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 plaqueforming 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 µg 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.