CULTIVATION OF BACTERIA 



FIG. 26 



hot in a basin of water which has been heated to about 45 C. 

 \Yhen the temperature of the agar in the tubes, as shown by a ther- 

 mometer in one, has fallen to 42 C., the water, milk, iVces, bacterial cul- 

 ture, or other substance to be tested is added to the other tubes in what- 

 ever quantity is thought to be proper up to 1 c.c. A greater quantity 

 of fluid would dilute and cool the nutrient agar too 

 much. After inoculation the contents of the tubes 

 are thoroughly shaken and poured out quickly into 

 round, flat-bottomed, glass Petri dishes (Fig. 26), 

 the covers of wliich are removed for the required 

 time only. Instead of placing the substance in 

 the tube it is often placed directly in the Petri 

 dish. The melted nutrient gelatin or agar is thus poured over it 

 and by gently tipping the dish mixed with it. The bacteria are now 

 scattered throughout the fluid, and as it quickly solidifies they are fixed 



FIG. 27 



Petri dish. 



Photograph of a large number of colonies developing in a layer of gelatin contained in a Petri 

 dish. Some colonies are only pinpoint in size ; some as large as a pencil. The colonies here appear 

 in their actual size. 



wherever they happen to be, and thus as each individual multiplies 

 clusters are formed about it at the spot where it was fixed at the moment 

 of solidification. The number of colonies of bacteria (Fig. 27) thus 

 indicate to us roughly the number of living bacteria in the quantity 

 of fluid added to the liquid agar. Groups or chains of bacteria 

 which in spite of shaking remain attached produce single colonies. 

 Nutrient gelatin is used exactly as agar, except that as the average 

 product does not congeal until cooled below 22 C. we have no fear 

 of its cooling too rapidly. In order not only to count the number of 

 colonies which develop, but also to obtain a characteristic 'growth, it is 

 desirable not to have them too near together. 



