426 



PATHOGENIC BACTERIA. 



good. A pellicle will be formed, a drop from which, 

 diluted in melted gelatin and poured upon plates, will 

 show typical colonies. 



Under the microscope the principal characteristics 

 can be made out. The. colony of the cholera spirillum 

 scarcely resembles that of any other organism. The little 

 colonies which have not yet reached the surface of the 

 gelatin begin very soon to show a pale-yellow color and 



FlG. 90. — Spirillum of Asiatic cholera: colonies two days old upon a gelatin 

 plate ; x 35 (Heim). 



to exhibit irregularities of contour, so that they are 

 almost never smooth and round. They are coarsely 

 granular, and have the largest granules in the centre. 

 As the colony increases in size the granules also increase 

 in size, and attain a peculiar transparent character which 

 is suggestive of powdered glass. The commencement 

 of liquefaction causes the colony to be surrounded with a 

 transparent halo. When this occurs the colony begins to 

 sink, from the digestion and evaporation of the medium, 

 and also to take on a peculiar rosy color. 



