Dissemination of Escherichia coli O157:H7 from a Contaminated Shredder or Conveyor Belt to Fresh-cut Iceberg Lettuce during Simulated Commercial Production

Introduction: Improperly cleaned and sanitized equipment for commercial production of fresh-cut leafy greens can foster the spread of Escherichia coli O157:H7 as evidenced by an earlier outbreak traced to a lettuce shredder.

Purpose: The goal of this study was to quantify the transfer of E. coliO157:H7 from a contaminated shredder and conveyor belt to previously uncontaminated iceberg lettuce during pilot-plant production.

Methods: Triplicate 50 lbs (22.7 kg) batches of unioculated iceberg head lettuce were processed using a commercial shredder (model TRS 2500 Urschel TranSlicer), Dorner step conveyor with a smooth ThermoDrive belt, 3.3-m flume tank without a chemical sanitizer, shaker table and centrifugal dryer (22.7 kg capacity). Heads of radicchio (9.1 kg), previously dip-inoculated with a 4-strain avirulent, GFP-labeled, ampicillin-resistant E. coli O157:H7 cocktail (10⁶ CFU/g), dewatered and air-dried for 1 h at 22°C, were then either shredded or shredded and conveyed to contaminate the shredder and conveyor belt, respectively, with the shredder thoroughly cleaned and sanitized before quantifying transfer from the conveyor. Thereafter, 200 lbs (90.7 kg) of iceberg lettuce were processed within 5 min during which time 15 radicchio-free lettuce samples were collected immediately after exiting the contaminated shredder or conveyor. These samples, along with additional lettuce, sanitizer-free water and equipment surface samples were appropriately diluted and surface-plated on TSAYE + ampicillin with or without membrane filtration to enumerate E. coliO157:H7.

Results: Based on triplicate experiments, average E. coli O157:H7 populations on the shredder and conveyor belt decreased from 3.73 to 0.69 and from 4.13 to 2.52 log CFU/100cm³, respectively, after processing the 200 lbs of iceberg lettuce. Populations on the 15 lettuce samples collected after exiting the contaminated shredder decreased significantly (P < 0.01) from 2.70 to 1.78 log CFU/g during 5 min of processing. In contrast, no significant difference (P < 0.05) was observed among the 15 lettuce samples collected after exiting the contaminated conveyor belt with E. coli O157:H7 populations ranging from 0.90 to 1.78 log CFU/g. 

Significance: These findings reinforce the importance of proper cleaning and sanitizing of shredders and conveyors to minimize the spread of bacterial pathogens during production of fresh-cut leafy greens.