Foodborne diseases / Staphylococcus aureus

Staphylococcus aureus

Staphylococcal food poisoning is an intoxication that follows ingestion of food containing enterotoxin produced by Staphylococcus aureus. Many foods contain low numbers of the organism and may only become hazardous when temperature abuse occurs, leading to growth and enterotoxin formation.

1. Bacteriological characteristics

Staph. aureus is a Gram-positive, catalase-positive organism, occurring as irregular clusters of cocci. Growth is possible under both aerobic and anaerobic conditions, although the rate of growth tends to be slower anaerobically. The enterotoxin shows greater heat resistance than the cells themselves. There are seven antigenic distinct types: A, B, C1, C2, C3, D and E. Most food poisoning involves types A and D toxins and as little as 0.1 μg/kg of toxin can cause human illness. Some strains produce more than one types of toxin. Distinctive properties of Staph. aureus include strong coagulase activity and the production of a thermostable nuclease.

2. Disease in humans

On raw foods, Staph. aureus is a poor competitor against other kinds of bacteria and seldom causes illness from this situation. Food poisoning usually arises when a cooked food is contaminated by an infected handler and then kept warm for several hours. Symptoms are likely to appear 2-4 hours after ingestion of toxin and commonly include nausea, vomiting, abdominal pain and diarrhoea. Recovery may be expected within 48 hours; in some cases, however, dehydration occurs and hospital treatment may be necessary.

3. Sources of infection and epidemiology

Staph. aureus is ubiquitous and is sometimes found in the nasopharynx and on the skin of man and all food animals. Carriage is generally asymptomatic, although boils, cuts and other lesions may be infected. Contamination of food occurs through poor handling practices, Human biotypes of the organism are more often producers of enterotoxin and up to half of the human population can be carriers. The foods most commonly implicated in staphylococcal food poisoning are cream cakes, cooked meats and shellfish, while certain cheeses and fermented sausages can be involved when faults occur in the production process, The organism grows well on cured meats.

4. Growth properties in relation to meat and meat products

Growth of Staph. aureus occurs at 7and 48°C. Conditions under which enterotoxin is formed are more restricted than those allowing growth. The organism is highly resistant to freezing and thawing and survives well in frozen foods; it is also tolerant to dry conditions and can multiply at water activity values down to 0.85. Growth also occurs in the presence of 25% w/w NaCl and at pH 4.3 or less, when other conditions are optimal. However, the organism is not usually resistant to chemical disinfection.

5. Isolation

It is seldom necessary to recover the organism from foods by enrichment (low numbers are usually unimportant) and isolation is normally by direct plating of samples on an agar medium such as that of Baird Parker. Apart from selective agents, this medium contains egg yolk which assists the recovery of any damaged cells.

6. Control measures in slaughterhouses and cutting plants

Low numbers of Staph. aureus are to be expected on raw meat products. Where foci of contamination occur, as in some poultry defeathering machines, counts may be increased and can lead to rejection of meat required for product manufacture, e.g. mechanically recovered meat. The problem can be minimised in this case by vigorous cleaning and disinfection of the machines and regular replacement of rubber “fingers” to avoid excessive wear and hence niches for staphylococcal growth. The presence of Staph. aureus is particularly important in relation to cooked products, where poor handling practices or inadequate temperature control may be indicated. Growth and toxin production in fermented meats, e.g. salami, would suggest a fault in the fermentation process.