Heterotrophic Plate Count Can Predict the Presence of Legionella spp. in Cooling Towers

Abstract

Legionella pneumophila (Lp) colonizes aquatic environments and is a potential pathogen to humans, causing outbreaks of Legionnaire’s disease. It is mainly associated with contaminated cooling towers (CTs). Several regulations, including Spanish legislation (Sl), have introduced the analysis of heterotrophic plate count (HPC) bacteria and Legionella spp. (Lsp) in management plans to prevent and control Legionella outbreaks from CTs. The 2003 Sl for CTs (RD 865/2003) considered that concentrations of HPC bacteria ≤10,000 cfu/mL and of Lsp ≤100 cfu/L are safe; therefore, no action is required, whereas management actions should be implemented above these standards. We have investigated to what extent the proposed standard for HPC bacteria is useful to predict the presence of Lsp in cooling waters. For this, we analyzed Lsp and HPC concentrations, water temperature, and the levels of chlorine in 1376 water samples from 17 CTs. The results showed that in the 1138 water samples negative for Legionella spp. (LN), the HPC geometric mean was significantly lower (83 cfu/mL, p < 0.05) than in the positive Lsp. samples (135 cfu/mL). Of the 238 (17.3%) LP samples, 88.4% (210/238) were associated with values of HPC ≤10,000 cfu/mL and most of them showed HPC concentrations ≤100 (53.7%). In addition, a relatively low percentage of LP (28/238, 11.6%) samples were associated with HPC bacteria concentrations >10,000 cfu/mL, indicating that this standard does not predict the colonization risk for Legionella in the CTs studied. The present study has demonstrated that a threshold concentration ≤100 cfu/mL of HPC bacteria could better predict the higher concentration of Legionella in CTs, which will aid in preventing possible outbreaks.

Heterotrophic plate counts (HPC) in drinking water distribution systems: A comprehensive review and meta-analysis

“…Higher levels of HPC would generally help to predict the presence of Legionella spp., but as HPC colony count units increase, HPC is more likely to suppress L. pneumophila regrowth (Donohue 2021). Although no correlation could be established, other studies found that high colony count units of HPC can be useful for determining the presence of L. pneumophila in cooling towers (Duda et al 2015;Pierre et al 2019;Sanchis et al 2023). Such findings suggest that HPC and L. pneumophila may have similar responses to environmental conditions and parameters in drinking water distribution systems such as water temperature (L. pneumophila growth occurs between 20 and 50 °C) and level of chlorine residual.…”