396 MICROORGANISMS 



several kinds are widely distributed in the soil and air and 

 it is these that are used in fermenting fruit juices in making 

 wines. 



Exercise 91. Study of Wild Yeast 



Place a little fresh apple cider or other available unfermented 

 fruit juice in a glass tumbler and allow it to stand in a warm place 

 for several days. After the fruit juice has shown signs of fermenta- 

 tion, mount a drop of it on a microscope slide and examine it under 

 the high power of the microscope for yeasts. You will probably 

 be able to notice that the yeast cells differ in shape somewhat from 

 the tame yeasts that you formerly studied. 



450. Yeasts and the Preservation of Food. It is obvious 

 that, since wild yeasts are floating in the air everywhere, they 

 will be constantly coming into competition with us for pos- 

 session of the sugar-containing foods that we try to preserve 

 for ourselves. Such foods as apple sauce, canned fruit, and 

 others that contain sugar are always in danger of being fer- 

 mented by yeasts unless they are protected by the low tem- 

 perature of the refrigerator or in other ways. It is a rather 

 peculiar fact, however, that some foods which contain very 

 much sugar, such as preserves, apple butter, and jelly are 

 seldom attacked by yeasts. Thus, while sugar is the principal 

 food of yeasts, strong concentrations of it serve as a preserva- 

 tive against yeast as well as against bacteria. Molds, however, 

 are much more likely to grow in such foods than are either of 

 the other two classes of organisms. 



BACTERIA 



We have seen that molds and yeasts are rather important 

 factors in our environment. In some ways they render us im- 

 portant service and in other ways they are the cause of great 

 inconvenience to us. Bacteria are of tremendously greater 

 importance to us in both these ways than are either molds or 

 yeasts. Bacteria are the smallest of living things and they 

 are doubtless the most widespread in their distribution. We 

 might well consider ourselves fortunate that we are living in 



