Sunlight Inactivation of Human Polymerase Chain Reaction Markers and Cultured Fecal Indicators in River and Saline Waters

Decay rates for sunlight inactivation of polymerase chain reaction (PCR) markers for total Bacteroidales, human-specific Bacteroidales, Escherichia coli, and Bifidobacterium adolescentis relative to cultured E. coli were investigated. The experiment used 100-L chambers of fresh water and seawater (paired with dark controls) seeded with human sewage and exposed to natural sunlight over three summer days. Culturable E. coli levels in sunlight-exposed chambers decreased by at least 3 logs on day 1, and by day 3 a total reduction of 4.5 to 5.5 logs was achieved in fresh water and seawater, respectively. In contrast, PCR detection of the four gene targets in sunlight-exposed chambers reduced by no more than 2 logs over the duration of the study (k(t) < 0.071 log(e) units h(-1)). Under these experimental conditions, PCR markers are considerably more conservative than culturable E. coli and can persist for extended periods of time following inactivation of E. coli.