Purpose: A study was conducted at the independent Expert Laboratory ISHA to validate this new method for detection and enumeration of Legionella in cooling towers and sanitary hot water, in comparison to the NF T90-431 and ISO 11731 reference methods and according to the NF Validation guidelines.
Methods: Water samples were analyzed following protocols of the references methods in which the MWY medium was compared to the GVPC medium. All presumptive-positive colonies were confirmed after streaking on BCYE (with and without L-cysteine) and then identified using the slidex Legionella kit.
Results: The MWY method showed satisfying relative linearity performances, with linear correlation coefficients superior to 0.95. Biases between both methods were characterized by low values. The intercepts close to 0 and the slopes close to 1 were validated for all the tested categories in the accuracy study. The selectivity and specificity of the MWY agar were shown to be good by testing 30 target and 30 non-target strains.
Significance: For all water evaluated, the MWY method demonstrated comparable results to the reference methods for enumeration of Legionella. Its higher selectivity reduces greatly the interfering bacterial growth and specially the fungal growth and facilitates the reading and the confirmation.