Protection Against Heat-Injury in Staphylococcus aureus by Solutes

The effect of solutes on heat-injury in Staphylococcus aureus 196E was studied in 25% ground beef (GB) slurry or distilled water equilibrated at 49 C. Exposure to 49 C for 90 min resulted in a 3-4 log cycle increase in injured cells. The number of injured cells was the difference between bacterial counts on tryptic soy agar (TSA) + 1% pyruvate and TSA + 9% NaCl. Increasing levels of NaCl (1-9%) added to GB slurry gave increasing protection against heat-injury and resulted in a decrease in the number of injured S. aureus ; glycerol and sucrose had a similar effect. At 0.85 M (equivalent to 5% NaCl), other compounds such as sodium citrate, KCl, NaNO₃, Na₂SO₄, Na₂HPO₄, NH₄Cl, CaCl₂, and LiCl were more effective than NaCl in protecting against heat injury; sodium acetate, MgSO₄, NaI, MnCl₂, MgCl₂, NaBr, NaH₂PO₄, and KI were less effective than NaCl. In the presence of 5% NaCl, it was necessary to raise the temperature from 49 to 55 C to obtain significant heat-injury to S. aureus . Addition of NaCl prevented the leakage of UV-absorbing materials and decreased the extent of magnesium ion leakage from heat-injured staphylococci.