**4. Conclusions**

The findings of this study are consistent with the understanding that *L. pneumophila* is not uncommon in municipal water flowing into buildings [49], but a range of water chemistry and premise plumbing conditions and disinfectant residuals can prevent their proliferation. Persistent disinfectant, including chlorine, is known to be a critical factor in reducing *Legionella* risk [35,37,39], and a recent study confirmed predicted associations between low levels of chlorine in Flint's distribution system and observed incidence of LD [5]. Consistent with our prior in-field observations [1,3], we further demonstrate under controlled laboratory conditions the importance of considering interactive effects with flow and pipe materials, particularly with respect to relative water corrosivity and influence on residual chlorine levels, in keeping *Legionella* levels low. Indeed, many individual factors can act as "two-edged swords" in terms of their net effect of controlling versus enhancing *Legionella* growth, depending on the status of other factors. For example, copper pipe achieves its best antimicrobial efficacy without corrosion control, but absence of corrosion control also leads to elevated iron and depleted chlorine residual, which in turn enhance *Legionella* growth. Such interactive effects can help

explain why prior studies reported relatively low levels of *Legionella* in single family homes, which tend to have greater stagnation and more copper in water from copper service lines and plumbing, compared to large multi-story buildings during the Flint Water Crisis.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-0817/9/9/730/s1, Table S1: PCR confirmation of unique morphologies that were identified morphologically as *Legionella*.

**Author Contributions:** Conceptualization, O.R.S., M.A.E., and A.P.; methodology, O.R.S., M.A.E., and A.P.; software, O.R.S. and R.L.M.; validation, O.R.S. and R.L.M.; formal analysis, R.L.M. and O.R.S.; investigation, O.R.S., R.L.M., M.A.E., and A.P.; resources, O.R.S.; data curation, R.L.M. and O.R.S.; writing—original draft preparation, R.L.M., O.R.S., M.A.E. and A.P.; writing—review and editing, O.R.S., R.L.M., M.A.E., and A.P.; visualization, O.R.S. and R.L.M.; supervision, M.A.E. and A.P.; project administration, M.A.E. and A.P.; and funding acquisition, M.A.E. and A.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Science Foundation for CBET via Awards 1336650, 1556258, and 1706733. The Alfred P. Sloan Foundation Microbiology of the Built Environment Program, and the Science and Engineering of the Exposome (SEE) Center supported by the Institute for Critical Technology and Applied Science at Virginia Tech, also provided financial support on this project.

**Acknowledgments:** We would like to thank Stephen Wolbert and LeeAnne Walters for their cooperation in collecting and shipping Flint water, as well as Laurel Strom, D. Otto Schwake, Emily Garner, Jacob Seymour, and Je ffrey Parks for their technical and laboratory assistance, and the Virginia Tech Library Open Access Subvention Fund.

**Conflicts of Interest:** The authors declare no conflict of interest.
