Abstract
Huwa-San peroxide (hydrogen peroxide; HSP) is a NSF Standard 60 (maximum 8mg/L-1) new generation peroxide stabilized with ionic silver suitable for continuous disinfection of potable water. Experiments were undertaken to examine the mechanism of HSP against planktonic and biofilm cultures of indicator bacterial strains. Contact/kill time (CT) relationships that achieve effective control were explored to determine the potential utility in primary disinfection. Inhibitory assays were conducted using both nutrient rich media and a medium based on synthetic wastewater. Assays were compared for exposures to three disinfectants (HSP, laboratory grade hydrogen peroxide (HP) and sodium hypochlorite) at concentrations of 20 ppm (therefore at 2.5 and 5 times the NSF limit for HP and sodium hypochlorite, respectively) and at pH 7.0 and 8.5 in dechlorinated tap water. HSP was found to be more or equally effective as hypochlorite or HP. Results from CT assays comparing HSP and HP at different bacterial concentrations with neutralization of residual peroxide with catalase suggested that at a high bacterial concentration HSP, but not HP, was protected from catalase degradation possibly through sequestration by bacterial cells. Consistent with this hypothesis, at a low bacterial cell density residual HSP was more effectively neutralized as less HSP was associated with bacteria and therefore accessible to catalase. Silver in HSP may facilitate this association through electrostatic interactions at the cell surface. This was supported by experiments where the addition of mono (K+) and divalent (Ca+2) cations (0.005-0.05M) reduced the killing efficacy of HSP but not HP. Experiments designed to distinguish any inhibitory effect of silver from that of peroxide in HSP were carried out by monitoring the metabolic activity of established P. aeruginosa PAO1 biofilms. Concentrations of 70-500 ppm HSP had a pronounced effect on metabolic activity while the equivalent concentrations of ionic silver (50- 375 ppb) had a negligible effect, demonstrating that the microbiocidal activity of HSP was due to peroxide rather than silver. Overall, it was found that the antimicrobial activity of HSP is enhanced over that of hydrogen peroxide; the presence of the ionic silver enhances interactions of HSP with the bacterial cell surface rather than acting directly as a biocide at the tested concentrations.
Highlights
Chlorine or sodium hypochlorite (NaOCl) has long been used as an effective disinfectant for drinking water [1], its use first coming into practice during the mid 1880s to help deal with typhoid fever epidemics in the port of Pola on the Adriatic Sea and in Maidstone, England [2]
The E. coli K12, B. subtilis, and environmental E. coli isolates were maintained on Luria Bertani (LB) agar while the S. aureus and P. aeruginosa PAO1 isolates were maintained on full strength Tryptic Soy agar (TSA)
A panel of Gram negative or positive laboratory and environmental isolates was selected to compare the Minimum inhibitory concentration (MIC) for HSP, H2O2, and NaOCl
Summary
Chlorine or sodium hypochlorite (NaOCl) has long been used as an effective disinfectant for drinking water [1], its use first coming into practice during the mid 1880s to help deal with typhoid fever epidemics in the port of Pola on the Adriatic Sea and in Maidstone, England [2]. Since that time, owing to the important public health outcome from the inactivation of microbial pathogens in drinking water supplies, the demands for cost effective, large-scale provision of potable water have driven an expansion in the use of chlorination worldwide. Communities ranging in size from large urban centres to individual family homes in rural settings all require access to safe drinking water that is provided through a wide variety of distribution system types. These systems experience a range of operational issues that are often of a microbial character, such as persistence of pathogens and development of microbial biofilms for which disinfection or microbial control strategies are important [3]. Alternative measures to chlorination or post-chlorination applications that mitigate the negative aspects could extend the usefulness of chlorine in providing safe drinking water
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