Plaque formation by avian infectious bronchitis virus in primary chick embryo fibroblast cells in the presence of trypsin.

Ten strains of avian infectious bronchitis virus (IBV) were titrated as plaque-forming units in primary chick embryo fibroblast cells. In the absence of trypsin, plaques were only formed by Beaudette-42 and Iowa-609 strains. When trypsin was incorporated in the overlay medium of cell monolayers, all the IBV strains tested produced plaques within 4 days after inoculation. Incorporation of 20-40 microgram of trypsin per ml of the overlay medium seemed to be suitable for plaque formation of IBV. A preliminary investigation was made of the mode of action of trypsin.