoroughly and keep it at safe temperatures: the cooking and refrigerating temperature should be above 140°F and below 40°F.
- Use safe water and raw materials.
- Wash hands before wearing gloves, and change gloves frequently.
- Reduce or avoid bare-hand contact of fresh produce and RTE food.
- Practice routine environmental cleaning and disinfection of equipment used in food processing.
- Avoid inadequate cooking and reheating.
- Implement an effective HACCP (Hazard Analysis and Critical Control Points) plan, GMPs (Good Manufacturing Practice), and GHPs (Good Hygienic Practices).
- Train employees to wash hands with soap and water before entering a food processing facility and handling and preparing food products.
- Have people who are infected with S. aureus seek medical attention and stay out of food handling.
- Strictly prohibit employees with wounds or skin infection from food-handling areas.
If an employee has an active infection, he or she should be excluded until the infection is healed. The simple act of touching or blowing the nose, coughing, or sneezing is enough to contaminate food products or food-contact surfaces with this disease-causing bacteria. If the employee does not have an active infection, then standard personnel hygienic practices, such as handwashing, should prevent transmission, and exclusion from food handling is not required.
The author is Director of Microbiology, AIB International..