396 Microbes and You 



non-living attributes of these parasitic agents. The word life has a 

 man-made definition, and man recognizes degrees of complexity 

 in living things. At the present stage of our knowledge it appears 

 that viruses are probably the simplest forms of matter exhibiting 

 at least some of the characteristics man assigns to living things. 

 Future research developments may disclose that even viruses have 

 "littler fleas to plague them," just as scientists who believed that 

 bacteria were the smallest forms of life had to concede that viruses 

 were still smaller. 



Since there are no saprophytic viruses recognized, attempts to 

 study viruses in pure culture completely removed from their host 

 cells have met with failure. In some diseases of virus etiology it 

 is possible to demonstrate granules, called elementary bodies, 

 which are smaller than most bacteria but are still visible under 

 compound microscopes. These granules aid in diagnosing some 

 diseases, since not all viruses display this granule formation. 

 There are strong indications that elementary bodies are aggregates 

 of the virus itself. 



Other objects called inclusion bodies are found either within 

 the nucleus or within the cytoplasm of affected cells, and these 

 bodies vary in size and in shape with the particular virus. They 

 probably represent colonies of the infecting virus or disintegration 

 products of the cell, and their presence aids in the diagnosis of 

 some virus diseases. In cases of rabies the inclusion bodies are 

 called Negri bodies, in honor of their discoverer, and they are 

 located in the cytoplasm of affected brain cells. 



Viruses have been likened to genes which also are nucleopro- 

 teins. A difference between the two, however, is that genes mul- 

 tiply only as fast as the cell nucleus, while viruses increase at a 

 rate independent of nuclear division. 



TECHNICS FOR CULTIVATION 



TISSUE CULTURE 

 Because of their high degree of parasitism, viruses can be cul- 

 tivated only in the presence of living cells. Furthermore, they 

 must have young, actively metabolizing cells for optimum growth. 



