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.
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).
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.
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.
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.
Optimum 35-40°C, range 10-45°C.
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.
Greatest toxin production is in the presence of oxygen. Less toxin is produced under anaerobic conditions.
Optimum for toxin production is ≥0.90 aw. Range 0.86 ≥0.99.
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.
S. aureus can survive in foods down to pH 4.2 but this is dependent on the type of acid present.
Cells survive longer under anaerobic conditions.
Survives for long periods in dried foods.
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30 min to 7 hours after eating food containing enterotoxins (mean 2-4 hours).
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%.
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.
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
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.
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.
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.
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.
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.
May colonise food-processing equipment in areas that are difficult to clean. Often found in ventilation system dust.
Ingestion of contaminated food.