Staphylococcus aureus

Staphylococcus aureus is important from both a medical and food perspective. S. aureus, also known as Golden staph, is resistant to some antibiotics and is responsible for many diseases in warm blooded animals. Illness is caused by the production of a toxin in the food as a result of bacterial growth. S aureus remains one of the most frequently reported causative agents of foodborne illness.

The most common source of the organism is the human body however, it can also be isolated from animals and poultry and hence can be present on raw meat . The human strains are more likely to be toxin producers. S. aureus can be part of the normal flora in the nose throat and on the skin of humans. They can also be present in skin eruptions such as boils acne and styes. Skin wounds, even minor ones, can harbour large numbers of S. aureus. Therefore the primary transfer of organisms is by the food handler.

Growth and toxin production can occur over a wide range of temperatures however, most cases occur from storage of susceptible foods at ambient temperatures for some period prior to consumption.

The organism is susceptible to heat but the toxin can show significant resistance so reheating foods in which the toxin has been allowed to develop will not necessarily inactivate the toxin.

This organism will be present in foods. Personal hygiene of the food handler is very important in preventing or minimizing transfer of the organism to foods. Handling of food should be kept to a minimum and tongs and implements used instead of hands. Infected wounds and skin eruptions should be covered.

Susceptible foods must not be stored in the temperature danger zone.

Description

S. aureus is a facultative anaerobic Gram-positive coccus, that is catalase positive and oxidase negative. Under the microscope they usually appear as grape-like clusters. They can be found in the air, dust, water and human faeces, and can be present on clothing and utensils handled by man. Staphylococci are a normal part of the microflora of the nose throat and skin. It can be found on other parts of the body but the nasal passage is the most significant site.

The carrier rate varies with different populations and studies have found a carriage rate of 10-40% in adults outside the hospital environment. Carriage may be intermittent or continuous over a long period of time. Approximately 15-20% of humans carry enterotoxin producing staphylococci.

Various types of skin eruptions and inflammations (boils acne styes etc) and wounds can harbour large numbers of these micro-organisms.

Animals and poultry can also carry S. aureus on various parts of their bodies. Udders and teat canals in cows are a source of S. aureus. It can be isolated from the milk of healthy cows and high levels are found in milk from cows suffering from mastitis.

High levels are also found on the skin of pigs and some strains have become endemic in poultry processing plants.

Strains from animal sources are less likely to produce endotoxins than strains from human sources.

Despite being a hardy organism S. aureus grows poorly in complex microbial ecosystems and it can be inhibited or overgrown by other organisms present in food. This poor competitive ability probably limits the growth of the organism in many foods. Those foods most at risk are cooked meats where other organism have been destroyed and cured meats where the high salt level limits the growth of other organisms.

Illness is not caused by the organism but by a toxin. To date 11 different toxins have been identified and implicated in foodborne illness.

The growth conditions for the organism and the conditions under which the toxin is produced vary slightly.

Parameter

 S aureus Growth

 Toxin production

Temperature

7-48ºC optimum 37ºC

10-48ºC optimum 40-45ºC

Water Activity

0.83->0.99 aerobic

0.90->0.99 anaerobic

0.85->0.99 aerobic

0.92->0.99 anaerobic

pH

4-10 optimum 6-7

4-9.6 optimum 7-8

 

S. aureus are fairly heat sensitive, however, heat can not be relied on to make food safe as the toxin is heat stable.

Characteristics of Illness

Staphylococcal food poisoning is caused by the ingestion of a heat-stable enterotoxin produced in the food by some strains of S. aureus.

  • Infective dose - It is thought that as little as 1.0 nanogram of the toxin per gram of food will produce symptoms of the illness. Levels of ingested toxin from 1-5 microgram have been associated with many outbreaks.
  • Time of onset - The onset of symptoms is usually 1-6 hours. The onset and severity of the illness is usually dependent on the individuals susceptibility to the toxin, the amount of contaminated food eaten, and the amount of toxin in the food ingested.
  • Symptoms - The most common symptoms are nausea, vomiting, diarrhoea and abdominal cramping. In more severe cases, headache, muscle cramping, and transient changes in blood pressure may occur.
  • Duration of illness - The illness is usually over within one to three days, however, in severe cases recovery can take longer.

Diagnosis of human illness

Due to the self-limiting nature of the illness, diagnosing foodborne disease caused by S. aureus relies on proper interviews with those affected, and effective gathering and analysis of epidemiological data. Incriminated foods should be collected and examined for staphylococci or the enterotoxin produced. The latter is especially important when foods that have been heated before consumption are implicated in the outbreak. For some outbreaks food handlers are also tested to ensure they are not the carriers of the strain responsible.

Complications

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

Susceptible individuals

All people are believed to be susceptible to this type of bacterial intoxication, however, the intensity of the symptoms may vary between individuals and their state of health.

Associated foods

Many foods have been incriminated in staphylococcal food poisoning including meat and meat products, ham; poultry and egg products; salads such as egg, tuna, chicken, potato, and pasta; bakery products such as cream-filled pastries and cakes; sandwich fillings; and milk and dairy products. Since S. aureus is a poor competitor in the presence of other micro-organisms, foods responsible for outbreaks are often those that have been heated to destroy micro-organisms, and then require some food handling and storage at room temperature. S. aureus usually contaminates the food during the handling stage after cooking.

Food analysis

S. aureus can easily be isolated from foods using traditional cultural techniques, although these techniques will not allow for the identification of strains capable of producing enterotoxin. The enterotoxin production by a strain can be determined by using serological methods. For detecting the staphylococcal enterotoxin in foods, the toxin must be extracted from the food and concentrated before testing.

Outbreaks

The incidence of food borne disease caused by S. aureus is thought to be much higher than reported. Since most people recover within 1 to 2 days, many do not seek medical advice.

In September 1997 in the USA, 13 people become ill after attending a luncheon. The main symptoms were nausea and vomiting, with some people also experiencing diarrhoea, sweating and chills. The onset of the illness was between 1 and 7 hours with most recovering within a few days. Investigations into the illness revealed the source of the outbreak to be baked ham. Baked ham is often implicated in foodborne disease outbreak associated with S. aureus because the baking destroys many of the micro-organisms present on the ham, and the ham becomes contaminated with S. aureus after baking. The salt content of baked ham is high enough to stop many other micro-organisms from growing, while still allowing S. aureus to grow.

Other foodborne disease outbreaks caused by S. aureus have occurred in institutions such as schools and hospitals where food was prepared in bulk and stored for more than 4 hours at ambient temperatures.

A number of studies suggest that food handlers can serve as a source of infection and that hand contact with the foods represent the means by which the contamination may occur.

Prevention

  • Cool cooked food quickly and minimize the time food is stored in the Temperature Danger Zone (50C to 60°C).
  • Use clean utensils NOT HANDS to handle food.
  • Wash hands with soap after going to the toilet, in between handling raw and cooked foods or using a handkerchief or tissue. (S aureus is carried in the nasal passage)
  • Skin lesions must be covered with an appropriate dressing and if necessary a water proof outer cover such as a finger stall.