[Chap. XLIl THE BIOLOGY OF BACTERIA 525 



plant to plant by insects, by man, and by other animals. Bacteria are less 

 frequently dispersed by rain and wind. 



Bacteria in milk. Among the natural media in which bacteria grow, 

 milk is ideal for most bacteria of decay and some pathogenic organisms 

 ( pathogens ) . Milk, then, must be handled carefully if it is to be used as 

 food. The cows, the stable, the vessels in which the milk is placed, the 

 persons who handle it are all possible sources of bacterial contamination. 

 It is of paramount importance that attention be paid to cleanliness and 

 to the retardation of bacterial growth by immediate refrigeration. 



In spite of all precautions, bacteria do occur in milk. To check their 

 multiplication, particularly during shipment from the dairy fami to 

 the cities, the milk is heated to 140° or 145° F. for 30 minutes.^ This 

 process, which is known as pasteurization, destroys nearly all the active 

 pathogens. Such heating, however, does not kill spores; but if the milk 

 is kept cool, their subsequent germination and growth are to a large 

 extent prevented. 



The bacteria of natural waters are discussed in Chapter XL VI. 



The science of bacteriology. The studv of bacteria has made great 

 strides since Leeuwenhoek in 1676 first saw through his crude magnifier 

 the "animalcules" from his teeth. Many years elapsed and many investi- 

 gations were made before the biological significance of these micro- 

 organisms was established. Even now a large portion of the world's 

 population has never heard of bacteria, a larger number is indifferent 

 to their importance, and too many people still like to believe that disease 

 results from mvsterious influences rather than from bacterial infection. 



Spontaneous generation. Since earliest times man has probably been 

 interested in the question of the origin of living organisms on the earth, 

 and has tried to fonnulate answers on the basis of what he imagined 

 or what he observed. Some of these observations, and also the inferences 

 drawn from them, we now know were erroneous. It was believed, for 

 example, that mice came from rags and meal, that mud produced frogs, 

 that putrid meat changed to bees and flies, and that water in some man- 

 ner gave rise suddenly to all sorts of fully formed aquatic organisms. 

 Although many people ardently subscribed to a special creation of each 

 kind of plant and animal, they at the same time accepted the idea of 

 the spontaneous production of such organisms from inorganic matter. 

 It required, of course, onlv simple experiments to show that screened 



^ Another method is to heat the milk for two minutes over a steam coil to a temperature 

 of 176° F. and cool it rapidly. 



