**5. Conclusions**

In conclusion, our data demonstrate that the quality of culture media is crucial in determining the level of *Legionella* spp. colonization in hospital water systems. As water remediation measures are based on quantitative *Legionella* spp. data obtained by culturing environmental samples, culture protocol standardization, as well as accurate quality control of the culture media, is essential to achieve intra- and interlaboratory reproducibility and accuracy.

Given the public health risk from *Legionella* spp., it is important that all water-testing laboratories carefully consider the following aspects: (1) there can be variability in *Legionella* spp. detection due to different types and brands of medium—of note, this variation can also be observed among different batches from the same supplier; (2) the medium should be purchased from a reputable company and fully validated in-house, with the inclusion of appropriate controls; (3) when switching to a different medium manufacturer, extensive validation should be performed in order to determine whether the new medium is "fit for purpose".

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-0817/9/7/523/s1. File: images of plate with background flora; paired images (BCYEα and MWY agar by two manufacturers) obtained during incubation period of inoculated plates.

**Author Contributions:** Conceptualization, S.D. and M.G.; methodology, S.D. and M.G.; formal analysis, M.G. and G.M.; data curation, M.G. and J.G.; funding acquisition, C.M.Z.; writing original draft preparation, S.D. and M.G.; writing, review, and editing, S.D., M.G., J.G., and C.M.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by a gran<sup>t</sup> from the Department fund ZOTC\_CT\_PREST\_18\_01, Department of Public Health and Pediatrics, University of Turin.

**Conflicts of Interest:** None of the authors declare to have a conflict of interest that could influence or bias the content of the paper.
