Parameters predictive of Legionella contamination in hot water systems: Association with trace elements and heterotrophic plate counts

The contamination of hot water samples with Legionella spp. was studied in relation to temperature, total hardness, trace element concentrations (iron, zinc, manganese, and copper) and heterotrophic plate counts (HPC) at both 22 and 37 °C. Factor analysis and receiver operating characteristic (ROC) curves were used to establish the cut-off of water parameters as predictors for Legionella contamination. Legionella spp. was isolated in 194 out of 408 samples (47.5%), with Legionella pneumophila being the most common (92.8%). After multiple logistic regression analysis, the risk for legionellae colonisation was positively associated with Mn levels >6 μg l−1, HPC at 22 °C >27 CFU l−1, and negatively with temperature >55 °C and Cu levels >50 μg l−1. Multiple regression analysis revealed that Legionella spp. counts were positively associated with Mn, HPC at 37 °C and Zn and negatively associated with temperature. Only 1 out of the 97 samples (1%) having a Mn concentration, an HPC at 22 °C and an HPC at 37 °C below the respective median values exhibited a Legionella spp. concentration exceeding 104 CFU l−1vs. 41 out of the 89 samples (46.1%) with the three parameters above the medians. Our results show a qualitative and quantitative relationship between Legionella spp., the Mn concentration and heterotrophic plate counts in hot water samples from different buildings, suggesting that these parameters should be included in a water safety plan. The role of manganese in biofilm formation and its possible involvement in the mechanisms favouring Legionella survival and growth in water niches should be investigated further.

Research highlights
► Frequency and amount of Legionella contamination in hot water distribution systems.
► Relationship between Legionella and trace elements, particularly manganese.
► Quali- and quantitative relationship between Legionella and heterotrophic bacteria.
► Confirm of a negative association of Legionella with temperature and copper.