Distribution, diversity, and seasonality of waterborne salmonellae in a rural watershed

Salmonella outbreaks from contaminated water and nonanimal foods (e.g., produce) are increasingly reported. To address the environment as a potential source of pathogenic Salmonella, we investigated levels of salmonellae and the geographic and temporal variation of Salmonella serotypes from surface waters in a region of Georgia (United States) with a history of high salmonellosis case rates. Monthly water samples were collected from six stations in the Little River (Upper Suwannee Basin) for 12 months (April 2005 to April 2006). Salmonellae were enumerated using a three-step most-probable-number (MPN) assay. Salmonellae were detected in 57 of the 72 water samples collected (79.2%). Monthly Salmonella densities ranged from an MPN of 2.5 liter(-1) in April 2005 to 36.3 liter(-1) in August 2005; concentrations were significantly higher in the summer months compared to other seasons (P<0.05). Concentrations were not significantly different between stations. Levels of salmonellae were correlated with average daily watershed rainfall for the 1 and 2 days preceding each sample collection (r=0.77 and 0.68, respectively; P<0.005). Additionally, water temperature was also positively associated with total Salmonella levels (r=0.44; P<0.05). In total, 13 S. enterica serotypes were identified among 197 Salmonella isolates. Eighty (40.6%) were identified as S. enterica subsp. arizonae. Muenchen and Rubislaw were the most frequently identified serotypes of the remaining 117 isolates (28 and 26 isolates, respectively). Serotype diversity peaked in the summer, with 9 serotypes observed in August compared to only one serotype (S. enterica subsp. arizonae) observed in April (2005 and 2006) (P<0.05). Furthermore, all samples collected in August (6/6) contained multiple serotypes (two to five per sample). The results of this study suggest that Salmonella abundance and diversity in the environment vary temporally and are strongly influenced by seasonal precipitation and water temperature.