162 ELEVENTH ANNUAL REPORT 



Certain bacteria grow and flourish only under such condi- 

 tions as are to be found in a living plant or animal; these are 

 the parasitic or disease-producing bacteria. It is largely to 

 the study of their relation to animal diseases that bacteriology 

 owes its, in late years, rapid development. The disease-pro- 

 ducing bacteria gain entrance to the animal body through the 

 breathing or digestive apparatus or through wounds. In many 

 cases when they gain entrance to the healthy body, though 

 they And most of the conditions favorable to their develop- 

 ment, they encounter a natural resistance which they are 

 unable to overcome, and, as a consequence, perish without 

 doing any serious damage. In other cases, owing to some 

 unhealthy condition of the body in which the natural resist- 

 ance is lost or weakened, the bacteria gain a foothold and de- 

 velop, producing their characteristic disease. Tuberculosis or 

 consumption, typhoid fever, diphtheria, cholera, lockjaw, 

 anthrax, hog-cholera, and glanders are well-known examples 

 of bacterial diseases. 



While the farmer or housekeeper may never have seen bac- 

 teria, or may even be ignorant of their existence, yet they do 

 things every day the fundamental reason for which has to do 

 in some way with these tiny plants. Food, after it is cooked, 

 if not used at once, is reheated or scalded after a few days ^^to 

 keep it from spoiling"; the original cooking destroyed all or 

 most of the bacteria j^resent in the material; after twenty-four 

 or forty-eight hours the few germs that may have been able to 

 withstand the heat used, or new germs that may have fallen 

 from the air, begin to develop, and would soon "spoil" the food 

 if they were not killed by reheating. The essential thing in 

 preserving fruit, meat, or other food by canning is to destroy 

 by heat all bacteria present and by sealing up while hot to 

 keep out any new ones. Food materials are kept in refrigera- 

 tors because the low temperature produced by the ice is unfa- 

 vorable to and retards the development of bacteria. The tem- 

 perature of ordinary refrigerators, however, is not usually low 

 enough to wholly check development, hence the things spoil 

 after a time. The reason for the various applications of heat 

 and cold in the dairy may readily be inferred. Corn and other 



