Bacteria, Helpful and Otherwise 449 



they will not survive in surroundings that are perfectly clean and 

 dry. It was this fact which Pasteur probably had in mind when 

 he said: "It is within the power of man to cause the parasitic 

 diseases to disappear from the surface of the globe." 



How to Kill Parasitic Bacteria. Bacteria may be killed by 

 the application of intense heat, by exposure to strong sunlight 

 and by the use of germicides. Heat may be applied to objects 

 suspected of harboring bacteria by boiling, by steaming or by 

 burning. Strong sunlight is especially efficient in killing disease 

 bacteria and will destroy all bacteria directly exposed to its rays. 

 Germicides, also called disinfectants, are chemical substances that 

 kill by contact. Among the most effective of these are bichloride 

 of mercury, formaldehyde and strong solutions of carbolic acid 

 or of iodine. All are poisonous and so should be used only on 

 the advice of a physician, or a health official. 



Some germicides are used in solutions. Others, such as 

 formaldehyde and sulphur dioxide, may be used as gas to dis- 

 infect a room or a building. Disinfection by poisonous fumes 

 or gases is called fumigation. 



SUMMARY 



Bacteria are minute plants of which there are three types, 

 spherical, rod-shaped and spiral. 



According to their method of obtaining food, bacteria are 

 divided into two classes, saprophytes and parasites. 



Bacteria, under favorable conditions, reproduce rapidly. 



Colonies of bacteria may readily be grown on gelatin or some 

 other medium under proper conditions. 



Yeasts and molds are forms of minute plant life closely related 

 to bacteria. 



Among bacteria useful to man are bacteria which give flavor 

 to butter and cheese, bacteria which release the valuable fibers 

 of flax and hemp, soil bacteria which change organic wastes to 

 plant food, and nitrogen-fixing bacteria which take nitrogen from 

 the air for use by plants. 



Among harmful bacteria are those which cause food to spoil 

 and those which cause disease. 



