THE INITIATION OF BACTERIOPHAGE INFECTION 205 



filterability (Elford, 1938), and radiation sensitivity (Lea, 1946; Pollard, 

 1953). However, with the advent of electron microscopy, and its refinement 

 in the hands of T. F. Anderson, R. W. G. Wyckoff, R. C. Williams, and 

 E. Kellenberger, this method now furnishes the definitive values of the 

 dimensions of phages. Fortunately, the sizes of all the known phages faU 

 within the limits of resolution of the most sensitive instruments; the smallest 

 phage, S13, appears to have a diameter of about 300 A (Tessman, personal 

 communication). Electron micrographs have revealed an array of intricate 

 forms (Fig. 1), each uniformly characteristic of a particular phage or group 

 of related phages. In general, the distinguishing features are a spherical or 

 polyhedral head and a narrower cylindrical tail (Ruska, 1941; Luria and 

 Anderson, 1942; Anderson, 1953; Williams, 1953). No clear-cut example of a 

 completely tailless phage is known; T3 and T7, once considered taiUess, had 

 to be reclassified with the development of more refined techniques (Eraser 

 and Williams, 1953). The tail does not appear to function as a flagellum, 

 since the diffusion constants of several phages can be accounted for satis- 

 factorily by their Brownian motion (Putnam, 1953). 



B. Chemical and Morphological Components 



1. Purification 



The problem of separating phage particles from the complex mixture of 

 bacterial debris in a lysate may be formidable for some phages (e.g. lambda) 

 and reasonably straightforward for others (e.g. T2). No phage has been 

 crystallized. Most purification procedures rely primarily on enzymatic diges- 

 tion of bacterial nucleic acids, filtration, precipitation, and differential centri- 

 fugation (Herriott and Barlow, 1952). Several sensitive tests for the purity 

 of a preparation are available (in addition to those standard in biochemistry, 

 such as ultracentrifugation and electrophoresis). One is the homogeneity of 

 the particles appearing in the electron microscope. Others are the degree to 

 which the material in the preparation attaches specifically to bacterial host 

 cells, or is precipitable by antiphage and not by antibacterial serum. 



The fraction of infective particles in purified phage preparations has been 

 determined for the T-even phages by correlating the number of particles 

 appearing in electron micrographs with the number of plaque-forming units 

 (Luria et ah, 1951). At least half of the particles were infective. Similar 

 results have been obtained on unpurified lysates of T2, T4, T5, and lambda 

 (KeUenberger and Arber, 1957). The availability of preparations containing 

 a large fraction of infective particles offers assurance that the chemical and 

 morphological details to be discussed below are pertinent to the mechanisms 

 of phage infection and multiplication. 



