Purpose: This study aimed to quantify Escherichia coli O157:H7 transfer from lettuce to water at different velocities in a closed pipe system.
Methods: One ml of a 4-strain nontoxigenic E. coli O157:H7 cocktail containing 8.1 log CFU/ml, or 5 x 5 cm pieces of iceberg lettuce dip-inoculated to contain E. coli O157:H7 at ~7 log CFU/g, were inserted into continuously flowing water pumped through a 10-cm dia pipe at 0.0756, 0.108 and 0.189 l/s. Water and lettuce samples were collected at 5- or 10-sec intervals for 2 min and quantitatively examined for E. coli by plating appropriate dilutions with/without membrane filtration on TSAYE with ampicillin. Data from triplicate experiments were assessed for significance using the Kenward-Roger correction.
Results: In the water study, cumulative populations of E. coli O157:H7 recovered were inversely related to flow rate. Using inoculated lettuce, E. coli O157:H7 populations in the water were significantly higher at a flow rate of 0.189 l/s (2.5 log CFU/ml) (P < 0.05) compared to 0.0756 l/s (2.1 log CFU/ml) and 0.108 l/s (1.9 log CFU/ml). E. coli O157:H7 populations in water peaked in the water/lettuce experiments at 50/45, 35/40 and 20/25 s for flow rates of 0.0756, 0.108, and 0.189 l/s, respectively, with no E. coli detected after 75/95, 65/100 and 30/80 s.
Significance: Bacterial removal rate from the lettuce increased significantly with water velocity, which is important to consider in future models of bacterial transfer and/or associated risk analyses.