**Critical Review: Propensity of Premise Plumbing Pipe Materials to Enhance or Diminish Growth of** *Legionella* **and Other Opportunistic Pathogens**

**Abraham C. Cullom 1, Rebekah L. Martin 1,2, Yang Song 1, Krista Williams 3, Amanda Williams 4, Amy Pruden 1 and Marc A. Edwards 1,\***


Received: 9 October 2020; Accepted: 13 November 2020; Published: 17 November 2020

**Abstract:** Growth of *Legionella pneumophila* and other opportunistic pathogens (OPs) in drinking water premise plumbing poses an increasing public health concern. Premise plumbing is constructed of a variety of materials, creating complex environments that vary chemically, microbiologically, spatially, and temporally in a manner likely to influence survival and growth of OPs. Here we systematically review the literature to critically examine the varied effects of common metallic (copper, iron) and plastic (PVC, cross-linked polyethylene (PEX)) pipe materials on factors influencing OP growth in drinking water, including nutrient availability, disinfectant levels, and the composition of the broader microbiome. Plastic pipes can leach organic carbon, but demonstrate a lower disinfectant demand and fewer water chemistry interactions. Iron pipes may provide OPs with nutrients directly or indirectly, exhibiting a high disinfectant demand and potential to form scales with high surface areas suitable for biofilm colonization. While copper pipes are known for their antimicrobial properties, evidence of their efficacy for OP control is inconsistent. Under some circumstances, copper's interactions with premise plumbing water chemistry and resident microbes can encourage growth of OPs. Plumbing design, configuration, and operation can be manipulated to control such interactions and health outcomes. Influences of pipe materials on OP physiology should also be considered, including the possibility of influencing virulence and antibiotic resistance. In conclusion, all known pipe materials have a potential to either stimulate or inhibit OP growth, depending on the circumstances. This review delineates some of these circumstances and informs future research and guidance towards effective deployment of pipe materials for control of OPs.

**Keywords:** non-tuberculous mycobacteria; *Pseudomonas*; *Acinetobacter*; amoebae; copper; iron; PEX; PVC; drinking water; disinfection
