Diagnostic testing for Legionnaires' disease.

Legionnaires' disease is commonly diagnosed clinically using a urinary antigen test. The urinary antigen test is highly accurate for L. pneumophila serogroup 1, however other diagnostic tests should also be utilized in conjunction with the urinary antigen as many other Legionella species and serogroups are pathogenic. Culturing of patient specimens remains the gold standard for diagnosis of Legionnaires' disease. Selective media, BYCE with the addition of antibiotics, allows for a high sensitivity and specificity. Culturing can identify all species and serogroups of Legionella. A major benefit of culturing is that it provides the recovery of a patient isolate, which can be used to find an environmental match. Other diagnostic tests, including DFA and molecular tests such as PCR and LAMP, are useful tests to supplement culturing. Molecular tests provide much more rapid results in comparison to culture, however these tests should not be a primary diagnostic tool given their lower sensitivity and specificity in comparison to culturing. It is recommended that all laboratories develop the ability to culture patient specimens in-house with the selective media.

Lack of correlation between Legionella colonization and microbial population quantification using heterotrophic plate count and adenosine triphosphate bioluminescence measurement.

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).

Monochloramine and chlorine dioxide for controlling Legionella pneumophila contamination: biocide levels and disinfection by-product formation in hospital water networks.

Legionella colonization in hospital hot water distribution networks was evaluated following 36 months of continuous treatment with monochloramine and compared with chlorine dioxide. Nitrite, nitrate, chlorite, chlorate, bromide, trihalomethanes and haloacetic acids as well as the biocide concentration at sampled points were measured. Only 8/84 samples treated with monochloramine were found contaminated and after the first 8 months of treatment no Legionella was isolated. Chlorine dioxide was associated with a strong reduction in Legionella contamination compared to pre-treatment, but differences according to the device were observed. Monochloramine between 2 and 3 mg l(-1) and chlorine dioxide between 0.50 and 0.70 mg l(-1) were needed to control Legionella colonization. Comparing no- and post-flush samples, a higher frequency of no-flush positive samples was noted using chlorine dioxide, suggesting an increased risk for patients when they open the tap. No increase in chlorite levels and no water nitrification occurred by using monochloramine. Chlorite at levels exceeding the limit requested for drinking water was measured when chlorine dioxide was applied. In conclusion, we highlight that continuous injection of monochloramine should be considered as an effective alternative to chlorine dioxide in controlling legionellae contamination inside hospital water distribution systems.

A bronchoscopy-associated pseudo-outbreak of Mycobacterium chelonae and Mycobacterium mucogenicum associated with contaminated ice machine water and ice.

A pseudo-outbreak of bronchoscopy-associated Mycobacterium chelonae and M. mucogenicum was traced to contaminated ice machine water and ice. A nonsterile ice bath was used to cool uncapped, sterile, saline syringes used to slow procedural bleeding. Joining the growing evidence of bronchoscopy pseudo-outbreaks, our investigation describes several lessons for future prevention.

Prevention of hospital-acquired legionellosis.

PURPOSE OF REVIEW: The incidence of hospital-acquired legionellosis appears to be increasing. Presence of Legionella in the hospital drinking water is the only risk factor known with certainty to be predictive of risk for contracting Legionnaires' disease. RECENT FINDINGS: Given the high frequency of infection by nonpneumophila and nonserogroup 1 species, both Legionella respiratory culture on selective media and urine antigen testing should be available in the hospital clinical microbiology laboratory. If the drinking water is contaminated by nonpneumophila or nonserogroup 1 species, Legionella culture on selective media must be available for patients with hospital-acquired pneumonia. The impact of PCR application for environmental water specimen remains to be elucidated. Its advantage is that it is a rapid test and its weakness is its low specificity. Copper-silver ionization disinfection and point-of-use (POU) filters have proved effective. Chlorine dioxide and monochloramine are under evaluation and their ultimate role remains to be elucidated. Routine Legionella cultures in concert with disinfectant levels are the best indicators for ensuring long-term efficacy. Percentage distal site positivity for Legionella in drinking water is accurate in predicting risk. Quantitative criteria (CFU/ml) have proven inaccurate and should be abandoned. SUMMARY: Infection control professionals, not healthcare facility personnel or engineers, should play the leadership role in selecting and evaluating the specific disinfection modality. Proactive measures of routine environmental cultures for hospital water and disinfection modalities allow for effective prevention of this high-profile hospital-acquired infection.

Evaluation of a new culture medium for isolation of nontuberculous mycobacteria from environmental water samples.

Nontuberculous mycobacteria (NTM) are waterborne pathogens commonly found in building water systems where they are a primary concern to vulnerable patient populations and can cause severe disease. The recovery of NTM from environmental samples can be a laborious undertaking and current pre-treatment methods and selective media lack sensitivity. We explored the use of the highly selective Rapidly Growing Mycobacteria (RGM) medium for culturing NTM from environmental water samples compared to existing methods. In total, 223 environmental water samples, including potable and non-potable water, were cultured for NTM using three culture media. In addition to direct culture on RGM medium, each sample was cultured on Middlebrook 7H10 medium and Mitchison 7H11 medium after pre-treatment with 0.2M KCl-HCl. Additionally, 33 distinct species of NTM were inoculated onto RGM medium and 7H10 medium in parallel to directly compare their growth. The use of RGM medium alone without pre-treatment provided a sensitivity (91%) comparable to that offered by culture on both 7H10 and 7H11 with acid pretreatment (combined sensitivity; 86%) with significantly less overgrowth and interference from other organisms on RGM medium. The average concentration of NTM observed on RGM medium alone was comparable to or greater than the NTM concentration on either medium alone or combined. Thirty-three species were examined in parallel and all tested strains of 27 of these species successfully grew on RGM medium, including 19 of 21 from the CDC's healthcare-associated infections species list. RGM medium was successful at recovering environmental NTM without a pre-treatment, greatly reducing labor and materials required to process samples. Simplification of culture processing for environmental NTM will allow for a better assessment of their presence in building water systems and the potential for reduced exposure of susceptible populations.

Evaluation of Recommended Water Sample Collection Methods and the Impact of Holding Time on Legionella Recovery and Variability from Healthcare Building Water Systems.

Water safety and management programs (WSMP) utilize field measurements to evaluate control limits and monitor water quality parameters including Legionella presence. This monitoring is important to verify that the plan is being implemented properly. However, once it has been determined when and how to sample for Legionella, it is important to choose appropriate collection and processing methods. We sought to compare processing immediate and flushed samples, filtration of different volumes collected, and sample hold times. Hot water samples were collected immediately and after a 2-min flush. These samples were plated directly and after filtration of either 100 mL, 200 mL, or 1 L. Additionally, unflushed samples were collected and processed immediately and after 1, 24, and 48 h of hold time. We found that flushed samples had significant reductions in Legionella counts compared to immediate samples. Processing 100 mL of that immediate sample both directly and after filter concentration yielded the highest concentration and percent sample positivity, respectively. We also show that there was no difference in culture values from time 0 compared to hold times of 1 h and 24 h. At 48 h, there were slightly fewer Legionella recovered than at time 0. However, Legionella counts were so variable based on sampling location and date that this hold time effect was minimal. The interpretation of Legionella culture results depends on the sample collection and processing methods used, as these can have a huge impact on the success of sampling and the validation of control measures.

Point-of-use filters for prevention of health care-acquired Legionnaires' disease: Field evaluation of a new filter product and literature review.

BACKGROUND: The Centers for Medicare & Medicaid Services requires that health care facilities assess their building water systems and minimize the risk of growth and spread of Legionella and other waterborne pathogens. Increasingly, point-of-use (POU) filters are being used to prevent exposure to these pathogens. This study provides efficacy and performance specifications (membrane size, pore size, and use restrictions), which will aid in selecting POU filters. METHODS: New faucet and shower filters rated for 62 days of use were evaluated at an acute care facility in Southwestern Ontario, Canada. Five faucets and 5 showers served as controls or were equipped with filters. Hot water samples were collected weekly for 12 weeks and cultured for Legionella, heterotrophic plate count, and Pseudomonas. Literature searches for articles on POU filters used in health care settings were performed using PubMed and Google Scholar. Filter specifications from 5 manufacturers were also compared. RESULTS: The 62-day POU filters installed on both faucets and showers eliminated Legionella and reduced heterotrophic plate count concentrations for 12 weeks. No Pseudomonas was recovered during this study. Twenty peer-reviewed studies are summarized, and 21 features of 53 POU filters have been compiled. CONCLUSIONS: The information provides infection preventionists and facility engineers with information to verify claims from manufacturers and compare differences among POU products, including validated efficacy, filter design, and operational specifications.

Water Quality as a Predictor of Legionella Positivity of Building Water Systems.

Testing drinking water systems for the presence of Legionella colonization is a proactive approach to assess and reduce the risk of Legionnaires' disease. Previous studies suggest that there may be a link between Legionella positivity in the hot water return line or certain water quality parameters (temperature, free chlorine residual, etc.) with distal site Legionella positivity. It has been suggested that these measurements could be used as a surrogate for testing for Legionella in building water systems. We evaluated the relationship between hot water return line Legionella positivity and other water quality parameters and Legionella colonization in premise plumbing systems by testing 269 samples from domestic cold and hot water samples in 28 buildings. The hot water return line Legionella positivity and distal site positivity only demonstrated a 77.8% concordance rate. Hot water return line Legionella positivity compared to distal site positivity had a sensitivity of 55% and a specificity of 96%. There was poor correlation and a low positive predictive value between the hot water return line and distal outlet positivity. There was no correlation between Legionella distal site positivity and total bacteria (heterotrophic plate count), pH, free chlorine, calcium, magnesium, zinc, manganese, copper, temperature, total organic carbon, or incoming cold-water chlorine concentration. These findings suggest that hot water return line Legionella positivity and other water quality parameters are not predictive of distal site positivity and should not be used alone to determine the building's Legionella colonization rate and effectiveness of water management programs.

Effect of pipe corrosion scales on chlorine dioxide consumption in drinking water distribution systems.

Previous studies showed that temperature and total organic carbon in drinking water would cause chlorine dioxide (ClO(2)) loss in a water distribution system and affect the efficiency of ClO(2) for Legionella control. However, among the various causes of ClO(2) loss in a drinking water distribution system, the loss of disinfectant due to the reaction with corrosion scales has not been studied in detail. In this study, the corrosion scales from a galvanized iron pipe and a copper pipe that have been in service for more than 10 years were characterized by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The impact of these corrosion scale materials on ClO(2) decay was investigated in de-ionized water at 25 and 45 degrees C in a batch reactor with floating glass cover. ClO(2) decay was also investigated in a specially designed reactor made from the iron and copper pipes to obtain more realistic reaction rate data. Goethite (alpha-FeOOH) and magnetite (Fe(3)O(4)) were identified as the main components of iron corrosion scale. Cuprite (Cu(2)O) was identified as the major component of copper corrosion scale. The reaction rate of ClO(2) with both iron and copper oxides followed a first-order kinetics. First-order decay rate constants for ClO(2) reactions with iron corrosion scales obtained from the used service pipe and in the iron pipe reactor itself ranged from 0.025 to 0.083 min(-1). The decay rate constant for ClO(2) with Cu(2)O powder and in the copper pipe reactor was much smaller and it ranged from 0.0052 to 0.0062 min(-1). Based on these results, it can be concluded that the corrosion scale will cause much more significant ClO(2) loss in corroded iron pipes of the distribution system than the total organic carbon that may be present in finished water.

Environmental sources of community-acquired legionnaires' disease: A review.

BACKGROUND: Most Legionnaires' disease in the US and abroad is community-acquired and believed to be sporadic, or non-outbreak associated. Most patients are exposed to numerous water sources, thus making it difficult to focus environmental investigations. Identifying known sources of sporadic community-acquired Legionnaires' disease will inform future sporadic Legionnaires' disease investigations as well as highlight directions for research. The objective is to summarize and rank sporadic Legionnaires' disease sources based on the level of linkage between the environmental source and cases. METHODS: A PubMed search was conducted using the search terms legion* and (origins or source or transmission) and (sporadic or community-acquired). Studies of nosocomial and/or outbreak-associated disease were excluded from this review. Definite, probable, possible and suspect ranks were assigned to sources based on evidence of linkage to sporadic Legionnaires' disease. RESULTS: The search yielded 196 articles and 47 articles were included in the final review after application of exclusion criteria. A total of 28 sources were identified. Of these, eight were assigned definite rank including residential potable water and car air-conditioner water leakage. Probable rank was assigned to five sources including solar-heated potable water and soil. Possible rank was assigned to nine sources including residential potable water and cooling towers. Suspect rank was assigned to 20 sources including large building water systems and cooling towers. CONCLUSION: Residential potable water, large building water systems and car travel appear to contribute to a substantial proportion of sporadic Legionnaires' disease. Cooling towers are also a potentially significant source; however, definitive linkage to sporadic cases proves difficult. The sources of sporadic Legionnaires' disease cannot be definitively identified for most cases.

Safety and efficacy of chlorine dioxide for Legionella control in a hospital water system.

In a 30-month prospective study, we evaluated the efficacy of chlorine dioxide to control Legionella organisms in a water distribution system of a hospital with 364 patient beds and 74 skilled nursing beds. The number of hot water specimens positive for Legionella organisms decreased from 12 (60%) of 20 to 2 (10%) of 20. An extended time (18 months) was needed to achieve a significant reduction in the rate of Legionella positivity among hot water specimens. At the time of writing, no cases of hospital-acquired Legionnaires disease have been detected at the hospital since the chlorine dioxide system was installed in January 2003. Use of chlorine dioxide was safe, based on Environmental Protection Agency limits regarding maximum concentrations of chlorine dioxide and chlorite.

Role of environmental surveillance in determining the risk of hospital-acquired legionellosis: a national surveillance study with clinical correlations.

OBJECTIVE: Hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the source is the water distribution system. Two prevention strategies have been advocated. One approach to prevention is clinical surveillance for disease without routine environmental monitoring. Another approach recommends environmental monitoring even in the absence of known cases of Legionella pneumonia. We determined the Legionella colonization status of water systems in hospitals to establish whether the results of environmental surveillance correlated with discovery of disease. None of these hospitals had previously experienced endemic hospital-acquired Legionella pneumonia. DESIGN: Cohort study. SETTING: Twenty US hospitals in 13 states. INTERVENTIONS: Hospitals performed clinical and environmental surveillance for Legionella from 2000 through 2002. All specimens were shipped to the Special Pathogens Laboratory at the Veterans Affairs Pittsburgh Medical Center. RESULTS: Legionella pneumophila and Legionella anisa were isolated from 14 (70%) of 20 hospital water systems. Of 676 environmental samples, 198 (29%) were positive for Legionella species. High-level colonization of the water system (30% or more of the distal outlets were positive for L. pneumophila) was demonstrated for 6 (43%) of the 14 hospitals with positive findings. L. pneumophila serogroup 1 was detected in 5 of these 6 hospitals, whereas 1 hospital was colonized with L. pneumophila serogroup 5. A total of 633 patients were evaluated for Legionella pneumonia from 12 (60%) of the 20 hospitals: 377 by urinary antigen testing and 577 by sputum culture. Hospital-acquired Legionella pneumonia was identified in 4 hospitals, all of which were hospitals with L. pneumophila serogroup 1 found in 30% or more of the distal outlets. No cases of disease due to other serogroups or species (L. anisa) were identified. CONCLUSION: Environmental monitoring followed by clinical surveillance was successful in uncovering previously unrecognized cases of hospital-acquired Legionella pneumonia.

Isolation of Staphylococcus aureus from the urinary tract: association of isolation with symptomatic urinary tract infection and subsequent staphylococcal bacteremia.

BACKGROUND: Staphylococcus aureus is frequently isolated from urine samples obtained from long-term care patients. The significance of staphylococcal bacteriuria is uncertain. We hypothesized that S. aureus is a urinary pathogen and that colonized urine could be a source of future staphylococcal infection. METHODS: We performed a cohort study of 102 patients at a long-term care Veterans Affairs facility for whom S. aureus had been isolated from clinical urine culture. Patients were observed via urine and nasal cultures that were performed every 2 months. We determined the occurrence of (1) symptomatic urinary tract infection concurrent with isolation of S. aureus (by predetermined criteria), (2) staphylococcal bacteremia concomitant with isolation of S. aureus from urine, and (3) subsequent episodes of staphylococcal infection. RESULTS: Of 102 patients, 82% had undergone recent urinary catheterization. Thirty-three percent of patients had symptomatic urinary tract infection at the time of initial isolation of S. aureus, and 13% were bacteremic. Eight-six percent of the initial urine isolates were methicillin-resistant S. aureus. Seventy-one patients had follow-up culture data; 58% of cultures were positive for S. aureus at > or =2 months (median duration of staphylococcal bacteriuria, 4.3 months). Sixteen patients had subsequent staphylococcal infections, occurring up to 12 months after initial isolation of S. aureus; 8 late-onset infections were bacteremic. In 5 of 8 patients, the late blood isolate was found to have matched the initial urine isolate by pulsed-field gel electrophoresis typing. CONCLUSIONS: S. aureus is a cause of urinary tract infection among patients with urinary tract catheterization. The majority of isolates are methicillin-resistant S. aureus. S. aureus bacteriuria can lead to subsequent invasive infection. The efficacy of antistaphylococcal therapy in preventing late-onset staphylococcal infection in patients with persistent staphylococcal bacteriuria should be tested in controlled trials.

Efficacy of new point-of-use water filter for preventing exposure to Legionella and waterborne bacteria.

BACKGROUND: Legionella species cause health care-acquired infections in which immunocompromised patients are disproportionately affected. Epidemiologic studies have demonstrated that point-of-use water fixtures are the reservoirs for these infections. The current approach to prevention is system-wide chemical disinfection of the hospital water system. These methods affect both low-risk and high-risk areas. A more effective approach to prevention may be a targeted approach aimed at protecting high-risk patients. One option is the application of a physical barrier (filter) at the point-of-use water fixture. OBJECTIVES: To evaluate the ability of point-of-use filters to eliminate Legionella and other pathogens from water. METHODS: One hundred twenty-milliliter hot water samples were collected from 7 faucets (4 with filters and 3 without) immediately and after a 1-minute flush. Samples were collected every 2 or 3 days for 1 week. This cycle was repeated for 12 weeks. Samples were cultured for Legionella, total heterotrophic plate count (HPC) bacteria, and Mycobacterium species. RESULTS: Five hundred ninety-four samples were collected over 12 cycles. No Legionella or Mycobacterium were isolated from the faucets with filters between T = 0 and T = 8 days. The mean concentration of L pneumophila and Mycobacterium from the control faucets was 104.5 CFU/mL and 0.44 CFU/mL, respectively. The filters achieved a greater than 99% reduction in HPC bacteria in the immediate and postflush samples. CONCLUSIONS: Point-of-use filters completely eliminated L pneumophila and Mycobacterium from hot water samples. These filter units could prevent exposure of high-risk patients to waterborne pathogens.

A proactive approach to prevention of health care-acquired Legionnaires' disease: the Allegheny County (Pittsburgh) experience.

BACKGROUND: The Allegheny County Health Department (ACHD) in Pennsylvania distributed the first guidelines for prevention and control of health care-acquired Legionnaires' disease (LD) by 1995. The proactive approach advocated in the guidelines differed notably from that of the Centers for Disease Control and Prevention (CDC) by recommending routine environmental testing of the hospital water distribution system even when cases of health care-acquired Legionnaires' disease had never been identified. OBJECTIVES: Our purpose was to (1) evaluate the impact of the ACHD guidelines on the Legionella diagnostic and preventive practices of health care facilities in Allegheny and surrounding counties and (2) compare the incidence of health care-acquired LD before and after issuance of the ACHD guidelines. METHODS: CDC case reports of LD from 1991 to 2001 were tabulated and compiled by the ACHD Infectious Disease Unit and the Association for Professionals in Infection Control and Epidemiology, Inc, Three Rivers Chapter. A survey was distributed to 110 hospitals and long-term care facilities in the region. The results were analyzed as occurring either in the preguideline period (1991-1994) or postguideline period (1995-2001). RESULTS: A significant decrease in the number of health care-acquired cases was demonstrated between the preguideline (33%) and postguideline (9%) periods (P=.0001). In contrast, community-acquired cases increased from 67% pre guideline to 91% post guideline. A total of 71% of the facilities were colonized with Legionella. Disinfection of the water distribution system was initiated by 44% of facilities. Use of urinary antigen testing significantly increased from 40% pre guideline to 79% post guideline (P=.0001). CONCLUSIONS: Health care-acquired LD declined significantly after the issuance of guidelines for prevention and control of health care-acquired LD. The decline was associated with health care facilities performing routine environmental monitoring of their water distribution systems followed by the initiation of disinfection methods if indicated. Two unanticipated benefits were (1) cases of LD in the community and long-term care facilities were uncovered as a result of increased availability of Legionella tests and (2) litigation and unfavorable publicity involving ACHD hospitals ceased.

Comparative activity of quinolones, macrolides and ketolides against Legionella species using in vitro broth dilution and intracellular susceptibility testing.

The comparative in vitro activity of quinolones (trovafloxacin, gemifloxacin, levofloxacin, ciprofloxacin, moxifloxacin and grepafloxacin), ketolides (ABT-773 and telithromycin) and macrolides (clarithromycin, azithromycin and erythromycin) were evaluated against Legionella pneumophila by broth dilution and an HL-60 intracellular model. The MIC90 of the quinolones, clarithromycin and ABT-773 were more than eight times lower than for erythromycin. Telithromycin, ABT-773 and azithromycin had significantly greater intracellular activity against L. pneumophila than erythromycin at 1xMIC and 8xMIC. The rank order of intracellular activity against L. pneumophila serogroup 1 was quinolones>ketolides>macrolides. Clinical trials to determine the clinical efficacy of ketolides for the treatment of Legionnaires' disease are warranted.

Fungal diversity and presence of potentially pathogenic fungi in a hospital hot water system treated with on-site monochloramine.

Currently, our knowledge of fungal ecology in engineered drinking water systems is limited, despite the potential for these systems to serve as a reservoir for opportunistic pathogens. In this study, hot water samples were collected both prior to and following the addition of monochloramine as an on-site disinfectant in a hospital hot water system. Fungal ecology was then analyzed by high throughput sequencing of the fungal ITS1 region. The results demonstrate that the genera Penicillium, Aspergillus, Peniophora, Cladosporium and Rhodosporidium comprised the core fungal biome of the hospital hot water system. Penicillium dominated the fungal community with an average relative abundance of 88.89% (±6.37%). ITS1 sequences of fungal genera containing potential pathogens such as Aspergillus, Candida, and Fusarium were also detected in this study. No significant change in fungal community structure was observed before and after the initiation of on-site monochloramine water treatment. This work represents the first report of the effects of on-site secondary water disinfection on fungal ecology in premise plumbing system, and demonstrates the necessity of considering opportunistic fungal pathogens during the evaluation of secondary premise plumbing disinfection systems.

Effect of monochloramine treatment on the microbial ecology of Legionella and associated bacterial populations in a hospital hot water system.

Opportunistic pathogens, including Legionella spp. and non-tuberculous mycobacteria, can thrive in building hot water systems despite municipal and traditional on-site chlorine disinfection. Monochloramine is a relatively new approach to on-site disinfection, but the microbiological impact of on-site chloramine use has not been well studied. We hypothesized that comparison of the microbial ecology associated with monochloramine treatment versus no on-site treatment would yield highly dissimilar bacterial communities. Hot water samples were collected monthly from 7 locations for three months from two buildings in a Pennsylvania hospital complex supplied with common municipal water: (1) a hospital administrative building (no on-site treatment) and (2) an adjacent acute-care hospital treated on-site with monochloramine to control Legionella spp. Water samples were subjected to DNA extraction, rRNA PCR, and 454 pyrosequencing. Stark differences in the microbiome of the chloraminated water and the control were observed. Bacteria in the treated samples were primarily Sphingomonadales and Limnohabitans, whereas Flexibacter and Planctomycetaceae predominated in untreated control samples. Serendipitously, one sampling month coincided with dysfunction of the on-site disinfection system that resulted in a Legionella bloom detected by sequencing and culture. This study also demonstrates the potential utility of high-throughput DNA sequencing to monitor microbial ecology in water systems.