PLATE CULTURES. 491 



(b) Gram's stain: All the organisms in the prepara- 

 tion are stained except the biscuit-shaped cocci and the 

 small, plump regular bacteria. The loss of color speaks in 

 favor of the cocci being gonococci, and the rods Bact. coli. 



(c) Stain for tubercle bacilli with carbol fuchsin: In 

 the differentiation the preparation is completely decolor- 

 ized with sulphuric acid. After counter-staining with 

 methylene-blue only organisms which are stained blue are 

 seen. Thus, in our case the tubercle bacillus and those 

 resembling it are excluded. 



If, as sometimes occurs, no micro-organisms can be seen 

 in the fuchsin preparation, then a preparation is also 

 stained by Gram's method because the stained cocci and 

 bacilli are more readily seen after the mucus and coag- 

 ulum have been decolorized. In any case if the examina- 

 tion of the slide is negative, the plate method is always 

 employed. 



II. Plate Cultures. 



In examining an animal body for micro-organisms the 

 nature of which we do not know, we employ "the best 

 nutrient* medium " : i. e., serum or ascitic fluid-agar, and, 

 as a substitute, glycerin-agar. l 



The usual plate method consists in placing the material 

 to be examined in liquefied gelatin or agar in various dilu- 

 tions and pouring it out into double dishes. This is not 

 especially suitable for the examination of materials which 

 contain relatively few germs. In our case we prefer 

 to pour the nutrient medium into plates, and, after it is 

 solidified, to carefully make several streaks over the sur- 

 face with a platinum loop which carries the pus or mucus, 

 etc. , to be examined. The double dishes are then turned 

 upside down (so the agar will not dry so rapidly) and 

 placed in the incubator. 



After forty-eight hours there appear upon the plate: 



1. Moist, white, yellow, and orange, roundish, slightly 

 elevated colonies, 2 which, when magnified sixty times, are 



1 On the contrary, many varieties from soil, water, etc. , grow only 

 upon nutrient media poor in nutrient substances, like the ordinary 

 nutrient media (see p. 200). 



2 The colonies here described are always such as lie on the surface 

 of the medium. 



