Il8 MORPHOLOGY AND CULTURE 0¥ MICROORGANISMS 



pleuropneumonia, cattle plague, swamp fever or infectious anasmia 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 pecuUar structure. The most powerful 

 microscopes wiU 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, o.ism in diameter) 

 and ther6 is no reason for beUeving that the size of organisms is limited 

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

 result from a pecuharity 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 filters 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 in- 

 ability to imitate exactly in the laboratory the conditions which exist 

 in the animal body. 



While the invisible microorganisms possess certain quahties 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 resistalit tJian 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 microorganisins 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 



