Main microbiological characteristics

A non spore-forming, non motile, facultative aeroanaerobic Gram-positive coccus, 0.5 to 1 micrometre in diameter, catalase- and coagulase-positive. S. aureus , the type species of the Staphylococcus genus is a golden yellow colony-forming Staphylococcus. It produces numerous toxins including staphylococcal enterotoxins (SEs) responsible for staphylococcal food-poisoning outbreaks (SFPO). To date, 21 different serotypes (SEA to SEE, SEG to SEV) have been described, six of them have been involved in SFPO: SEA (the most commonly detected in SFPO) to SEE and SEH. Unlike the bacterium, SEs are stable to heat treatments generally applied during food processing.
Other species of coagulase-positive staphylococci (CPS) can also produce SEs, but their role in SFPO has still not been clarified.

Zoonotic character

S. aureus is a zoonotic bacterium. However, the strains isolated in cases of SFPO are for the most part of human origin due to a human contamination of a food during the process.

Reservoir (telluric, environmental, animal, human)

Staphylococci are ubiquitous bacteria found on the skin, nose and throat in warm-blood animals (mammals and birds) and especially in humans. They have also been isolated in the natural environment (soil, water, dust and air), at home (kitchen, fridge), in hospitals environment, in food processing plants and from foodstuffs. Since S. aureus are found on the skin and mucous membranes of humans and animals, the presence of this germ in the environment is likely to have been caused by contamination by humans or animals.

Food(s) involved
S. aureus may be isolated from a wide variety of food. The most “at-risk” foods are:
•    recontaminated foods after heat treatment or any other process eliminating the competitive microfl ora. The more the food is handled, the higher the risk. For example, cooked and sliced meat, poultry and ham, mixed salads including rice and vegetable salads, cream cakes and cooked dishes handled after cooking;
•    fermented foods, where slow acidification permits growth of S. aureus during fermentation. For example, cheese or salami. The risk is enhanced when starters cultures are not used or poorly controlled;
•    dried and intermediate-moisture products where growth of S. aureus is favoured at some stage in processing or storage by a combination of temperature and reduced aw. For example, powered milk, pasta and dried fish.
Dishes requiring human handling (mixed salads, prepared dishes) and secondly, dairy products are the most commonly associated with SFPO reported in France.

Conditions leading to contamination
Four conditions are required for food contamination to occur:
•    a source of enterotoxin-producing staphylococci (raw material, healthy or infected carrier, etc.);
•    a transfer of staphylococci from source to food (dirty cutting tool, healthy or infected carrier, etc.);
•    a food presenting suitable physicochemical characteristics for S. aureus growth and toxinogenesis (see C.a, the most “at-risk” foods) and;
•    a suitable temperature and a sufficient time for sizeable bacterial growth and for toxin production.

Control measures (4) in the food sector
Good hygiene practices (GHP)
The prevention of SFPO is based on hygiene measures in order to avoid or limit the contamination of food by S. aureus. These must include monitoring of the health of animals (mastitis, for example), good manufacturing practices (GMPs) and cleaning and disinfection of equipments and environmental surfaces.
Moreover for people involved in food processing, the hand care as well as the wear of hair protection covering the whole of the scalp are essential good hygiene practices that must be followed. Food handlers presenting skin lesions must be excluded from handling non-packaged foods, while the lesions are not correctly covered (gloves).
Likewise, a mask must be worn when suffering from any throat type symptom. Regarding preventive measures carried out by at-risk food handlers, it is not necessary to screen S. aureus carrier before recruitment, nor to exclude a healthy carrier (nose or throat carriage), provided that these GHPs are applied.
Since these measures are not enough to achieve zero contamination, it is necessary to destroy staphylococci through suitable treatment, heat or other, before they have multiplied, or to prevent their growth by food storage under +6°C. Complying with the cold chain is vital as far as staphylococci are concerned. All food technological processes applied in a hazardous temperature zone must be short or rely on parameters other than temperature to stop bacterial growth. Since staphylococci are heatsensitive, whereas their enterotoxins are thermostable, making safe a highly contaminated product by S. aureus is not guaranteed by heat treatment. This will destroy the bacteria but not their enterotoxins if these are present.
It must be considered that, once preformed in food, enterotoxins cannot be eliminated effectively.
Characteristics of sanitary treatments
Destruction of S. aureus : heat treatment higher than pasteurisation, radiation at doses of around 5 kGray, treatment by aldehydes and alkaline products. For foods with an aw of 0.99 and pH between 6.5 and 7.0, the usual values of D (decimal reduction time) at 60°C and 72°C vary from 0.8 to 10 minutes and 0.1 to 1 second respectively, depending on the food and strain tested.
Destruction of SEs: no compatible treatment with food processes can guarantee complete inactivation.
Depending on the amount of enterotoxins initially present, sterilisation (of milk for example, UHT-type treatment) can only partially destroy the SEs, leaving enough in the food to cause SFPO.