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

Routine water quality monitoring practices based on the enumeration of culturable Escherichia 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 consistently observed in seawater microcosms of 3 different conditions that culturable E. coli decayed faster (T₉₉ = 1.14 – 4.29 days) than Bacteroidales DNA markers did (T₉₉ = 1.81 – 200.23 days). 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 T₉₉ of E. coli was reached) was faster than during the later stage (after T₉₉ 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 noncompliance to the single sample maximum criterion that mandates high frequency follow-up monitoring.