Water Research & Technology, Feb 1, 2021

Background: Antimicrobial resistance is a well-documented public health concern. The role that drinking water distribution pipes have as sources of antibiotic resistance genes (ARGs) is not well known. Metals are a known stressor for antibiotic resistance development, implying that aging metal-pipe infrastructure could be a source of ARGs. The objective of this study was to determine if ARGs, metal resistance genes (MRGs), and intI1 were pervasive across various pipe biofilm sample types (biomass surfaces, pipe surfaces, corrosion tubercles, and under corrosion tubercles) and if the resistance genes associated with particular microbial taxa.

Methods: Eight sample types in triplicate (n=24) were taken from inside a >100-year-old, six ft. section of a full-scale chloraminated cast iron drinking water main. Droplet digital PCR (ddPCR) was employed as a novel approach to quantify ARGs in pipes from full-scale drinking water distribution systems (DWDS) because it yielded higher detection frequencies than quantitative PCR (qPCR). Illumina sequencing was employed to characterize microbial community based on 16S rRNA genes.

Results: ARGs and MRGs were detected in all 24 pipe samples. Interestingly, the mean absolute abundances of ARGs and MRGs only varied by approximately one log value across microenvironments, but the mean relative abundances (copy numbers normalized to 16S rRNA genes) varied by over two log values. The ARG and MRGs concentrations were not significantly different between sample types, despite significant changes in dominant microbial taxa. The most abundant genera observed in the biofilm communities were Mycobacterium (0.2-70%), and β-lactam resistance genes blaTEM, blaSHV, and the integrase gene of class 1 integrons (intI1) were positively correlated with Mycobacterium.

Conclusions: The detection of ARGs, MRGs, and class 1 integrons across all sample types within the pipe indicates that pipes themselves can serve as sources for ARGs in DWDS. Consequently, future work should determine the role of pipe material and interactions between corrosion inhibitors and pipe materials to determine how engineering decisions can mitigate ARGs in drinking water that stem from pipe materials.

PRJNA692495: Cast iron drinking water pipe biofilm community analysis