Effects of growth at low water activity on the thermal tolerance of Staphylococcus aureus

Staphylococcus aureus is the most osmotolerant foodborne pathogen, and outb reaks of staphylococcal food poisoning are often linked to foods of reduced water activity (a(w)) values. While it is generally known that the thermal tolerance of microorganisms increases as the a(w) of the heating menstruum is decreased, surprisingly little research has examined the influence of g rowth medium a(w) on microbial thermal tolerance. In the present study, we show that growth of S. aureus at an a(w) value of 0.94 leads to the develop ment of dramatically enhanced thermal tolerance (i.e., less than 1 log redu ction after heating for 20 min at 60 degrees C). We further show that the i dentity of the accumulated compatible solute within cells grown at low a(w) can also influence the overall level of thermal tolerance of S. aureus. Fi nally, we provide evidence that the synthesis of general stress and/or osmo tic stress proteins is required for the development of enhanced thermal tol erance of S. aureus at low a(w).