Specific interference among strains of Newcastle disease virus. I. Demonstration and measurement of the interference.
A non-plaque-forming vaccine strain of NDV (NDV-B1) interferes with the infection of cultured chick embryo cells by the virulent plaque-forming NDV-L strain. The level of interference increases when the concentration of interfering virus is increased or when the time between infection and superinfection is increased. Only at multiplicities of superinfection of less than one does one obtain an accurate measure of the degree of interference. The direct assay technique for measuring the extent of interference is compared with the infective center assay, and the superiority of the latter in interference experiments is demonstrated. A procedure for plating infective centers is described which insures that the majority of plaques formed in the infective center assay are due to infected cells and not reversibly attached or free virus.
Bratt
M A
MA
Virus Laboratory, University of California, Berkeley, California 94720, USA.
Rubin
H
H