Antimicrobial Activity of Electrolyzed NaCl Solutions: Effect on the Growth of Streptomyces spp.

Abstract

An acidic solution (pH 2.5∼2.6) with a high oxidation-reduction potential (ORP; about +1,170 mV) and an alkaline solution (pH 11.5∼11.7) with a low ORP (about −880mV) that resulted from electrolysis of 20 mM NaCl (dissolved in a pure water) were tested for their effect on the growth of Streptomyces spp. When spores (∼2×107) were exposed to the electrolyzed solutions (2 ml) for 1 minute, colony formation was totally inhibited by the acidic solution, but little by the alkaline solution although extending the exposure (10 minutes) resulted in a marked inhibition. The 1 minute exposure to their mixture (1:1, v/v) showed a strong inhibition (but weaker than that of the acidic solution). When the unexposed spores were streaked and incubated on ISP No. 4 (inorganic salts - starch medium) agar plate containing a cross density gradient of the acidic and alkaline solutions, a biased growth inhibition toward the acidic solution side was observed although the pH range of the acidic solution end of the plate was around 6.2. It seemed thus unlikely that low pH value contributed to the antimicrobial activity of the acidic solution. It was notable that S. griseus SS-1198 formed a unique morphology on the cross gradient plate.In addition, clear growth inhibition by the acidic solution was observed without direct contact with spores, probably because of chlorine gas release. Acidic solutions (pH 2.6∼2.7) resulting from the electrolysis of 20 mM of Na2SO4 show no significant antimicrobial activity when tested by the cross gradient plate method. It thus seemed likely that chlorine played a key role for the antimicrobial activity of the acidic electrolyzed NaCl solution.