DUSTS, MOLDS, AND BACTERIA OF THE AIR 87 



95. Bacteria and disease. Bacteria that are able to grow 

 parasitically ill the bodies of men, animals, and plants fre- 

 quently produce more or less serious diseases. In many cases 

 the bacteria are easily transferred from one person to another, 

 when the diseases are said to be infectious. Other bacteria of 

 this class are distributed through the agency of air or of water. 



Examples of bacterial diseases are boils and carbuncles, 

 blood poisoning, lockjaw, pneumonia, diphtheria, typhoid, in- 

 fluenza, tuberculosis, leprosy, cholera, and the plague. 



96. Useful bacteria. Bacteria are of direct use in many 

 manufacturing processes, as in vinegar-making, for the fer- 

 mentation that produces the sour substance in vinegar is the 

 result of bacterial action. Others of these organisms cause the 

 change in cream known as " ripening," and the flavor of but- 

 ter is due, in part at least, to the kind of bacteria that ripen 

 the cream. In cheese-making the work of bacteria is indis- 

 pensable, and a large part of the difference in flavor of the 

 different varieties is due to the different kinds of bacteria which 

 have assisted in the ripening, though some kinds of cheese are 

 ripened by certain molds. 



97. Bacteria in cultivation. On account of the value of 

 certain kinds of bacteria they are sometimes cultivated, in 

 order to have them always on hand. For instance, it has 

 been found that if milk is sterilized and then inoculated with 

 certain kinds of bacteria, the cheese made from this milk will 

 always have the desired flavor, whereas if this is not done, 

 the flavor will depend upon the bacteria that may chance "to 

 get into it from the air, and the results may not be at all 

 desirable. It is quite probable that in the future we shall 

 domesticate many kinds of bacteria for which we find use. 



