How can Legionella pneumophila transmit through the hot water plumbing of residences and office buildings?

In a recent study published in the International Journal of Hygiene and Environmental Health, researchers investigate the transmission of the causative agent of legionellosis, Legionella pneumophila, through hot water plumbing systems in commercial and residential buildings.

Study: Hot water plumbing in residences and office buildings have distinctive risk of Legionella pneumophila contamination. Image Credit:


Legionella pneumophila is a bacterium that spreads through inhaled contaminated soil and water particles. This environmental bacterium causes legionellosis, which manifests in a mild form as Pontiac fever or Legionnaires’ disease, which has severe symptoms similar to pneumonia. Known legionellosis outbreaks have been linked to exposure through aerosolization in the air-conditioning systems, but studies have also found traces of L. pneumophila in hot water samples in buildings.

Hot water systems provide an ideal environment for the microbes to thrive by decreasing disinfectant concentrations and providing temperatures conducive to microbial growth. Since chemical disinfectants might not have uniform efficacy in preventing microbial growth, identifying the physical and chemical factors that contribute to microbial contamination is important.

About the study

The present study measured the detection frequency and concentration of L. pneumophila in three types of structures — commercial buildings, single-family residences, and apartments. Samples were collected approximately every three months, between January 2011 and October 2019, to determine if the occurrence pattern was sporadic (single positive sample) or persistent (multiple positive samples).

Additional data such as the structure’s age and the disinfectant residual in the hot water were collected to find other contributors to contamination risk. The final sample set included 296 water samples collected from 66 single-family homes, 30 office buildings, and four apartments.

Deoxyribonucleic acid (DNA) was extracted from membrane filters using standard methods. Quantitative polymerase chain reaction (qPCR), using primer-probes for the L. pneumophila 16S ribosomal ribonucleic acid (rRNA) gene and the L. pneumophila Serogroup 1 (Sg1) wzm gene, was used to detect the presence of L. pneumophila and L. pneumophila Sg1 in the hot water samples.

Genomic DNA from a known strain of L. pneumophila Sg1 was used as the positive control, and all assays were duplicated. Samples were classified as positive if both replicates showed quantification cycle values were less than 39.  


The study results showed that commercial buildings had a higher detection frequency of L. pneumophila than residential ones (single-family homes and apartments). One or more water samples from 36% (25 out of 70) of residences tested positive for L. pneumophila, while for office buildings, the detection frequency was 56% (16 out of 30). Ten of the 25 residential buildings that tested positive were persistent in L. pneumophila contamination; of those, L. pneumophila Sg1 was detected sporadically in five. Of the 16 commercial buildings that tested positive, 11 had persistent L. pneumophila contamination, and all 11 were also persistently positive for L. pneumophila Sg1.

Among commercial buildings, the age of the building did not seem to be a significant predictor of contamination, but the area of the building was a predictor. Buildings that were greater than 100,000 square feet in area had a higher incidence of L. pneumophila contamination. In residential buildings, the trend was reversed, with the building’s age being a significant predictor of L. pneumophila contamination, but not its area. The secondary disinfectant (chlorine or chloramine) did not affect the detection frequency or concentration of L. pneumophila in residential buildings but did affect the frequency or concentration in commercial ones.

The researchers believe that a building’s water age, the time the water circulates in the plumbing before being used, can determine water quality. With increasing water age, the disinfectant concentrations reduce, allowing microbial and biofilm colonies to grow and pipe corrosion to occur. They attribute commercial buildings with complex plumbing systems and greater water age as one of the possible reasons for the greater frequency of L. pneumophila contamination in office buildings.


To summarize, the study results showed that hot water systems in commercial and residential buildings are potential reservoirs of L. pneumophila, and pose a risk of legionellosis. Old and new commercial buildings, especially large ones, have a higher incidence of L. pneumophila contamination than residential ones. Older residential buildings, however, have an increased risk of L. pneumophila contamination in their hot water systems than newer ones. Secondary disinfection methods are ineffective in controlling L. pneumophila contamination in commercial buildings. Further research and studies with larger sample sizes are required to collaborate the findings.

Leave a Reply

Your email address will not be published. Required fields are marked *