496 BACTERIA AND SANITATION 



killed by exposure to high temperature, such as the boiling- 

 point of water. They require water to develop in, and 

 when there is no water present they become inactive, and 

 in a dried condition may be easily borne about by the winds. 

 This drying, however, does not kill them, and when trans- 

 planted to a moist place they become active again and 

 grow. 



The most remarkable feature of their action is the ra- 

 pidity with which they reproduce. Like all single cells, 

 they reproduce by division. In bacteria, however, this pro- 

 cess of division is much simpler than in higher forms of 

 cells and is accompanied by no nuclear changes. When a 

 bacterium has reached a certain size the walls begin to fold 

 in at the middle, a partition forms, and the two halves break 

 apart to form two individuals. Each of these halves ab- 

 sorbs food until it becomes of normal size and then splits 

 again. In active forms this whole process may be repeated 

 as frequently as once in thirty minutes, and it has been 

 calculated that one bacterium can produce at this rate a 

 colony of over 200,000,000,000,000 in twenty-four hours. 

 It is this marvellous rate of development that makes the 

 introduction of a single disease causing bacterium into our 

 system so dangerous. A single diphtheria bacillus can 

 alone produce little damage, but if allowed to develop the 

 increase may become sufficient to cause serious damage in 

 a short time. 



Occurrence of bacteria and infection. — Bacteria are found 

 in earth, air, and water, and in fact may be present prac- 

 tically everywhere on the earth's surface. Certain condi- 

 tions are, however, more favorable to their development 

 than others, and fortunately the malignant forms arise only 

 under conditions which are subject to control. 



