122 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



layer of sterile paraffin oil to exclude oxygen. The material from which 

 cultures are to be made is introduced into the bottom of the tube by 

 means of capillary pipettes. * 



While the filtrable microorganisms possess certain qualities in com- 

 mon, in some respects they differ widely from one another. Some will 

 pass only through the coarsest of bacteria-proof filters, while others pass 

 readily through the densest filters, thus indicating wide differences in 

 size or in structure. Some are very susceptible to the action of germici- 

 dal agents, whereas others are more resistant than the ordinary bacteria. 

 Some produce disease in only one species of animal, while others show 

 little or no limitation in this respect. The diseases produced by these 

 microorganisms likewise differ markedly, some being comparatively 

 benign and local in character, whereas others appear as the most pro- 

 found septicaemias. Some are extremely contagious, while others can 

 be transferred from one animal to another only by means of an inter- 

 mediate host. In fact these invisible microorganisms seem to differ 

 among themselves quite as widely as do those which are visible to us. 

 The existence of a filtrable microorganism is determined as follows: 

 The infectious agent must pass through a bacteria-proof filter, which 

 is free from imperfections as shown by tests with visible organisms of 

 small size. Pressure exceeding one atmosphere should not be employed 

 during filtration. The time of filtration should not exceed one hour. 

 The filtrate should remain free from all visible bacteria as shown by 

 microscopic examination and cultural tests. The filtrate should 

 possess the specific disease-producing qualities of the unfiltered material. 

 Animals infected with the filtrate should yield material which, after 

 filtration, will in its turn possess the attributes of the original unfiltered 

 material. Recent suggestive developments have thrown some light on 

 the possible nature of filtrable viruses. The reader is referred to the 

 work of Flexner and Noguchi since 1912, published in the Journal 

 of Experimental Medicine; he is also requested to read the article by 

 Lohms and Smith already mentioned on page 99. 



* For details of this method see J. Exp. Med., 1911, et seq. 



