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 w ater 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-Hint on 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 try ptic soy broth alone after incubation for 24 h (P < 0.01). A membranous str ucture (an immature biofilm) was formed on the coverslips of tissue culture dishes by S. all,eus cells in plasma after incubation for 24 h, although t he colony counts of S. aureus cells in the immature biofilms in plasma cont aining acidic hot-spring water (60%, pH 5.4) were about eight limes lower t han 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 ov er 1000 times lower than those in plasma alone after incubation for 24 h. T hese results suggest that 50% acidic hot-spring water has a bacteriostatic effect, 60% acidic hot-spring water has a moderate bactericidal effect agai nst floating S. aureus cells and those cells in a biofilm, and, 60% acidic hot-spring water has an inhibitory effect on plasma coagulation and attachm ent of S. aureus cells. Furthermore, our present results suggest that a sma ll amount of some ions in hot-spring water such as manganese and iodide ion s are very important for a bactericidal activity of hot-spring water as wel l as the low pH condition. (C) 2000 Elsevier Science Ireland Ltd. All right s reserved.