ARS | Publication request: Effectiveness of lytic bacteriophages in reducing E. coli O157:H7 populations introduced through cross-contamination on fresh cut lettuce

Submitted to: Annual Meeting of the Institute of Food Technologists
Publication Type: Abstract Only
Publication Acceptance Date: February 10, 2012
Publication Date: June 25, 2012
Citation: Ferguson, S.E., Roberts, C.L., Sharma, M. 2012. Effectiveness of lytic bacteriophages in reducing E. coli O157:H7 populations introduced through cross-contamination on fresh cut lettuce. Annual Meeting of the Institute of Food Technologists.

Technical Abstract: Previous research has shown that lytic bacteriophages (phages) can kill E. coli O157:H7 on produce surfaces. The role of lytic bacteriophages in preventing cross contamination of produce has not been evaluated. A cocktail of three lytic phages specific for E. coli O157:H7 (EcoShield) at 10^8 PFU/ml or a control (phosphate buffered saline, PBS) was sprayed on to lettuce pieces (9 cm^2). For suspension studies, lettuce pieces were immersed in either 500 ml of 10^8 PFU/ml Ecoshield or PBS for 30 s and 2 min. Phage-treated lettuce was spot-inoculated with 50 ul of 10^4 CFU/ml E. coli O157:H7. Phage-treated, inoculated lettuce pieces were stored at 4oC for and analyzed for E. coli O157:H7 populations on days 0, 1, 2, 3, 6, and 7. Spraying phages on to cut lettuce reduced E. coli O157:H7 counts to 1.76 and 1.46 log CFU/cm^2, compared to spraying with PBS, which resulted 2.38 and 2.35 log CFU/cm^2, on days 0 and 1, respectively. Immersing lettuce pieces in a suspension of EcoShield for 2 min was more effective in reducing counts of E. coli O157:H7 than for 30 s. Lettuce pieces immersed in EcoShield for 2 min contained lower E. coli O157:H7 populations, 0.61 and 0.52 log CFU/cm^2, compared to pieces immersed in PBS, 2.27 and 2.04 log CFU/cm^2, on days 2 and 3. Phage titers from both spray and immersion treatments were ca. 4 log PFU/ cm^2 on lettuce stored for 7 days. Spraying phages on to fresh cut lettuce resulted in a more immediate reduction of E. coli O157:H7 introduced to lettuce post- phage treatment than immersion in the phage suspension. However, reductions were greater from the 2 min immersion treatment than from spraying. Our results show that phage treatments have the potential to reduce E. coli O157:H7 populations on leafy greens introduced through cross contamination.