Electron Microscope Studies of Bacterial Viruses

Abstract

Four strains of bacterial viruses were studied with the electron microscope. In all cases the particles of virus could be identified on the micrographs. Three of these strains show "sperm-shaped" particles, consisting of a head and a tail; for the 4th strain, a tail is not visible on the micrographs. The particles of one of these viruses show a distinct structure in the head. The particle sizes agree well with the sizes inferred by some of the indirect methods. The interaction between the virus and its host was studied in detail in the case of 2 viruses which act upon the same strain of Escherichia coli. The micrographs demonstrate adsorption of virus on the host and, after the predicted time, lysis of the host with the liberation of virus particles of the infecting type. There is quantitative agreement between the numbers of particles visible on the micrographs and the numbers predicted on the basis of growth expts. for which plaque count assays were used. Along with the virus particles, the lysing cells shed protoplasmic material of uniform granular structure. These granules are much smaller than the viruses and their size is independent of the virus under whose influence the bacterium is lysed. Upon lysis the virus particles are liberated from the interior of the bact. cell, for they are not visible on its surface up to the moment of lysis. In cases of multiple infection, the infecting particles of virus, or at least the majority of them, seem not to enter the cell but to remain attached to the outside of the bacterial cell-wall. The bearing of these results on the problems of the nature of viruses and of their systematic position is discussed.