what is Staphylococcus aureus?

S. aureus is a Gram-positive, non-spore-forming spherical bacterium belonging to the Staphylococcus genus. The Staphylococcus genus is subdivided into 32 species and subspecies. S. aureus produces staphylococcal enterotoxin (SE) and is responsible for almost all staphylococcal food poisoning (Montville and Matthews 2008; FDA 2012). S. intermedius, a Staphylococcus species commonly associated with dogs and other animals, can also produce SE and has been rarely associated with staphylococcal food poisoning (Talan et al. 1989; Khambaty et al. 1994; Le Loir et al. 2003). 

 

The growth and survival of S. aureus are dependent on a number of environmental factors such as temperature, water activity (aw), pH, the presence of oxygen and the composition of the food (refer to Table 1). These physical growth parameters vary for different S. aureus strains (Stewart 2003). The temperature range for growth of S. aureus is 7–48°C, with an optimum of 37°C. S. aureus is resistant to freezing and survives well in food stored below -20°C; however, viability is reduced at temperatures of -10 to 0°C. S. aureus is readily killed during pasteurisation or cooking. Growth of S. aureus occurs over the pH range of 4.0–10.0, with an optimum of 6–7 (ICMSF 1996; Stewart 2003). S. aureus is uniquely resistant to adverse conditions such as low aw, high salt content and osmotic stress. In response to low aw, several compounds accumulate in the bacterial cell, which lowers the intracellular aw to match the external aw (Montville and Matthews 2008). As such, most S. aureus strains can grow over an aw range of 0.83 to >0.99 (FDA 2012). S. aureus is a poor competitor, but its ability to grow under osmotic and pH stress means that it is capable of thriving in a wide variety of foods, including cured meats that do not support the growth of other foodborne pathogens (Montville and Matthews 2008). S. aureus is a facultative anaerobe so can grow under both aerobic and anaerobic conditions. However, growth occurs much slower under anaerobic conditions (Stewart 2003). For a non-sporing mesophilic bacterium, S. aureus has a relatively high heat resistance (Stewart 2003). The observed average decimal reduction value (D-value, the value at which the initial concentration of bacterial cells would be reduced by 1 log10 unit) was 4.8–6.6 min at 60°C when heated in broth (Kennedy et al. 2005). The bacteria have a higher heat resistance when it is encapsulated in oil, with a D-value at 60°C of 20.5 min for S. aureus in fish and oil (Gaze 1985). An extremely heat-resistant strain of S. aureus (D-value at 60°C of >15 min in broth) has been recovered from a foodborne outbreak in India (Nema et al. 2007).

 

Staphylococcal food poisoning symptoms generally have a rapid onset, appearing around 3 hours after ingestion (range 1–6 hours). Common symptoms include nausea, vomiting, abdominal cramps and diarrhoea. Individuals may not demonstrate all the symptoms associated with the illness. In severe cases, headache, muscle cramping and transient changes in blood pressure and pulse rate may occur. Recovery is usually between 1–3 days (Stewart 2003; FDA 2012). Fatalities are rare (0.03% for the general public) but are occasionally reported in young children and the elderly (4.4% fatality rate) (Montville and Matthews 2008). S. aureus can cause various non-food-related health issues such as skin inflammations (e.g. boils and styes), mastitis, respiratory infections, wound sepsis and toxic shock syndrome (Stewart 2003; Montville and Matthews 2008)

 

Staphylococcal food poisoning occurs when food is consumed that contains SE produced by S. aureus. Food handlers carrying enterotoxin-producing S. aureus in their noses or on their hands are regarded as the main source of food contamination via direct contact or through respiratory secretions (Argudin et al. 2010).

 

All people are believed to be susceptible to staphylococcal food poisoning. However, the severity of symptoms may vary depending on the amount of SE consumed in the food and the general health of individuals. The young and elderly are more likely to develop more serious symptoms (FDA 2012).