Staphylococcus aureus produces heat-stable toxins in food that, when ingested, can result in a severe but short-lived gastroenteritis (primarily abdominal cramps and vomiting). Humans carry the organism in moist areas such as the nose or in skin lesions. Poor hygiene may result in contamination of cooked foods, and temperature abuse allows the organism to grow to high numbers and produce toxins. Most cases are associated with outbreaks and only a few sporadic cases are detected. On average, there are 5 - 10 outbreaks per year in the UK (down from a peak of 150 outbreaks/year in the 1950s).
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Incubation Period, Symptoms & Duration
 Incubation Period
1 - 6 hours, typically 2-4 hours.
Symptoms
Symptoms depend on individual susceptibility and health, the amount of contaminated food eaten, and the amount of toxin in the food. Severe vomiting and abdominal cramps are the predominant symptoms (with nausea, diarrhoea, sweating, and sometimes collapse/prostration).
Duration 24 - 48 hours
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Source & Spread
Sources/Associated Foods
Staphylococcus aureus is a bacterium that occurs frequently on the skin of humans. It is particularly associated with the nasal tract (nose and throat) where it can be found in 20 - 50% of healthy individuals. Additionally, when the skin is breached or body resistance is low, Staph. aureus can cause minor skin abscesses such as boils, septic cuts and carbuncles. The organism can also be found on hair, where carriage rates may be as high as those for the nose and throat.
Foods implicated in outbreaks have included cooked meats (especially hams), poultry products, bakery products with cream fillings, sandwich fillings, raw milk, cream and cheese.
Spread
The five most frequently involved contributory factors are:
- Inadequate refrigeration
- Preparing food too far in advance of planned service
- Infected persons practising poor personal hygiene, contaminating food through direct contact with (infected) skin or hair, or from sneezing or coughing
- Inadequate cooking or heat processing
- Holding food in warming/hot holding devices at an incorrect temperature (i.e. at bacterial growth temperatures).
Institutional catering and buffet meals, with food prepared some time in advance and stored at room temperature or inadequately chilled, provide typical outbreak settings. Consequently, outbreaks can affect a large number of people.
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Control
Control measures include:
- adequate temperature control, i.e. hot storage above 63oC, chill/cold storage at or below 4oC.
- reducing food handling time (from initial preparation to eating) to a minimum
- good personal (hand/skin) hygiene
- thorough cooking of foods
- effective training and supervision
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Exclusion/Return to Work Parameters
Risk group 1 - exclude food handlers with septic lesions from work until successfully treated. Nasal carriers do not need to be excluded.
[Group 1 - food handlers; Group 2 - health care workers; Group 3 - children under 5 in nurseries etc.; Group 4 - older children and adults who may find it difficult to implement good standards of personal hygiene]
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Additional Information
- Staphylococcus aureus is a Gram +ve, non-sporeforming, spherically-shaped facultative anaerobic organism. The cells appear in pairs, short chains or typical 'bunched grapes' under the microscope.
- S. aureus is a poor microbial competitor. Problems therefore tend to arise with foods that have few competitor organisms (i.e. foods that have been cooked or which have a low pH or reduced aw) which are then contaminated by a food handler and subjected to temperature abuse afterwards.
- Salted meats such as ham are particularly vulnerable, since the levels of salt and/or nitrite in the product will inhibit a large proportion of microbial competitors, leaving Staph. aureus unaffected.
- Although the acidity (low pH) of mayonnaise is inhibitory to the growth of staphylococci, growth may occur in salads (such as egg or chicken) where the otherwise low pH of the mayonnaise is raised or 'buffered' by other salad ingredients.
- A toxin dose of less than 1 microgram in contaminated food will produce symptoms. In order to reach this level, there needs to be at least 106 organisms per gram of food. With the generation time of S. aureus being 30-40 minutes it can be seen that contamination is necessary but on its own is insufficient for an outbreak to occur. In particular, time/temperature abuse of the food following contamination must take place in order for the necessary bacterial growth to occur.
- Different staphylococcal enterotoxins continue to be identified. By the year 2000, 11 different protein enterotoxins had been identified: Staphylococcal Enterotoxin (SE)-A, B, C1, C2, C3, D, E, G, H, I and J. About two thirds of the enterotoxin-producing strains form one toxin only (usually type A) and the majority of the remainder form only two.
- The enterotoxins are exotoxins, being produced within the cell and then released/excreted into the food. The toxins are pepsin and trypsin resistant (this allows stability in the intestine) and very heat resistant - they are not inactivated by heat processing or cooking.
- Staphylococcal enterotoxins do not act directly on intestinal cells and therefore differ from classic enterotoxins, such as cholera toxin. The mode of action resembles that of a neurotoxin, in that they trigger vomiting through impulses (via the nervous system) to the vomiting centre of the brain.
Growth Factors
- For cell growth: >0.83 (NB Very tolerant to salt (up to 15 - 20% - the lowest level that can be withstood by a vegetative bacteria) and reduced aW.
- For toxin production: >0.86
- pH range: 4.2 - 9.3
- Optimum pH: 6.5 - 7.5
- Temperature range:
- For cell growth: 7oC -48oC (optimum 37oC); typical mesophile.
- For toxin production: 10oC - 46oC (optimum 35-40oC).
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