DEPARTMENT OF BACTERIOLOGY 163 



green fodder packed in the silo "keeps" because the packing 

 excludes to a greater or less degree the oxygen of the air which 

 is necessary for the development of the decay-producing bac- 

 teria. Certain substances, such as salt, sugar, smoke, are used 

 to preserve food materials because they are unfavorable to the 

 bacteria. The terms "pasteurization" and "sterilization" are 

 frequently used in connection with the handling of various 

 food products. A sterilized substance is one which has been 

 subjected to a degree of heat, usually the boiling temperature, 

 sufficient to kill all the bacteria and their spores which it may 

 have originally contained. Many substances, e. g., milk, can- 

 not bear such a temperature' without injury; a lower degree of 

 heat, however, may be applied to them in such a way as to 

 destroy most of the germs present, especially those likely to 

 produce disease, without injury; this is pasteurization. 



Anything which kills bacteria is a disinfectant. One of the 

 best disinfectants is heat at the boiling temperature' or higher. 

 Besides heat certain chemical disinfectants are in frequent 

 use, especially in dealing with disease gerins; the most com- 

 mon are corrosive sublimate, formalin, carbolic acid, fumes 

 of burning sulphur, and "chloride of lime." These are poi- 

 sonous to animal life as well as to bacteria, and therefore must 

 be used with care. A substance, which, while not necessarily 

 fatal to bacteria, retards their growth, is called an antiseptic; 

 sugar and salt, mentioned above, are such substances. Cold is 

 to be considered as an antiseptic. Salicylic acid and boric 

 acid are antiseptics which are sometimes used to preserve food; 

 but any chemical food-preservative other than sugar and salt 

 should be looked upon with suspicion as likely to be injurious 

 to health. 



Leaving out of account the disease-producing ones, bacteria 

 are of great interest to those engaged in agriculture, from the 

 intimate relation they sustain to many of the operations which 

 are continually in progress on the farm, some of which have 

 already been hinted at. In dairy work bacteria in great variety 

 have to be dealt w^ith; not only is the souring of the milk and 

 the ripening of the cream due to their activity, but various 

 other changes, desirable or the contrary, are brought about by 



