One strain of Listeria innocua and six strains of L. monocytogenes were combined into a cocktail for use in this study. See Table 1 for details on strains used. Stock cultures were maintained frozen (–80°C) in tryptic soy broth containing 0.6% yeast extract (TSB-YE; Bacto,
Becton Dickinson Co., Sparks, MD) supplemented with 16% glycerol. Each culture was inoculated from frozen stocks onto plates of Bacto tryptic soy agar containing 0.6% yeast extract (TSA-YE; Bacto, Becton Dickinson Co.) and incubated at 37°C for 24 h. Overnight cultures of each strain were prepared by inoculating a colony into 10 ml of TSB-YE and incubating the mixture at 37°C for 18 to 20 h. Cocktails for inoculation were prepared by placing equal aliquots of each overnight culture in a single sterile tube and mixing by use of a vortex mixer.