Graduate Program

Biological Sciences

Degree Name

Master of Science (MS)

Semester of Degree Completion

Fall 2025

Thesis Director

Sanghoon Kang

Thesis Committee Member

Gary Bulla

Thesis Committee Member

Kai Foong Hung

Abstract

Domestic hot-water systems quietly grow biofilms under elevated temperatures that can serve as shelter to Opportunistic Premise Plumbing Pathogens. Although focus has been placed on microbes in domestic systems, there has been no clear conclusion on the mechanism of survival and persistence of opportunistic pathogens despite various disinfection strategies. In this study, we determined the distinction between showerhead and drip-flow biofilms at the whole-community level and analyzed the coexistence of thermotolerant organisms with opportunistic genera, such as Legionella. We sampled eight showerheads and simulated four biofilm samples in the laboratory using the drip-flow biofilm reactor.

We observed a strong overlap in both showerheads and drip flow indicating a shared community backbone. The results revealed a core microbiome of thermotolerant organisms dominated by Methylobacterium, Sphingobium, and Sphingomonas which were observed to be highly prevalent across samples. Opportunistic pathogens were observed to be consistent in all the samples although they were low in abundance. Mycobacterium contributed the largest pooled share and displayed higher abundance in the drip flow reactor which contrasted with Acinetobacter which showed higher abundance in showerheads. Pseudomonas and Legionella and other opportunistic pathogens were consistently detected at low relative abundance. This finding illustrated that the low abundances observed reflected persistent, low-level colonization in domestic systems. These data collectively suggest a model in which a stable, thermotolerant base exists across household water systems, acting as a reservoir for opportunistic species. This study clarifies the persistence of low-abundance opportunists in domestic hot-water systems, thereby improving risk assessment and informing control measures aimed at both biofilm and fixture conditions.

Keywords: domestic water biofilms, thermotolerant core, opportunistic pathogens, showerhead

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