Inhibitory Effect of Hexametaphosphate on the Growth of Staphylococcus aureus

Abstract

A longer-chain polyphosphate exhibited a stronger antibacterial activity on Staphylococcus aureus, although other polyphosphates had about the same chelation values. Hexametaphosphate (HP), a long-chain polyphosphate, showed a minimum inhibitory concentration of 0.05% on the growth of S. aureus and had the strongest antibacterial activity among the polyphosphates examined. The chelation value and growth inhibitory effect of HP, which was hydrolyzed into lower-molecular-weight substances by heating, gradually decreased during heating, but the patterns of decrease differed. These findings suggest that the antibacterial effect of HP on S. aureus depends not only on its chelating ability, but also on its polymeric nature. At 0.05 to 1.0%, HP exhibited no bactericidal action against S. aureus cells on incubation for 8hr; its action was bacteriostatic. The viability of HP-treated cells gradually decreased on an agar plate containing 7.5% sodium chloride but not on a normal agar plate during treatment. This result suggests that HP acts on the cell membrane and decreases the salt tolerance of S. aureus. HP induced leakage of magnesium from cells into the medium, and increased the leakage of amino acids and low-molecular nucleic acid-related substances from cells into deionized water. The cells slowly leaked large amounts of magnesium and lowmolecular substances, a small amount of proteins and no nucleic acids during HP treatment for 6hr. These results suggest a weak action of HP on the cell membrane and some damage to the cell membrane by HP. It was thus assumed that the antibacterial action of HP on S. aureus was caused by the loss of osmoregulation and selective permeability resulting from membrane damage, as well as a lowering of metabolic function arising from leakage of substrates.