GENERAL CHARACTERS OF BACTERIA IT 
them are shown in Fig. 6. The shape and appearance of the 
colonies produced by different kinds of bacteria are often very 
different, showing, indeed, greater varieties than can be seen in 
the bacteria themselves with a microscope. As a result the shape 
and appearance of these colonies are often used to separate the 
numerous bacteria from each other and to classify them. A 
colony, when it comes from the multiplica- 
tion of a single individual bacterium, is 
made of one kind of bacteria only. This 
colony may easily be picked out with a 
sterile needle, and when properly placed 
in another culture medium it becomes a 
pure culture. The starting-point in prac- 
tical bacteriological study is thus the colony rather than the indi- 
vidual bacterium (see Laboratory Work). 
Form of Bacteria.—Bacteria are of three quite different shapes, 
but are all very simple. (1) Simple spheres (see Fig. 7, a). Such 
spherical forms are called Cocci. In common microscopical 
preparations no internal structure can be seen, the bacteria appear- 
Fre. 6.—Colonies of bacteria. 
Fic. 7.—General shape of bacteria. a, spheres; 0, rods; c, spirals. 
ing as deeply stained balls. The Cocci, however, differ somewhat 
in their method of growth, thus enabling the microscopist to distin- 
guish different kinds, as will be mentioned presently. (2) The rod- 
formed bacteria (Fig. 7, b). These organisms are longer than they 
are broad, sometimes only slightly so, but at other times very much 
longer, forming, indeed, long, slender threads. (3) The spiral- 
