Mechanism of the temperature protective effect of salts on Staphylococcus aureus

Abstract

In the previous paper we reported that the maximum temperature for growth of Staphylococcus aureus was about 2 °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 (1 M) gave about half the biomass of 1 M 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 °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.