Purpose: The potential of biological (Aerobic Plate Count, E. coli and coliforms), chemical (pH, turbidity, conductivity and oxidation-reduction (OR) potential) and physical (water and air temperature) indicators to predict the levels of Salmonella in surface water was evaluated.
Methods: Samples were taken monthly for 12 months from 18 locations (216 samples). Air and water temperature, pH, OR potential, turbidity and conductivity were measured. Weather data was obtained from nearby weather stations. A three tube modified FDA-BAM Salmonella MPN was used to determine Salmonella levels. Salmonella isolates were confirmed using PCR for invA or oriC genes. Aerobic plate counts were determined by spread plating on tryptic soy agar; a three tube Colisure MPN was used for coliform/E. coli determination.
Results: Bivariate scatter plots of all indicator parameters and Salmonella levels were done. There were weak correlations (r2≈0.1) between E. coli and/or coliforms levels with Salmonella levels, suggesting that these biological indicators could not predict Salmonella levels. Similarly, chemical indicators could not predict (r2<0.1) Salmonella levels. Scatter plots between biological and chemical indicators and biological and physical indicators did not indicate significant relationships. The average rainfall (previous month) before sampling did not correlate well with bacterial levels. Contour plots showed that most observations of Salmonella, E. coli and coliforms occurred at around 1, 2 and 4 log MPN/100 ml respectively.
Significance: The role of index microorganisms and chemical indicators in determining the prevalence of Salmonella in at least some surface water may be limited. In the absence of a rapid indicator, direct screening of surface water for Salmonella may be necessary where Salmonella levels are high and where waters are applied close to harvest.