138 MICRO-ORGANISMS AND DISEASE [chap. 



matrix contains the pigment. Zoogloea masses always present 

 themselves as uniformly granular, the granules or micrococci 

 being either of the same size or differing considerably. 



True micrococci never elongate to form rods, although in 

 certain rod-like bacteria the individual elements owing to 

 rapid division have the shape of spherical elements (see 

 below). 



Some species of micrococci form after some days a pellicle 

 on the surface of fluid nourishing material, although there is 

 also an abundance of these micrococci in the depth of the 

 nourishing material. This pellicle is composed of zoogloea, 

 and after some time bits of it, or the whole, sink to the 

 bottom of the fluid medium. Micrococci that thus form 

 pellicles are pre-eminently aerobic (Pasteur), i.e. require 

 a great deal of free oxygen, which they receive from the air 

 to which they are exposed on the surface of the nourishing 

 material. Other species do not require free oxygen 

 (anaerobic, Pasteur), and therefore grow well in the depth 

 and do not form a superficial pellicle. There is a marked 

 distinction in this respect between different species. The 

 micrococci occurring in connection with disease are 

 facultative anaerobic. 



When cultivated in suitable fluids they produce after a day 

 or two general turbidity ; growing in solid nutritive gelatine 

 some produce liquefaction of the gelatine, others do not, 

 and it is with micrococci as with other bacteria that identi- 

 fication of different species is possible by their mode of 

 . growth in and on solid media and in fluids, in plate cultiva- 

 tions, in their power of liquefying gelatine, and in their 

 behaviour in the animal body. 



Besides those mentioned in connection with certain 

 special fermentative changes (micrococcus ureae), and others 

 to be mentioned in connection with disease, various species 



