Detection of Legionella by quantitative-polymerase chain reaction (qPCR) for monitoring and risk assessment

Abstract

Background: Culture and quantitative polymerase chain reaction (qPCR) assays for the detection of Legionella were compared on samples from a residential area before and after two interventions. A total of 84 samples were collected from shower hoses and taps as first flush samples and at constant temperature. Samples were grouped according to the origin of the sample, a) circulation water b) water from empty apartments c) water from shower hoses. The aims were to investigate the usefulness of qPCR compared to culture for monitoring remedial actions for elimination of Legionella bacteria and as a tool for risk assessment.

Results: In water collected from the apartments Legionella spp were detected by qPCR in the concentration range from LOQ to 9.6*105GU/L while L. pneumophila were detected in a range from LOQ to 6.8*105 GU/L. By culturing, the legionellae were detected in the range from below detection limit (> 10 CFU/L) to 1.6*106 CFU/L. In circulating water and in first flush water from shower hoses, culture and qPCR showed the same tendencies. The overall correlation between the bacteria number detected by culture and the two developed qPCR assays (L. spp and L. pneumophila) was relatively poor (r2 = 0.31 for culture and Legionella spp. assay, r2 = 0.20 for culture and L. pneumophila assay).

Conclusion: Detection by qPCR was suitable for monitoring changes in the concentration of Legionella but the precise determination of bacteria is difficult. Risk assessment by qPCR only on samples without any background information regarding treatment, timing, etc is dubious. However, the rapid detection by qPCR of high concentrations of Legionella - especially Legionella pneumophila - is valuable as an indicator of risk, although it may be false positive compared to culture results. On the other hand, the detection of a low number of bacteria by qPCR is a strong indication for the absence of risk.

Figure 1

Circulation water. Comparison of the amount of Legionella detected by culture and by qPCR. Legionella species and the Legionella pneumophila assay in circulation water before and after the two interventions. LOQ: Limit of quantification. Water samples were collected from both bathroom and kitchen hot water taps. Each triangle, dot and square represents one sample.

Figure 2

Empty apartments first flush. Comparison of the amount of Legionella detected by culture and by qPCR. Legionella species and the Legionella pneumophila assay in first flush samples from empty apartments before and after the second intervention. Collected from both bathroom and kitchen hot water taps. Each triangle, dot and square represents one sample

Figure 3

Shower hoses first flush. Comparison of the amount of Legionella detected by culture and by qPCR. Legionella species and the Legionella pneumophila assay in first flush samples from shower hoses before and after the two interventions. LOQ: Limit of quantification. Each triangle, dot and square represents one sample