CULTIVATION OF BACTERIA 59 



is an advantage, as many forms of bacteria develop more character- 

 istically on the surface than in the midst of the media, and it is easier 

 to remove them free from other bacteria with the platinum needle. 

 Instead of streaking the material by means of the platinum wire over 

 the agar, a loopful may be deposited on the agar and then smeared over 

 its surface by a sterile swab or bent glass rod. The method of using 

 glass plates upon a cooling stage has now been practically given up for 

 the more convenient one of Petri dishes. In warm weather the dishes 

 may be cooled before using, so as to harden quickly the agar or gelatin 

 that is poured into them. 



An old method, which is still sometimes used to find the number of 

 living bacteria, is, instead of pouring out the media which has been 

 inoculated, to congeal it on the sides of the test-tubes. This is best 

 done by laying the tube flat on its side on a cake of ice and rotating it. 

 Tubes come especially formed for this by having a slight neck, which 

 prevents the media running up to the plugged end of the tube. This 

 method, Esmarch's, is used only when the Petri dishes are not obtain- 

 able or cannot easily be transported. 



Study of Colonies in Plate Cultures in Nutrient Agar. The plates 

 should be removed after twelve to twenty-four hours' growth at blood 

 temperature and after one to three days at 70 F. (21 C.). The 

 special time allowed varies according to the rapidity of the growth of 

 the varieties developing; thus, bacteria, such as the streptococci and 

 influenza bacilli, reach the characteristic development of their colonies 

 in from ten to sixteen hours, while others continue to spread for several 

 days. If we wait too long where numerous varieties of bacteria are 

 growing the colonies of heavier growth may cover up the finer and 

 more delicate ones. As a rule, the younger colonies are more charac- 

 teristic, except where the development of pigment is sought. 



The colonies are first examined with the eye (Fig. 27), then with 

 magnification of about 60 diameters (Figs. 30 to 35), and then again, 

 if necessary, at from 400 to 500 diameters (Fig. 44). We note every- 

 thing we can about them, such as their size, surface elevation, form, 

 internal structure, edges, and optical characters. If grown in gelatin, 

 whether they have or have not caused liquefaction. The accompanying 

 schematic representations from Lehman and Neumann (Figs. 36 to 43) 

 illustrate some of these points. 



At the higher magnification we begin to detect the individual bac- 

 teria (Fig. 44). After studying the colonies we remove a few of the 

 bacteria from one or more of them by touching them with the tip of a 

 sterile platinum needle (Fig. 45), and thus transfer them to a cover- 

 glass for microscopic examination, or to new media where they may 

 develop in pure cultures and show their growth characteristics. 



Hanging Block Cultures. In order to study the morphology and 

 manner of multiplication of bacteria to better advantage than in 

 the hanging drop, Hill devised the following procedure: Melted 

 nutrient agar is poured into a Petri dish to a depth of about one-eighth 

 to one-quarter of an inch. When cool a block is cut out about one- 



