Comparison of selective procedures for isolation and enumeration of Legionella species from hot water systems

Various sample pre-treatment techniques and different growth media for the isolation of Legionellae from hot water supplies in public buildings were c ompared. A total of 102 hot water samples from taps and showers was examine d, The highest recovery frequency was obtained with the heat pre-treatment method and using the selective medium GVPC, However, the results differed a ccording to the concentration of legionellas. In the case of low plate coun ts (less than or equal to 5000 cfu l(-1)), the heat pre-treatment technique gave a significantly higher percentage of positive samples compared with o ther techniques (P < 0.05). With increasing concentration, the differences between the procedures decreased until they became statistically not signif icant for concentrations above 50 000 cfu l(-1). The direct inoculum method allowed a significantly higher detection of concentrations (P < 0.001) com pared with heat and acid decontamination methods, which brought about a 67- 68% reduction in detectable Legionellae. Heat decontamination techniques sh ow greater sensitivity and specificity. However, they underestimate the num ber of legionellas, In environmental surveillance programmes, this underest imate must be taken into consideration when assessing the health risk.