OUR SMALLEST LIVING THINGS 



217 



out, they become inactive and may die. Some bacteria 

 remain alive for months and perhaps years after dry- 

 ing. Fortunately most disease-producing bacteria are 

 soon killed by drying. 



Light. Most bacteria are killed by exposure to di- 

 rect sunlight for even a few hours. Hence bacteria 

 live in places where continued direct exposure to sun- 

 light is not likely to occur, such as in soil, in foods, or 

 inside the bodies of plants and animals. The killing 

 action of sunlight on bacteria aids in the purification 

 of streams and in the destruction of bacteria in our 

 homes and on our streets and sidewalks. 



Temperature. The best temperatures for bacterial 

 growth are between 70 F. and 100 F. Very few bac- 

 teria grow well above 115F., though in hot springs 

 certain bacteria have been found growing at tempera- 

 tures as high as 175F. 



Within certain limits a rising temperature speeds 

 up the life processes in bacteria, whereas a falling 

 temperature retards their life processes. Ordinarily 

 all active bacteria are killed by boiling water. Their 

 life activities are checked as the freezing point of 

 water is approached and cease entirely when the 

 water freezes. Freezing, however, does not kill bac- 

 teria ; they may live in ice for months and become 

 active again when the ice melts. 



Food supply. Most bacteria require foods prepared 

 by other living things. These foods are oxidized in 

 their bodies by oxygen they obtain from air dissolved 

 in the water surrounding them. The nitrifying bac- 

 teria of soils that break down dead plants and ani- 

 mals are exceptions to these statements. These nitri- 

 fying bacteria resemble green plants in that they are 

 able to build their own foods from carbon dioxide, 

 water, and mineral salts. They differ from green 

 plants, however, in that they are able to manufac- 

 ture their foods without the aid of sunlight. They 

 obtain their energy from chemical action. 



How do bacteria reproduce? Bacteria reproduce by 

 dividing. One divides in the middle, forming two ; each 



of these divides, making 

 four (see Fig. 345). The 

 cells grow quickly to full 

 size and in many forms the 

 dividing may take place 

 every half hour. It is inter- 

 esting to calculate how 

 many bacteria would be 

 formed from a single bac- 

 terium at the end of three 



days, if they divided every half hour and if all lived. 

 It has been calculated that at the end of the first 

 day (24 hours) there would be 281,000,000.000,000 of 

 them, about one pint, formed from one bacterium. 

 At the end of the second day there would be 



FIG. 345. 

 BACTERIA DIVIDING 



281,000,000,000,000 pints, and at the end of the third 

 day there would be enough to make 33,000 bodies 

 as large as the earth. 



Obviously they do not multiply so fast, otherwise 

 the world would be covered with bacteria in a short 

 time. Various agencies keep them down. Unfavor- 

 able conditions of moisture, temperature, and light 

 kill many of them. Millions of them are eaten by 

 microscopic animals. Harmful substances that pre- 

 vent their growth are produced by the bacteria them- 

 selves, in the liquids surrounding them. 



If conditions are unfavorable many bacteria de- 

 crease in size and form a thick wall around them- 

 selves. In this form they remain inactive until con- 

 ditions are again favorable for their development. 

 A bacterium in this dry and hard-walled state is called 

 a spore, and may easily be carried about by the wind. 

 Fortunately most disease-producing bacteria do not 

 form spores ; therefore, they are not likely to be 

 found in dust. Spore formation in bacteria is not a 

 method of reproduction, because there is no increase 

 in numbers. It is a method by which certain bacteria 

 live through unfavorable conditions. Spores are not 

 killed by drying, and certain forms withstand the 

 heat of boiling water. 



How do bacteria aid man in industry, in food manu- 

 facture,*and in maintaining soil fertility? Many bac- 

 teria are beneficial. In fact, in many ways the bene- 

 ficial effects of bacteria are greater than their injuri- 

 ous effects. 



Bacteria are useful in certain industries. In tanning 

 leather, the hides are put into solutions of tan bark, 

 where bacteria assist in making leather soft. Bacteria 

 are valuable in preparing linen fibers. Linen fibers 

 come from the flax plant. Flax is cut, put into pits, 

 and kept moist. Bacteria cause rotting of the sub- 

 stances that hold the fibers of the flax plant together. 

 The fibers are then separated and used in the manu- 

 facture of linen. This natural process is used in Ire- 

 land, where the best grades of linen are produced. In 

 the United States an artificial process that requires 

 less time is employed, but a poorer grade of flax 

 fibers is obtained. 



Bacteria are useful in the preparation of certain 

 foods. Certain bacteria change alcohol into vinegar. 

 The sugars in fruit juices are converted into alcohol 

 by yeast ; then the alcohol is converted into vinegar 

 by bacteria. Certain other bacteria convert the sugar 

 (lactose) in milk to lactic acid. This process is com- 

 monly spoken of as smiriiui of milk. The presence of 

 lactic acid in milk is necessary in the making of sour- 

 milk cheeses such as our common Swiss cheese. Lactic 

 acid bacteria are also useful in the making of sauer- 

 kraut. When cabbage is finely chopped, salted, and 

 packed tightly in containers, the juice of the cabbage 



