40 IMMUNOLOGY 



varies from 10 ni/i, for poliomyelitis virus to 250 m/A for psittaco- 

 sis virus. He places the size of vaccine virus at 150 mju,. 



New instruments and techniques such as the ultracentrifuge, 

 the electron microscope, and x-ray, are beginning to extend our 

 knowledge concerning the size, morphologj^ and structure of both 

 viruses and bacteria. By means of the ultracentrifuge, Bauer and 

 Pickels^ (1940) determined the minimal diameter of yellow fever 

 virus to be 14 m.[x while Gratia- (1940) reports the recovery of 

 polyhedral bodies and of minute granules possessing virulences 

 from serum of diseased silkworms by differential centrifugation. 



Borries, Ruska and Ruska (1938) describe a new electron 

 microscope witli which they have obtained photographs of several 

 viruses and of bacterial structures previously unrecognized. Ac- 

 cording to Mudd^ (1941), magnification of 50,000 diameters has 

 been obtained with instruments in the RCA laboratories. He 

 mentions a few limitations of the electron microscope such as the 

 in vacuo requirements, the opacity of specimens of Iju, or more in 

 thickness, and the possible effect of drying and electron bombard- 

 ment on the specimen. Bernal (1940) submits x-ray evidence to 

 show that both spherical and rodlike forms exist among the viruses. 



There has been much discussion concerning the living or non- 

 living nature of viruses. Rivers (1928) suggests three possi- 

 bilities as to the nature of viruses. Some may be miniature 

 bacteria; others may be as yet unrecognized forms of life, while 

 a third group may be inanimate transmissible agents of disease. 



Green* (1935), Laidlaw (1938) and Gortner (1938) suggest that 

 viruses are living entities that through retrograde changes have 

 lost their enzyme systems and are therefore dependent upon the 

 enzyme system of the host cell. Zinsser and Bayne-Jones (1039) 

 state, however, that tliey regard the bacterial origin of viruses as 

 highly improbable. 



Chemical Composition of Bacteria. — Bacteria have been investi- 

 gated more extensively from the standpoint of chemical composi- 

 tion and metabolism than any other group of infectious agents. 

 The modern era began with Pasteur's studies in fermentation and 

 his investigation of the germ theory of disease. Koch (1891) 

 obtained from tubercle bacilli a crude group specific protein which 



iProc. Third International Congress for Microbiology, 1940, p. 287. =Ibid., p. 

 288. 'J. Bact. 41: 415. 1941; Ibid. 43: 251, 1941. "Science 82: 443, 1935; Bio- 

 dynamics No. 39, 1, 1938. 



