Abstract
Physical and chemical disinfection methods have been proposed with the aim of controlling Legionella water contamination. To date, the most effective procedures for reducing bacterial contamination have not yet been defined. The aim of this study was to assess the long-term effectiveness of various disinfection procedures in order to reduce both culturable and nonculturable (NC) legionellae in different hospital water networks treated with heat, chlorine dioxide, monochloramine, and hydrogen peroxide. The temperature levels and biocide concentrations that proved to give reliable results were analyzed. In order to study the possible effects on the water pipes, we verified the extent of corrosion on experimental coupons after applying each method for 6 months. The percentage of positive points was at its lowest after treatment with monochloramine, followed by chlorine dioxide, hydrogen peroxide, and hyperthermia. Different selections of Legionella spp. were observed, as networks treated with chlorine-based disinfectants were contaminated mainly by Legionella pneumophila serogroup 1, hyperthermia was associated with serogroups 2 to 14, and hydrogen peroxide treatment was associated mainly with non-pneumophila species. NC cells were detected only in heat-treated waters, and also when the temperature was approximately 60°C. The corrosion rates of the coupons were within a satisfactory limit for water networks, but the morphologies differed. We confirm here that chemical disinfection controls Legionella colonization more effectively than hyperthermia does. Monochloramine was the most effective treatment, while hydrogen peroxide may be a promising alternative to chlorine-based disinfectants due to its ability to select for other, less virulent or nonpathogenic species.
Highlights
The number of cases of Legionnaires’ disease has increased steadily over the years, especially in Italy and Europe [1, 2]
Regarding Legionella concentrations, no differences in the geometric mean were observed according to chemical treatments (2.2 ϫ 102 CFU literϪ1 for monochloramine, 3.0 ϫ 102 CFU literϪ1 for chlorine dioxide, and 1.3 ϫ CFU literϪ1 for hydrogen peroxide), while a significantly higher geometric mean (1.7 ϫ CFU literϪ1) was observed for the heat-treated positive samples than for the samples treated with all biocides (P Ͻ 0.05 by the Bonferroni test)
The bacterial load did not differ according to the biocide/temperature level, and only a limited number of samples exceeded 1.0 ϫ 104 CFU literϪ1
Summary
The number of cases of Legionnaires’ disease has increased steadily over the years, especially in Italy and Europe [1, 2]. A range of physical and chemical disinfection methods have been proposed, but to date, the most effective procedures for controlling contamination have not been defined [6] Their impacts on pipe deterioration/corrosion have not been documented extensively and are studied mainly in model water distribution systems [7,8,9,10]. The main aim of the present study was to assess the long-term effectiveness of monochloramine, chlorine dioxide, hydrogen peroxide, and heat in reducing/eliminating both culturable and nonculturable (NC) legionellae in various hospital water networks. For this purpose, traditional culture was used with qPCR alone and in combination with EMA. In order to verify the possible corrosive action of each disinfection procedure, carbon steel coupons were inserted along the water distribution systems and were periodically examined for weight loss and morphology
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