Combined Effect of Water Activity, pH and Sub-optimal Temperature on Growth and Enterotoxin Production of Staphylococcus aureus

Staphylococcus aureus growth and enterotoxin production in relation to a, (0.99–0.87), pH (4.0–7.0) and temperature (8–30°C) was determined in Brain-heart-infusion broth with respectively NaCl and sucrose as humectants. Growth of S. aureus was not observed at a_w 0.85, at pH 4.3 or at 8°C. At 12°C no growth occurred at a, 0.90 or at a, 0.93 in combination with pH < 5.5. At a_w 0.96 no growth occurred at pH ≤ 4.9. Production of staphylococcal enterotoxin type A (SEA) occurred under nearly all conditions allowing growth. Production of SEB appears to be determined by a_w; at a_w 0.96 SEB was produced at all temperatures allowing growth, while at a, 0.93 SEB was hardly produced. Production of SEC and SEF was affected both by a_w and temperaure. Production of these toxins was rarely observed at a, 0.93. No SEC and SEF were produced at 12°C. Extrapolation of these findings to food is discussed.