BACTERIA A. YD SAX/TAT/OX I 59 



animals. The green plants and the animals now upon the 

 earth have proved their fitness to survive by successfully 

 resisting these one-celled vegetable germs, or bacteria. 

 Microbe diseases attack only the weaker individuals of the 

 human species, or those who have gone to regions where 

 there are microbes which their bodies have not yet ac- 

 quired the power of resisting. 



Usefulness of Bacteria. — Their chief work is to destroy 

 dead tissue and return it to the soil and air for the use of 

 green plants again, otherwise the earth would be filled 

 with carcasses, etc. They are indispensable in soil forma- 

 tion. They give the agreeable flavors to butter and cheese, 

 and cause milk to sour. A rod-shaped bacterium is called 

 a bacillus (Fig. 130); a spherical one is a coccus. 



Multiplication of Bacteria. — This is by division or fis- 

 sion. Sometimes, instead of dividing, a little rounded mass 

 known as a spore appears. The spore breaks out and the 

 bacterium itself perishes. Species which do not produce 

 spores are readily destroyed, but spores have a hard, tough 

 shell, and they may be dried or heated even to boiling with- 

 out being killed. Spores float through the air and start 

 new colonies. Most common bacteria grow best between Jo° 

 and 95 F. They render it difficult to preserve foods, espe- 

 cially proteid foods (cheese, lean meat, eggs, etc.). Food 

 decays slowly if at all below yo° and above 125 . Direct 

 sunlight, or the temperature of boiling water (21 2° F.) 

 kills bacteria but not spores. Pantries, kitchen, and sick- 

 rooms should have bright walls and all the light possible. 

 Boiling water should be poured into the sink, and dish 

 cloths should be thoroughly washed in boiling water. 



Diseases due to Bacteria. — A germ disease is usually due 

 partly or wholly to substances called toxins produced by 

 the bacteria. Most disease germs attack a single organ 



