22 ESSENTIALS OF BACTERIOLOGY 



Classifications. Various ones have been proposed: Mor- 

 phologic, as micrococci, spirilla, and bacilli. Physiologic, ac- 

 cording to their activities and functions, as acid bacteria, 

 alkali and indol bacteria; then subdivisions, according to 

 motility or need for oxygen, but none are satisfactory. 

 The tendency to place bacteria similar in their disease- 

 producing manifestations in one group is growing, as, for in- 

 stance, the colon group, the pus-producers, the pneumonic 

 group, etc. 



Structure. Bacteria are cells; they appear as round or 

 cylindric, of an average diameter on transverse section of 

 o.ooi mm. (=i micromillimeter), written M = 2"g o o mcn - 

 The cell, as other plant-cells, is composed of a membranous 

 cell-wall and cell-contents or cytoplasm. 



Cell- wall. The cell- wall is composed either of hemi- 

 cellulose, or a form of albumin, since it is less permeable than 

 cellulose membrane. The membrane is firm, and can be 

 brought plainly into view by the action .of iodin upon the 

 cell-contents, which contract them. 



Cell-contents. The contents of the cell consist mainly of 

 protoplasm, usually homogeneous, but in some varieties 

 finely granular, or holding pigment, chlorophyl, fat-droplets, 

 and sulphur in its structure. The protoplasm permits osmo- 

 sis, and is like that of other plant-ceils in its structure. 



Chemic Composition of Bacteria. The ash is mostly 

 phosphoric acid; potassium, chlorin, and calcium are present 

 to a small extent; 80 to 90 per cent, is water. The bacteria 

 resemble the lower animals, rather than plants, in chemic 

 composition. 



Nuclein, hypoxanthin, and other nitrogen compounds are 

 found in most bacteria. Varies with media in which grown; 

 the, proteids are about 10 per cent.; fats, i per cent.; ash, 

 0.75 per cent. 



Gelatinous Membrane. The outer layer of the cell- 

 membrane can absorb water and become gelatinoid, forming 

 either a little envelop or capsule around the bacterium or 

 preventing the separation of the newly branched germs, 



