Some factors affecting the formation of plaques by fowlplague virus in chick embryo cells

Summary

• 1. Fowl-plague virus makes plaques in agar suspensions of chick embryo cells. The number of plaques formed is directly proportional to the quantity of inoculum.

• 2. The plaque count is not affected by allowing time for adsorption of virus before mixing with the agar. Plaques are visible with a cell population which is about one quarter that of the optimum, but the plaques are less distinct and fewer. Many batches of neutral red are toxic to the cells, and either destroy them completely, or reduce the plaque count. The count is affected by differences in the material used to enrich the agar, chick embryo extract proving better than a mixture of various biological materials.

• 3. With sufficient cells, agar enriched with chick embryo extract, and four days' incubation, the counts of plaque-forming units were consistent and reproducible, and were approximately 1.1 × 10⁹ plaqueforming units per ml of the allantoic fluid seed used.

• 4. The relation of plaque-forming units to egg-infective units (EID₅₀'s) was approximately one to four.