SYLLABUS 105 



affected by osmotic shock (Anderson, 1949). Viruses treated by intense 

 sonic vibrations show the same effect (Anderson et al., 1948). 



5. Chemical Composition of Phages. — Biochemical analytical work 

 has been done on T2 (Cohen, 1946b; Taylor, 1946), T4 (Poison et al., 

 1948; Cohen, 1946a), T6 (Putnam et al., 1950), and T7 (Kerby et al., 

 1949). The simplest and most reliable method for purification of 

 phages is differential centrifugation of lysates at low and high speed. 

 Uniformity of preparations is controlled by electron micrographs or 

 in the analytical ultracentrifuge. In many cases it is difficult to be 

 sure of dealing with homogeneous material especially where the 

 samples taken are small. Generally, proteins and great amounts of 

 nucleic acid are found in phages, the nucleic acid consisting entirely, 

 or almost entirely, of DNA. The quantitative composition of a bac- 

 teriophage seems to vary with the medium in which the infected cells 

 were kept. Phage particles of a given strain apparently are not merely 

 uniform molecules of a highly complicated chemical substance. For 

 this reason alone, a detailed comparison of quantitative differences in 

 composition between different phage strains would not be very signif- 

 icant, let alone our lack of knowledge about the possible meaning of 

 such differences. The amino acid content of T4 showed no striking 

 deAdations from that of the host E. coli (Poison and Wyckoff, 1948). 

 A significant distinction, however, exists in the nucleic acid type, the 

 ribopentose type being predominant in the bacterium. 



6. Stability. — There are many factors which affect the stability 

 of phages. Temperature, pressure, chemical action, irradiation, and 

 ionic environment are just a few of these. Studies have been confined 

 mostly to the conditions used in experimental work with phages. 



The most detailed study is by Adams (1949). 



The seven T phages are relatively stable in broth. At temperatures 

 up to 50°C the rate of inactivation is hardly measurable. Above 5o°C, 

 the rate increases. The kinetics of inactivation is of the first order. 



In dilute sodium salt solutions most of the phages are inactivated 

 at a measurable rate at room temperatures. For example, at 20°C in 

 o.iN NaCl, about 78% of phage T5 is inactivated in 60 minutes, and 

 at 30°C about 95% of this phage is inactivated during the same time 

 in the same concentration of sodium ions. The rates of inactivation 

 are not significantly different, whether the anion is chloride, citrate, or 

 phosphate. As the concentration of sodium ions is increased above .iN, 

 the inactivation rate of the phages is decreased rapidly. At a concen- 

 tration of sodium ions of iN, the rate has decreased by a factor of 

 10^ and phage T5 is then as stable as it is in broth. 



