Antimicrobial effects of acidic hot-spring water on Staphylococcus aureus strains isolated from atopic dermatitis patients.

The present study examined the antimicrobial effects of acidic hot-spring water on Staphylococcus aureus strains isolated from atopic dermatitis (AD) patients. Plasma coagulation by S. aureus cells was not detected in plasma containing acidic hot-spring water (60%, pH 5.4) or hydrochloric acid (pH 5.0) after incubation for 24 h. S. aureus cells did not grow in Mueller-Hinton broth with acidic hot-spring water (50%, pH 4.4) after 24 h incubation. The colony counts of S. aureus cells in tryptic soy broth containing acidic hot-spring water (60%, pH 3.9) were over ten times lower than those in tryptic soy broth alone after incubation for 24 h (P<0.01). A membranous structure (an immature biofilm) was formed on the coverslips of tissue culture dishes by S. aureus cells in plasma after incubation for 24 h, although the colony counts of S. aureus cells in the immature biofilms in plasma containing acidic hot-spring water (60%, pH 5.4) were about eight times lower than those in plasma alone after incubation for 24 h (P<0.01). The colony counts of S. aureus cells that attached on coverslips in plasma containing acidic hot-spring water (60%, pH 5.4) or hydrochloric acid (pH 5.4) were over 1000 times lower than those in plasma alone after incubation for 24 h. These results suggest that 50% acidic hot-spring water has a bacteriostatic effect, 60% acidic hot-spring water has a moderate bactericidal effect against floating S. aureus cells and those cells in a biofilm, and, 60% acidic hot-spring water has an inhibitory effect on plasma coagulation and attachment of S. aureus cells. Furthermore, our present results suggest that a small amount of some ions in hot-spring water such as manganese and iodide ions are very important for a bactericidal activity of hot-spring water as well as the low pH condition.