Enterotoxins and Food Products

The primary source of staphylococcal food contamination is human. However, cows are an additional source - staphylococci are causative agents of mastitis in cows and so may contaminate dairy products (Ombui et al., 1992). Although staphylococci are frequently present in pets (e.g., in dogs, which may suffer from mouth acne), these animals are not critical vehicles of food contamination. Besides dairy products, staphylococci are also present in almost 30% of raw pork, salted meat, and uncooked smoked ham (Atanassova et al., 2001). Meat is frequently contaminated with S. aureus and may contain high numbers of colony-forming units (cfu) per gram (Surkiewicz et al., 1975). Home-made cakes, creams, and vegetable salads are important vehicles of staphylococci in central Europe.

Compared with other human pathogens, staphylococci are resistant to low aw and so their outgrowth in cakes and sweets is possible.

Contamination of food products with staphylococci is not synonymous with the presence of enterotoxins. During production and maturation of Burgos cheese prepared from sheep milk, an increase in the number of S. aureus cfu by up to x103 to x104 units was observed. However, enterotoxins were not present (Otero et al., 1988). The enterotoxin production in maturing Camembert cheese produced from raw goat milk revealed that it depended on the initial S. aureus cfu (Meyrand et al., 1998). The SE synthesis may be inhibited by pH and glucose (Smith et al., 1986). The outgrowth of enterotoxic S. aureus is partially inhibited by starters containing Lactobacillus sake, Pediococcus pentosaceus, and Staphylococcus xylosus, used in sausage manufacturing. The production of SEs: A, B, C1 and D is completely inhibited (Gonzalez-Fandos et al., 1994). SEA production depends on the staphylococ-cal growth rate.

Temperature also influences the synthesis of enterotoxins - they are usually produced at temperatures close to optimal (Yang et al., 2001). Out of 77 S. aureus strains isolated from food, 58% produced SEA, SEB, or SEE. The lowest temperature limit for the growth was 6.5 to 12.5°C, the highest, 39.5 to 48.5°C. The lowest level for enterotoxin production was 14 to 38°C, the highest, 35 to 44°C (Schmitt et al., 1990).

The amino acid composition of the environment also plays an important role. Valine is required for the outgrowth of S. aureus, arginine and cysteine, for the production of SEA, SEB, and SEC (Onoue and Mori, 1997). A low concentration of oleuropein (0.1%) - a phenol compound extracted from olives - retards the growth of S. aureus. Higher concentrations (>0.2%) inhibit completely the growth and production of enterotoxins (Tranter et al., 1993).

In the U.S., SFPs are not obligatorily reported diseases. SFPs comprise 14% of all foodborne diseases and it is estimated that only a small percentage of cases are noted by official sanitary and medical authorities. In other countries, such as Hungary, the number of cases is estimated to be about 40% of the total (Bergdoll, 1979). The frequency of SFPs in Japan is 20 to 25%, and rice balls are usually responsible for the disease occurrence. Corned beef and, less frequently, milk are reported in many countries as the main sources of infections. As Staphylococcus are mesophilic microorganisms, conditions for growth are better in summer. However, the major factors promoting foodborne poisonings are improper hygiene during food processing, distribution, and storage.

The most frequently observed symptoms include vomiting (82%), nausea (74%), abdominal cramps (64%), diarrhea (64%), and also headaches and muscular cramping. The onset of symptoms usually starts after 6 to 10 hours after food ingestion, however they may be reported earlier. The lethal cases are rare and occur among infants and elderly people. Only 10% of affected people need medical treatment - treatments include analgesics, antidiarrheic drugs, and administration of fluids.

The precise infectious dose has not been established. It probably ranges from 105 to 108 cfu per g and depends on the environmental conditions and strain properties. The SE concentration of 1ng per g of contaminated food is necessary to trigger the disease symptoms. However, some cases have been described where the dose activating disease symptoms was significantly lower. Tests on human volunteers revealed that 20 to 25 mg (0.4 mg per kg of body weight) led to vomiting.

In addition to SFPs, enterotoxins may participate in the development of atopic eczema (Morishita et al., 1999; Wehner and Neuber, 2001) and menstrual toxic-shock syndrome (MTSS) (Morishita et al., 1999).

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