If disinfection of some kind was used it is more difficult to correlate results from the two methods since some or all Legionella could have been killed. However, on some occasions it could be interesting to monitor the level of dead or unculturable Legionella, since a high level measured by qPCR could
indicate a current or recent colonisation of the system, which could indicate a potential risk even though the bacteria do not grow. As also discussed in Joly et al 2006  a negative or low level of Legionella detected by qPCR is a quite good predictor of a negative culture result. Unfortunately, this selection is difficult to establish based on detection of Legionella species since all tested samples were found to contain Legionella DNA. Using the Legionella pneumophila assay, eight of ten samples
found selleck chemical negative by qPCR were also negative by culture. selleck inhibitor It has been suggested to improve the usefulness of qPCR by pre-treatment with the DNA-dye Propidium monoazide to discriminate between dead and live bacteria . Previous work with dying DNA of membrane compromised cells focused on the use of the dye ethidium monoazide  but Propidium monoazide has been found to show less cytotoxicity . Nevertheless, optimization of the use of the dyes is still needed. Conclusion We found that detection of Legionella in water samples by qPCR was suitable for monitoring changes in the concentration of Legionella Ergoloid over time, whereas the specific number measured by qPCR was difficult to use for risk assessment. Results for both culture and qPCR followed the same decreasing tendencies for circulating water and first flush water samples from shower hoses. In first flush samples from empty apartments,
before the second intervention, culture and qPCR results were generally at the same level, but the two samples collected after the second intervention showed different tendencies with the two methods. Background information about the water system is necessary to interpret the qPCR results, but low amounts of Legionella pneumophila detected by qPCR is a good indicator of low risk, and detection of high levels in untreated water systems is a good indicator of colonisation and risk. Acknowledgements and Funding We would like to thank laboratory technicians of the Dept. of Microbiological Diagnostics, Statens Serum Institut, Berit Larsen, Bente Tangvig and Gitte H. Riisgaard for their practical help with the culturing of water samples. Louise Hjelmar Krøjgaard was partly financially supported by the Graduate School UrbanWaterTech. All authors declare no conflicts of interest. Parts of the results have been presented as a poster at the 25th European working group for Legionella Wortmannin in vivo Infections meeting in Copenhagen, Denmark, 15-17 September 2010. References 1. Rosa F: Legionnaires’ Disease prevention and Control.