60 MICRO-ORGANISMS AND DISEASE. [CHAP. 



ment, and thus form smaller or larger continuous masses, 

 zoogtosa or colonies, in which the individuals appear embedded 

 in a hyaline gelatinous matrix ; the amount of this varies in 

 the different species ; in some there is little of the matrix 

 actually visible, the micrococci being in close juxtaposition, 

 in others it is easily recognised, the interstices between the 

 individuals being measurable. 



In some of the pigmented species (see below) the interstitial 

 matrix contains the pigment. Zoogloea masses always present 

 themselves as uniformly granular, the granules or micrococci 

 being of the same size. 



True micrococci never elongate to form rods, although in 

 certain rod-like bacteria the individual elements sometimes 

 assume 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 zooglcea, 

 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 

 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. 

 Micrococci may be divided, according to their chemical 



