66 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



pleuropneumonia, cattle plague, canine distemper, swamp fever or in- 

 fectious anaemia of horses, chicken pest, sheep pox, and horse sickness. 



The invisibility of this group of microorganisms may depend upon 

 either their minute size or their peculiar structure. The most powerful 

 microscopes will not enable us to discern with distinctness objects which 

 are less than o.i/i in diameter. We know of bacteria which in size 

 approach this limit quite closely (M. progrediens, 0.15^ in diameter) 

 and there is no reason for believing that the size of organisms is limited 

 by our ability to see them. As already stated, invisibility may also 

 result from a peculiarity of structure, such as complete transparency 

 and failure to stain with the reagents ordinarily used for this purpose. 



The ability of microorganisms to pass through niters is dependent 

 upon a variety of factors. The size and plasticity of the organism, 

 the fineness of the pores, and the thickness of the walls of the filter as 

 well as the conditions under which the filtration is performed, will all 

 influence the result. 



The failure of the invisible microorganisms to develop under artificial 

 conditions is to be attributed to their strict parasitism and to our inability 

 to imitate exactly in the laboratory the conditions which exist in the animal 

 body. 



While the invisible 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 germicidal 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 microorgan- 

 isms likewise differ markedly, some being comparatively benign and local 

 in character, whereas others appear as the most profound septicaemias. 

 Some are extremely contagious, while others can be transferred from 

 one animal to another only by means of an intermediate 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 an invisible 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 



