Mechanism of the temperature protective effect of salts on Staphylococcus aureus

In the previous paper we reported that the maximum temperature for growth of Staphylococcus aureus was about 2 degrees C higher in media supplemented with NaCl. We now show that MgCl2 was the most effective protectant at 0.4 M. NaCl and KCl were as effective as MgCl2 when tested at 1 M. NH4Cl was less effective at all concentrations and LiCl was not protective. Sucrose and glucose (1M) gave about half the biomass of 1M NaCl. Glycerol, Na2SO4, NaNO2, NaNO3, and CH3COONA were not protective. Protection is probably due to the nonpenetrating solute (sucrose) or the nonpenetrating Cl- anion. Mg2+ had an effect additional to that attributable to Cl- because MgSO4 and to a slight extent (CH3COO)2Mg were protective. The morphology of the cells grown at 45 degrees C in 1 M NaCl was abnormal: septation became irregular, cell walls were thickened, and the cells occurred in irregularly sized clumps surrounded by capsular material.