CLASSIFICATION OF THE BACTERIA 17 



Multiplication of Bacteria.— Bacteria multiply by a 

 process of simple fission. A rod elongates to about double 

 its original length and splits in the middle to form two in- 

 dividuals. Spherical cells frequently elongate somewhat 

 before they divide. 



Bacteria in many eases may multiply with considerable 

 rapidity. Certain of the common bacteria, for example, 

 when placed under the most favorable conditions for 

 growth, that is, with an abundance of food, moisture, the 

 right temperature, and all conditions optimum, may grow 

 to their full size and divide once every twenty minutes. 

 Probably most of the more active bacteria can grow to their 

 full size and divide to form two individuals within one- 

 half hour. If it is assumed that this process continues for 

 two days the number of bacteria will be 2"°, a number 

 having some twenty-eight figures. A rough calculation of 

 the weight of such a bacterial mass will show it to be more 

 than a trillion tons. Obviously no such bacterial masses 

 can ever be formed, inasmuch as the optimum conditions 

 for growth cannot long continue. The principal factors 

 which limit the rapidity of growth are the disappearance 

 of the available food material, and the production of sub- 

 stances by the bacteria which are more or less harmful to 

 their own development. This rapidity of multiplication, 

 however, explains why fermentative changes, such as the 

 souring of milk, may take place so rapidly. A compara- 

 tively small number of bacteria introduced into a bottle 

 of milk and kept at a suitable temperature will cause it to 

 become sour within a few hours. 



Cell Groupings of Bacteria. — Bacterial cells do not al- 

 ways become detached from each other immediately follow- 

 ing cell division. In many cases the presence of capsules 

 or of mucilaginous or gummy walls may cause the cells to 

 cling together. It is obvious that rod-shaped bacteria, in- 

 asmuch as they can divide or multiply only by splitting 



