Bacterial Colonization of Point-of-Use (PoU) Drinking Water Filters, Selection of Opportunistic Pathogens and Presence of Antibiotic Resistance Genes

Abstract

Commercial activated carbon block (ACB) point-of-use (PoU) drinking water filters are widely used to remove chemicals of concern from tap water. ACB PoU filters, however, are an incubator of bacteria because they remove disinfectant residual and adsorb organic compounds that can serve as bacterial substrates. The chemicals adsorbed on the ACB, such as disinfection byproducts (DBPs), may interact with colonizing bacteria. Bacterial growth in ACB PoU filters raises questions about whether waterborne bacteria that pose health risks can be enriched, such as enteric-related bacteria, opportunistic pathogens, and antibiotic resistant gene (ARG)-carrying bacteria. The goal of this dissertation is to understand how ACB PoU filters change the microbial ecology of drinking water using culture-based and culture-independent methods. A lab-scale ACB PoU filter system experiment was conducted to study the bacterial composition shifts across the filters with and without the addition of pentachlorophenol (PCP), a surrogate chlorinated phenolic disinfection byproduct. The results show that the PoU filter has a primary effect on altering the bacterial composition and increasing the abundance of Mycobacterium, while the influence of PCP was subtle. A field study was conducted to evaluate the prevalence of ARGs across the ACB PoU filters installed at different households in Flint, Michigan. The study indicates that ACB PoU filters raised the occurrence and abundance of some ARGs in the tap water. The increase in ARGs across filters was also detected in a lab-scale system in Ann Arbor. Lastly, the mechanism of bacterial migration through the ACB PoU filters was studied by spiking inert bacterial-sized particles (1 μm) and fluorescent-tagged Escherichia coli and Pseudomonas aeruginosa into the system. We found that preferential flow pathways existed in the ACB, allowing bacteria to pass through rapidly. Differences in growth, biofilm formation, and physical properties of the spiked species determine their migration and survival through the ACB PoU filters.