2 BACTERIA, YEASTS, AND MOLDS 



if it is kept warm; while if she bakes her own bread she 

 must know that the dough has to be warm or it will not 

 rise. Such practical information as this is possible without 

 realizing that bacteria sour milk or that yeasts cause bread 

 to rise ; but there is no question that she can control bacteria 

 and yeasts more intelligently if she knows something about 

 the habits of these tiny beings. 



These microscopic beings, commonly known as micro- 

 organisms or, more popularly, as microbes or germs, are 

 minute plants consisting of the three groups — bacteria, yeasts, 

 and molds — each of which is the subject of one of the later 

 sections of this book. They have been with mankind since 

 the earliest days. Some of them have always been spoiling 

 our food or even bringing sickness upon us ; others, in a less 

 conspicuous way, have been benefiting us by acting as scaven- 

 gers or by f lurnishing food for our crops or by giving desirable 

 flavors to some of our foods. But the human race lived for 

 thousands of years without knowing of their existence or 

 realizing how much more comfortable life on earth could be 

 if we became acquainted with our microscopic friends and 

 enemies. This acquaintance actually began only about 

 seventy-five years ago ; and during that brief period man has 

 made more progress in conquering the ills that flesh is heir 

 to than since human life on earth began. Nevertheless, even 

 in the earlier days mankind had learned how to control some 

 of the activities of microorganisms, although without sus- 

 pecting that there were any such creatures ; and one of the 

 first practical lessons in their control was probably made by 

 a housewife. 



It is a question whether the housewife or the wine-grower 

 was the first to obtain practical knowledge of microorganisms. 



