Tracking the relative concentration between Bacteroidales DNA markers and culturable Escherichia coli in fecally polluted subtropical seawater: potential use in differentiating fresh and aged pollution

Abstract

Routine water quality monitoring practices based on the enumeration of culturableEscherichia coli provides no information about the source or age of fecal pollution. An emerging strategy is to use culturable E. coli and the DNA markers of Bacteroidales complementarily for microbial source tracking. In this study, we observed consistently in seawater microcosms of three different conditions that culturable E. coli decayed faster (T 99 = 1.14 -4.29 day) than Bacteroidales DNA markers did (T 99 = 1.81 -200.23 day).Concomitantly, the relative concentration between Bacteroidales DNA markers and culturable E. coli increased over time in all treatments. Particularly, the increase during the early stage of the experiments (before T 99 of E. coli was reached) was faster than during the later stage (after T 99 of E. coli was attained). We propose that the tracking of the relative concentration between Bacteroidales DNA markers and culturable E. coli provides an opportunity to differentiate a pollution that is relatively fresh from one that has aged. This method, upon further investigation and validation, could be useful in episodic pollution events where the surge of E. coli concentration causes non-compliance to the single sample maximum criterion that mandates high frequency follow up monitoring. Keywords:Bacteroidales, E. coli, decay, fecal pollution, age (HKEPD 2014). It is a common practice for routine water quality monitoring programs that the water of a given location is sampled and analyzed on a weekly or even monthly basis (HKEPD 2014; USEPA 2015). Very often non-compliance to water quality standard would not come to a regulatory agency's attention until the next scheduled monitoring. When monitoring is infrequent, it is often not known whether the non-compliance is due to a pollution incident that has occurred shortly prior to the monitoring or an incident that has occurred days or weeks before (Sokolova et al. 2012).The differentiation of fresh and aged pollution can be useful for health risk assessment, since the persistence of fecal pathogens varies widely in the aquatic environment. Ideally, the persistence of cEC as fecal indicator bacteria should be comparable to that of the most recalcitrant pathogens. Nevertheless, as the more labile pathogens deactivate more quickly, a pollution event that has occurred recently should carry pathogens of higher diversity and infectivity than an event that has aged. However, the concentrations of cEC obtained in routine water quality monitoring, as a function of input scale, population decay and dispersal, provide little information whether a pollution being detected is fresh or aged (Sokolova et al. 2012).Another limitation of cEC counting-based routine water quality monitoring is the lack of information about the source(s) of fecal pollution (e.g. humans, farm animals and wildlife).This hinders not only the identification of the source of impairment (e.g. sewage discharge vs. farm runoff) but also the assessment of health risks, since different animal species ...