One-step growth curve of western equine encephalomyelitis virus on chieken embryo cells grown in vitro and analysis of virus yields from single cells

A method for assay of animal viruses by plaque count was developed re-cently (1), based on the property of Western equine encephalomyelitis (WEE) virus to produce necrosis of infected cells derived from the chicken embryo and cultivated in vitro as a monolayer of cells on glass. This method was extended to the system formed by poliomyelitis viruses and monkey cells grown in vitro (2); in this system plaque production has been used for titration and purification of the virus. The plaque method has now been applied to a detailed study of the growth characteristics of the WEE virus along lines similar to those followed in bac-teriophage work. The present study concerns the one-step growth curve of the virus, both on a cell layer formed in vitro and on cells in suspension, and the distribution of virus yields from single infected cells. An outline of the growth cycle of the WEE virus, as inferred from these data, will be given in the dis-cussion. Both for the WEE and for the poliomyelitis ystem, a linear relationship between virus concentration and number of plaques has been found. This linear dependence has been discussed and shown to prove that a single virus particle is sufficient to produce a plaque. The number of plaque-forming doses contained in a virus sample is, therefore, directly proportional to the number of the particles it contains. It will be shown in the present article that most of the WEE virus particles, able to produce a plaque, do so under the conditions of our experiments; a plaque-forming dose, can, therefore, be considered approximately equivalent to a plaque-forming particle. It has been already pointed out in a previous article that the definition of a plaque-forming particle of poliomyelitis virus is an operational one, and that the morphological nd genetical properties of such a particle are unknown. The same definition and the same reservations apply to the plaque-forming particle of WEE virus