Relationship between loss of magnesium and loss of salt tolerance after sublethal heating of Staphylococcus aureus

The effect of sublethal heating on Staphylococcus aureus was followed by plate counting on trypticase soy agar and by calculating decimal reduction times (D 52C ). Pseudodecimal reduction times (D ′ 52C ) were obtained from plate counts on trypticase soy agar containing 7.5% NaCl. D 52C was a measure of lethality and the ratio D′ 52C /D 52C was used as a measure of injury. Sublethal heating was carried out in various 100 mM (pH 7.2) solutions.Orthophosphates of potassium, ammonium, and sodium were more lethal than the corresponding chlorides (average D 52 values of 18.2 and 29.6 min respectively). Orthophosphates also appeared to be more injurious than chlorides (average D′ 52C /D 52C values of 0.43 and 0.62 respectively). 'Phosphate'-heated cells lost 28.4% of their cellular Mg while 'chloride'-heated cells lost 11% of their Mg. Good correlation exists between D′ 52C /D 52C values and the magnesium lost. Kinetic studies in potassium buffer also showed good correlation between loss of salt tolerance and loss of cellular Mg. This, and other evidence, suggests that loss of Mg is one of the primary events in the sublethal heat injury of S. aureus.