Staphylococcus Aureus

S. aureus is a bacterium with strains that are capable of producing a highly heat-stable protein toxin that causes illness in humans. Staphylococcal food poisoning (staphyloenterotoxicosis; staphyloenterotoxemia) is the name of the condition caused by the enterotoxins which some strains of S. aureus produce.

What are the symptoms of staphylococcal food poisoning?

The onset of symptoms in staphylococcal food poisoning is usually rapid and in many cases acute. Severity of symptoms depend on individual susceptibility to the toxin, the amount of contaminated food eaten, the amount of toxin in the food ingested, and the general health of the victim. The most common symptoms are nausea, vomiting, retching, abdominal cramping, and prostration. Some individuals may not always demonstrate all the symptoms associated with the illness. In more severe cases, headache, muscle cramping, and transient changes in blood pressure and pulse rate may occur. Recovery generally takes two days, However, it us not unusual for complete recovery to take three days and sometimes longer in severe cases.

How is staphylococcal food poisoning diagnosed?

In diagnosing staphylococcal foodborne illness, it is important that victims be interviewed and data gathered and analyzed. Suspected foods should be collected and examined for staphylococci . The presence of relatively large numbers of enterotoxigenic staphylococci is good circumstantial evidence that the food contains toxin. The most conclusive test is the linking of an illness with a specific food. In cases where multiple foods are suspected, confirmation is obtained from the detection of the toxin in the food sample(s). In cases where the food may have been treated to kill the staphylococci , as in pasteurization or heating, direct microscopic observation of the food may be an aid in the diagnosis.

What foods are associated with staphylococcal food poisoning?

Foods that are frequently implicated in staphylococcal food poisoning include:

• Meat and meat products
• Poultry and egg products
• Salads such as egg, tuna, chicken, potato, and macaroni
• Bakery products such as cream-filled pastries, cream pies, and chocolate eclairs
• Sandwich fillings
• Milk and dairy products

Foods that require considerable handling during preparation and that are kept at slightly elevated temperatures after preparation are frequently involved in staphylococcal food poisoning. Staphylococci exist in air, dust, sewage, water, milk, and food or on food equipment, environmental surfaces, humans, and animals. Humans and animals are the primary reservoirs. Staphylococci are present in the nasal passages and throats and on the hair and skin of 50 percent or more of healthy individuals. This incidence is even higher for those who associate with or who come in contact with sick individuals and hospital environments. Although food handlers are usually the main source of food contamination in food poisoning outbreaks, equipment and environmental surfaces can also be sources of contamination with S. aureus . Human intoxication is caused by ingesting enterotoxins produced in food by some strains of S. aureus , usually because the food has not been kept hot enough (60°C, 140°F, or above) or cold enough (7.2°C, 45°F, or below).

How common is staphylococcal food poisoning?

The true incidence of staphylococcal food poisoning is unknown for a number of reasons, including:

• Poor responses from victims during interviews with health officials
• Misdiagnosis of the illness, which may have similar symptoms to other types of food poisoning (such as vomiting caused by Bacillus cereus toxin)
• Inadequate collection of samples for laboratory analyses
• Improper laboratory examination.

How serious is staphylococcal food poisoning?

Death from staphylococcal food poisoning is very rare, although such cases have occurred among the elderly, infants, and severely debilitated persons.

Who is susceptible to staphylococcal food poisoning?

All people are believed to be susceptible to this type of bacterial intoxication; however, intensity of symptoms may vary.

Example of a typical outbreak

1,364 children became ill out of a total of 5,824 who had eaten lunch served at 16 elementary schools in Texas. The lunches were prepared in a central kitchen and transported to the schools by truck. Epidemiological studies revealed that 95% of the children who became ill had eaten a chicken salad. The afternoon of the day preceding the lunch, frozen chickens were boiled for 3 hours. After cooking, the chickens were deboned, cooled to room temperature with a fan, ground into small pieces, placed into l2-inch-deep aluminum pans and stored overnight in a walk-in refrigerator at 42-45°F. The following morning, the remaining ingredients of the salad were added and the mixture was blended with an electric mixer. The food was placed in thermal containers and transported to the various schools at 9:30 AM to 10:30 AM, where it was kept at room temperature until served between 11:30 AM and noon.

Bacteriological examination of the chicken salad revealed the presence of large numbers of S. aureus . Contamination of the chicken probably occurred when it was deboned. The chicken was not cooled rapidly enough because it was stored in l2-inch-deep layers. Growth of the staphylococcus probably occurred also during the period when the food was kept in the warm classrooms. Prevention of this incident would have entailed:

• Screening the individuals who deboned the chicken for carriers of the staphylococcus
• More rapid cooling of the chicken
• Adequate refrigeration of the salad from the time of preparation to its consumption