Phylogenetic Characterization of Viable but-not-yet Cultured Legionella Groups Grown in Amoebic Cocultures: A Case Study using Various Cooling Tower Water Samples

Interpreting the Results of the Conventional Plate Culture and Gene Detection Methods for Legionella Detection in Environmental Water Samples

The conventional plate culture method is widely used as a method for detection of Legionella in environmental water samples, but to obtain results takes more than a week. Because it is much quicker, the gene detection method has become widespread as an alternative detection method. However, the results of gene detection and plate culture methods may differ even when the same sample is examined; the gene detection method shows a higher detection ratio than the plate culture method. The reason for this difference is that the plate culture method detects Legionella cells that have the ability to form colonies on an agar plate, whereas the gene detection method detects any Legionella genes present regardless of the state of the Legionella. In this paper, we consider the factors that cause differences between the results of the plate culture and gene detection methods, and how to interpret the results of each.

Investigations on Contamination of Environmental Water Samples by Legionella using Real-Time Quantitative PCR Combined with Amoebic Co-Culturing

We analyzed the contamination of environmental water samples with Legionella spp. using a conventional culture method, real-time quantitative PCR (qPCR), and real-time qPCR combined with an amoebic co-culture method. Samples (n = 110) were collected from 19 cooling towers, 31 amenity water facilities, and 60 river water sources of tap water in Japan. Legionella was detected in only three samples (3/110, 2.7%) using the culture method. The rate of Legionella detection using amoebic co-culture followed by qPCR was 74.5%, while that using qPCR without amoebic co-culture was 75.5%. A higher than 10-fold bacterial count was observed in 19 samples (19/110, 17.3%) using real-time qPCR subsequent to amoebic co-culture, compared with identical samples analyzed without co-culture. Of these 19 samples, 13 were identified as Legionella spp., including L. pneumophila and L. anisa, and the non-culturable species were identified as L. lytica and L. rowbothamii. This study showed that the detection of Legionella spp., even in those samples where they were not detected by the culture method, was possible using real-time qPCR and an amoebic co-culture method. In addition, this analytical test combination is a useful tool to detect viable and virulent Legionella spp..