The multiplication of vaccinia virus in suspended KB cells

Abstract

The multiplication of vaccinia virus was investigated in suspensions of KB cells, assays being made by infectivity titrations and by staining cells with fluorescent antibody. Under the conditions described, 50–60% of the input virus was absorbed in 30 minutes, free virus was reduced to less than 1% by a single wash, and virus multiplication was restricted to a single cycle. Adsorbed virus became serum resistant, and soon afterward there was a fall of 80–90% in the infectivity titer of cell-associated virus. Assay of single cells in the early stages of the growth cycle indicated the existence of a true eclipse phase. Infectivity began to rise between the fifth and seventh hours, and when all infected cells were producing virus the infectivity titer rose logarithmically between the tenth and thirtieth hour. The maximum titer reached was approximately 150 PFU per infected cell and was independent of the multiplicity of infection between m = 1.5 and m = 15 at least. Most of the newly formed infectious virus was not accessible to antibody, a result showing that maturation was an intracellular process and mature virus remained in an intracellular location. The proportion of infected cells could be determined by a fluorescent cell count at the end of the growth cycle, and from this an estimate of the multiplicity of infection could be made.