Aliquots (100 lL) of serial 10-fold dilutions and 200 lL of a suspension of isolate QH (1.0 1010 CFU mL1) were added to 10 mL of 0.6% agar melted and kept at 55 C and then mixed and poured onto a pre-prepared 1.5% agar plate until solidification. The double-layer agar was then incubated at 30 ± 1 C for four days, and the plaques were recorded.