Advanced technologies for monitoring the microbial quality of drinking water

Abstract

Flow cytometry (FC) has been adopted by the water industry as a tool for the rapid evaluation of the microbial status of drinking water. FC is primarily used to measure bacterial cell counts. However, there is a lack of consensus in interpretation of results. My hypothesis was that nucleic-acid linked fluorescence emission values, measured by FC, indicate the genome size of cells. To test my hypothesis, I first examined pure cell cultures of bacteria with known genomes sizes, and subsequently used fluorescence-activated cell sorting of cells from drinking water service reservoirs (SR) based on fluorescence emission, followed by DNA sequencing. The results did not support my hypothesis. I then considered SRs as continuous reactors. I combined bacterial cell counts and 16S rRNA gene sequences to model growth rates of the bacteria. The bacterial community in some SRs had similar communities with a stable distribution of net growth rates, with a mean of zero. I consider such SRs, to be in a dynamic steady state. One SR, that had an unusual distribution of nucleic-acid linked fluorescence emission values, had most taxa exhibiting positive net growth. I propose that bacterial populations in a dynamic steady state are desirable from a water quality perspective, and this is a more realistic engineering goal than biological stability, which implies no growth or change in microbial populations. The patterns seen in FC are specific to the particular bacterial species present, but bacterial identity cannot be inferred from flow cytometry analysis of mixed communities. As a result, it is beneficial to combine flow cytometry data with DNA sequencing. I suggest Oxford Nanopore technology as a rapid, accessible platform which could be used routinely in the water sector. I present a business case to support this proposition.