Cultured and molecular measures of fecal indicator microbes in Gulf of Mexico recreational waters

Recreational waters are routinely assessed by enumeration of fecal indicator bacteria, such as enterococci, but research suggests viral indicators may be better suited for recreational water quality applications. Somatic and F+ coliphage were proposed as predictors of viral pathogens in recreational waters, but concurrent measurements of enterococci and coliphages and comparisons across different risk-based beach action values (BAVs) are limited. We collected paired measurements of enterococci, E. coli, somatic and F+ coliphage from three Gulf of Mexico beaches. Enterococci (colony forming units: CFU) were enumerated on mEI agar and by qPCR (Entero1a: target sequence [TS]). E. coli was enumerated by qPCR (EC23S587, TS). Coliphages (plaque forming units: PFU) were enumerated using dead-end hollowfiber ultrafiltration and single agar layer assay. The largest correlation observed was between Entero1a and cultured enterococci (r ≥ 0.51, p ≤ 0.0001) and the smallest (r ≤ 0.01, p = 1.0) between F+ coliphage and cultured enterococci. Applying equivalent BAVs for culturable enterococci (60 CFU/100 mL), Entero1a (9658 TS per 100 mL) and suggested somatic (14 PFU/100 mL) and F+ (3 PFU/100 mL) thresholds resulted in the overall greatest agreement in beach advisory status between coliphages and Entero1a (74–82 %) and lowest between cultured enterococci and somatic coliphage (65 %). This rich data set not only provides valuable insights on the incidence of coliphage and FIB in the Gulf of Mexico but will also provide a foundation for future research on fecal source identification and water quality forecast modeling in sub-tropical marine waters.