The Fluorescent In Situ Hybridization Test in Comparison with Culture for Detection of Legionella pneumophila in Water Samples

This chapter evaluates the fluorescent in situ hybridization (FISH) test with a specific 16S rRNA-targeted probe for detecting the presence of viable Legionella pneumophila in water. The results of this FISH test are available after 6 h (without inactivation) or 24 h (with activation). A large number of water samples, mostly from hot water systems, were tested with both test varieties and results were compared with those obtained with the culture method. FISH is a detection technique using a specific fluorescence-labeled DNA probe targeting the rRNA of the cells of the selected microorganism(s). Effect of activation on the result of the FISH test for detecting L. pneumophila in water samples is discussed in the chapter. Interpretation of the FISH test results, in terms of hygienic significance, is even more complicated than such interpretation of colony counts. On the basis of the FISH results following conclusions can be drawn: (i) FISH allows detection of L. pneumophila in samples of water and biofilms within 24 h after sampling, but activation is needed, (ii) in many cases the numbers of cells detected with FISH are much higher than numbers detected with the culture method, and (iii) further improvement of the FISH test, e.g., a level of detection similar to the culture method, is needed.