6 BACTERIA, YEASTS, AND MOLDS 



they are useful servants rather than undesirable foes. 

 Even bacteria, which are in general looked upon as 

 dreaded foes, and as agents only of evil, are, under 

 some circumstances, our friends rather than our enemies. 

 Bacteria, for example, produce the delicate flavors in 

 butter and the stronger but equally delicious flavors of 

 cheese. Bacteria also are solely responsible for the man- 

 ufacture of vinegar ; for although vinegar might be made 

 by chemical means, the vinegar of our tables is produced 

 by the agency of bacteria. Molds also, though generally 

 looked upon as unmitigated nuisances, are, in some places, 

 of decided use. The utility of molds, however, has little 

 to do with household products, being confined chiefly to 

 the production of certain types of cheeses. The flavor 

 of Roquefort cheese, for example, is due chiefly, if not 

 wholly, to the growth of certain types of molds within 

 the cheese. These illustrations will serve to show that 

 microorganisms, even in the household, must occasionally 

 be looked upon as friends rather than enemies. 



3. MICROORGANISMS AND DISEASE 



Certain species of microorganisms are harmful to 

 human health and are the cause of contagious diseases. 

 They are generally known as disease germs or pathogenic 

 bacteria. Fortunately they are few in number. While 

 large numbers of species of microorganisms may be 

 troublesome in the household because of their action 

 upon our foods, very few species, comparatively, are able 

 to do harm in the human body or to produce disease if 

 they should find entrance. The great majority of species 



