Staphylococcus aureus is very common in the environment and can be found in soil, water, and air, and on everyday objects and surfaces. It can live in humans and animals.
Everyone immediately thinks of MRSA but don’t forget staph is also found in foods and can make toxins (enterotoxins) that might not be destroyed by cooking, although the bacterium itself can be destroyed by heat. These toxins can cause nausea, stomach cramps, vomiting, and diarrhoea. In more severe cases, the toxins may cause dehydration, headache, muscle cramps, and temporary changes in blood pressure and heart rate. Its toxins are fast‐acting; they cause symptoms within 1 to 7 hours after contaminated food is eaten. Outbreaks often have been linked to foods that require a lot of handling when they’re being prepared.
Growth and toxin production is best in the presence of oxygen but can grow anaerobically. It is not regarded as a good competitor with other bacteria. Infected food handlers are a significant cause of food poisonings.
Growth and Control
Temperature: Optimum 37°C, range 6-48°C. Upper limit of growth can be extended above 44°C by addition of NaCl, monosodium glutamate (MSG) and soy sauce.
pH: Optimum pH for growth is 7.0-7.5. Minimum pH for growth is 4.2, maximum 9.3. Growth is inhibited in the presence of 0.1% acetic acid (pH 5.1).
Atmosphere: Grows best in the presence of oxygen. Capable of growing anaerobically. Growth is retarded in the presence of 80% CO2 compared to growth in an air atmosphere.
Water Activity: The low aw at which S. aureus grows is particularly significant. The organisms are resistant to drying and may grow and produce enterotoxins in foods with aw as low as 0.85. Can grow in up to 25% NaCl. Grows well in 7-10% NaCl. Optimum aw for growth is 0.99. Its ability to grow at low aw means that it has a competitive advantage on low aw foods.
Toxin Production: Combinations of different inhibitory factors (e.g. NaCl content and pH) can be used to control toxin production (and growth), i.e. multiple hurdles can be used. Basically organisms exposed to an extreme of one inhibitory factor become more susceptible to others.
Temperature: Optimum 35-40°C, range 10-45°C.
pH: Optimum pH for toxin production is 5.3-7.0, minimum 4.8, maximum around 9.0. Toxin production is inhibited more effectively when the pH is reduced by lactic acid rather than hydrochloric acid.
Atmosphere: Greatest toxin production is in the presence of oxygen. Less toxin is produced under anaerobic conditions.
Water Activity: Optimum for toxin production is ≥0.90 aw. Range 0.86 ≥0.99.
Temperature: The organisms is usually readily killed at cooking and pasteurisation temperatures. Heat resistance is increased in dry, high-fat and high-salt foods. Survives frozen storage.
Toxins are extremely resistant to heat. For example the D time of enterotoxin B at 149°C is 100 min at an aw of 0.99, and 225 min at an aw of 0.90.
pH: S. aureus can survive in foods down to pH 4.2 but this is dependent on the type of acid present.
Atmosphere: Cells survive longer under anaerobic conditions.
Water Activity: Survives for long periods in dried foods.
Inactivation (CCPs and Hurdles)
Temperature: D60 is approx. 2 min. However, the D60 for salty foods, e.g. cheese, bacon and ham, is considerably longer (can reach >50 min). Heat resistance is reduced at high and low pH.
pH: Rapid destruction of S. aureus has been demonstrated in lemon and lime juices at pH 2.3. During food fermentations, lactic acid bacteria produce substances that are inhibitory to S. aureus including lactic acid, hydrogen peroxide and bacteriocins.
Water Activity: Withstands desiccation well.
Preservatives: S. aureus shows no unusual resistance to common food preservative methods except for its osmotolerance (permits survival and growth in high concentrations of NaCl). Cells grown in high salt foods at high temperatures are less sensitive to some food preservatives. When reduced pH and aw are used in combination to control S. aureus, less stringent levels of these parameters can be applied. Sorbate and benzoate are effective inhibitors of S. aureus with a minimum inhibitory concentration at pH 6.1 of 1000 mg/kg. The effectiveness of these preservatives increases as pH is reduced.
Methyl and propyl parabens are also effective.
High concentrations of CO2 substantially reduce growth.
Sanitisers/Disinfectants: See here for guidance. Most chemical sanitisers used routinely in the food industry, such as chlorine, other halogens, and quaternary ammonium compounds will destroy S. aureus on surfaces with ease.
Radiation: Relatively resistant to ionising radiation, but not to UV irradiation, when compared with other non-sporulating bacteria such as Salmonella and E. coli. D value of 0.45 kGy.
Incubation: 30 min to 7 hours after eating food containing enterotoxins (mean 2-4 hours).
Symptoms: Symptoms usually include nausea, vomiting and abdominal cramps and may be followed by diarrhoea. In severe cases, headaches, sweating and fever may occur. In mild cases there may be nausea and vomiting without diarrhoea, or cramps and diarrhoea without vomiting. Recovery is rapid, usually within 2 days.
Estimated hospitalisation rate = 18%, case fatality rate = 0.02%.
Condition: Staphyloenterotoxaemia; enterotoxin causes inflammation of the intestinal tract lining. Staphylococcal food poisoning is seldom fatal but fatalities have been reported occasionally in young children and elderly.
Toxins: Illness results from consuming toxins in foods.
At Risk Groups: All people are believed to be susceptible to staphylococcal intoxication, but the intensity of symptoms may vary depending on the amount of food ingested and the susceptibility of the individual to the toxin.
Long Term Effects: None.
Dose: Less than 1.0 μg of toxin in contaminated food can produce symptoms. This toxin level is reached when S. aureus populations exceed 105 per gram. Small numbers of S. aureus in food are not a direct hazard to health.
Treatment: Usually no treatment is given. Fluids may be administered when diarrhoea and vomiting are severe.
Reservoirs / Sources
Human: Humans are the main reservoir for staphylococci involved in human disease. Human contamination of food can occur by direct contact, indirectly by skin fragments, or through respiratory tract droplets.
Animal: Animals and poultry carry S. aureus on parts of their body which can lead to infections. Cows udders and teats, tonsils and skin of pigs, and skin of chickens and turkeys are known sources.
Food: S. aureus competes poorly with other bacteria and therefore seldom causes food poisoning in raw products. Unpasteurised milk may cause food poisoning if numbers of organisms are very high, such as when a cow has mastitis. Foods which present the greatest risk are those in which the normal flora has been destroyed (e.g. cooked meats) or inhibited (e.g. cured, salted meats). Staphylococci grow well in cooked foods which are high in protein, sugar or salt, low in acid, or food with moist fillings.
Environment: May colonise food-processing equipment in areas that are difficult to clean. Often found in ventilation system dust.
Transmission Routes: Ingestion of contaminated food.
Plague and Pestilence
Outbreaks: Most outbreaks are caused by eating foods in which enterotoxin has been produced because of time and temperature screw ups following preparation. Control measure failures include contamination of food by handler, inadequate re-heating and hot holding of cooked foods, inadequate thawing prior to cooking, inadequate cooling of cooked product, prolonged storage at ambient temperature.