Electron Micrographic Counts of Bacteriophage Particles

All measurements of virus activity give titers expressed in infectious units, that is, in multiples of the smallest amount of virus needed for virus infection under the conditions of the measurements. Statistical arguments have been used (Feemster and Wells, 1933; Parker, 1938; Ellis and Delbruick, 1939; Luria, 1940; Lauffer and Price, 1945) to show that the number of infectious units is a linear function of the number of material particles supposedly bearing virus activity. This relation is not unequivocally established in all cases, however (Bryan and Beard, 1940; Kleczkowski, 1950), and the factor of proportionality between in-fectious units and particles is in some cases fairly low-between 10-4 and 10-8 for viruses such as tobacco mosaic virus or rabbit papilloma virus. Some doubts may, therefore, be justified as to the general validity of the identification of the minimal infectious unit with one material virus particle. Bacteriophage particles, as identified by electron microscopy (Ruska, 1940; Luria and Anderson, 1942), come closer to representing an ideal case. Phage activity assays by plaque count give a perfectly linear relation with the amount