The samples were homogenized in 180 mL of sterile phosphate
buffered saline in a stomacher 400 Biomaster (Seward Limited,
London, UK). Populations of E. coli were enumerated by plating
serially diluted samples using a spiral plating device (Wasp II Spiral
Plater, DW Scientific, West Yorkshire, UK) onto Luria Bertani (LB)
agar (Difco) supplemented with 100 mgmL1 streptomycin. The LB