Drinking water distribution systems, including premise plumbing, contain a diverse microbiological community that may include opportunistic pathogens. On-site supplemental disinfection systems have been proposed as a control method for opportunistic pathogens in premise plumbing. The majority of on-site disinfection systems to date have been installed in hospitals due to the high concentration of opportunistic pathogen susceptible occupants. The installation of on-site supplemental disinfection systems in hospitals allows for evaluation of the impact of on-site disinfection systems on drinking water system microbial ecology prior to widespread application. This study evaluated the impact of supplemental monochloramine on the microbial ecology of a hospital’s hot water system. Samples were taken three months and immediately prior to monochloramine treatment and monthly for the first six months of treatment, and all samples were subjected to high throughput Illumina 16S rRNA region sequencing. The microbial community composition of monochloramine treated samples was dramatically different than the baseline months. There was an immediate shift towards decreased relative abundance of Betaproteobacteria, and increased relative abundance of Firmicutes, Alphaproteobacteria, Gammaproteobacteria, Cyanobacteria and Actinobacteria. Following treatment, microbial populations grouped by sampling location rather than sampling time. Over the course of treatment the relative abundance of certain genera containing opportunistic pathogens and genera containing denitrifying bacteria increased. The results demonstrate the driving influence of supplemental disinfection on premise plumbing microbial ecology and suggest the value of further investigation into the overall effects of premise plumbing disinfection strategies on microbial ecology and not solely specific target microorganisms.
This investigation compared biological quantification of potable and non-potable (cooling) water samples using pour plate heterotrophic plate count (HPC) methods and adenosine triphosphate (ATP) concentration measurement using bioluminescence. The relationship between these measurements and the presence of Legionella spp. was also examined. HPC for potable and non-potable water were cultured on R2A and PCA, respectively. Results indicated a strong correlation between HPC and ATP measurements in potable water (R = 0.90, p < 0.001). In the make-up water and two cooling towers, the correlations between ATP and HPC were much weaker but statistically significant (make-up water: R = 0.37, p = 0.005; cooling tower 1: R = 0.52, p < 0.001; cooling tower 2: R = 0.54, p < 0.001). For potable and non-potable samples, HPC exhibited higher measurement variability than ATP. However, ATP measurements showed higher microbial concentrations than HPC measurements. Following chlorination of the cooling towers, ATP measurements indicated very low bacterial concentrations (<10 colony-forming units (CFU)/mL) despite high HPC concentrations (>1000 CFU/mL) which consisted primarily of non-tuberculous mycobacteria. HPC concentrations have been suggested to be predictive of Legionella presence, although this has not been proven. Our evaluation showed that HPC or ATP demonstrated a fair predictive capacity for Legionella positivity in potable water (HPC: receiver operating characteristic (ROC) = 0.70; ATP: ROC = 0.78; p = 0.003). However, HPC or ATP correctly classified sites as positive only 64 and 62% of the time, respectively. No correlation between HPC or ATP and Legionella colonization in non-potable water samples was found (HPC: ROC = 0.28; ATP: ROC = 0.44; p = 0.193).
This is the first evaluation in the United States of a commercially available monochloramine system installed on a hospital hot water system for Legionella disinfection, and it demonstrated a significant reduction in Legionella colonization. Significant increases in microbial populations or other negative effects previously associated with monochloramine use in large municipal cold water systems were not observed.
This article reviews how Legionella and other waterborne pathogens can present a risk to consumers of potable water, secondary disinfection options, and a case study on chlorine dioxide.
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