Purpose: To evaluate effectiveness of seven bacteriophages isolated from beef cattle to reduce E. coli serogroups O26, O45, O103, O111, O121 and O145 and O157 over a range of temperatures (2 °C, 10 °C and 25 °C) .
Methods: Seven VTEC strains at exponential growth, were treated individually with different dilutions of one of seven lytic phages (phage stocks concentrations 10^7 -10^9 log PFU ml) in a duplicate micro-plate assay at 2, 10 and 25°C. Romaine lettuce leaves were washed and cut into pieces (3 x 3 cm) and spot inoculated with a specific VTEC strain. Phages were individually sprayed on each lettuce piece with contact for 1 h (25°C). VTEC survival was detected using total plate counts on MacConkey agar (n= 100).
Results: Phages showed lytic activity against the seven VTEC to differing degrees. As expected, an interaction existed among phage lytic activity and temperature (p=0.003). All phages were effective at 25°C, however only six were able to kill VTEC at 10 and 2 °C at high concentrations (>10^7 -10^9 log PFU/ml), suggesting the phages were causing lysis from without (lysis effected directly by extracellular supplied agents such as tail-associated lysozyme). On lettuce, five phages targeting O26, O45, O121, O145 and O157 effectively reduced VTEC by more than 90%.
Significance: VTEC phages showed lytic activity at refrigeration temperatures and reduced the number of VTEC on the surface of lettuce. Although concentration of phage needs to be determined for optimal phage activity on lettuce, these results are promising for future application of phages to reduce VTEC on fresh vegetables.