258 BACTERIA AND DISEASE 



What Bacteria do to Foods. — ^Tien bacteria feed, they dis- 

 solve the food substances by means of enzjTxies which they secrete. 

 The food is decomposed and eventual!}^ rots. The material left 

 behind after the bacteria have finished their meal is quite different 

 from its original form. It is broken down by the action of the 

 bacterial enzjmies into gases, fluids, and some solids. It has a 

 characteristic '^ rotten " odor, and contains poisons which come 

 as a result of the work of the bacteria. 



Bacteria affected by Light. — If we cover with black paper one 

 half of a Petri dish in which bacteria are growing, and then place 

 the dish in a light warm place for a few days, the growth of bac- 

 teria in the exposed part of the dish will be found to be checked, 

 while growth continues in the covered part. It is a matter of com- 

 mon knowledge that disease germs thrive where dirt and darkness 

 exist and are killed by long exposure to sunlight. This shows us 

 the need of light in our homes, especially in our bedrooms. 



Bacteria and Air. — We have seen that plants need oxygen 

 in order to perform the work that they do. This is equally 

 true of all animals. But not all bacteria need air to live ; in fact, 

 some are killed bj^ the presence of air. Bacteria which live with- 

 out free oxj^gen are called anaerob'ic bacteria. They need oxj^gen, 

 as do aU other hving things, but they obtain it by breaking down 

 the foods on which they live, and utilizing the oxygen freed in this 

 process. 



Sterilization. — Bacteria grow very slowly, if at all, in the tem- 

 perature of an ice box, very rapidly from 70° to 98°, and much less 

 rapidly (or are killed) at a higher temperature. Those bacteria 

 which form spores resist a great deal of heat and may even be 

 boiled for some time without injury. The practical lessons drawn 

 from these facts are many. We boil our drinking water if we are 

 uncertain of its purity ; we cook foods that we believe might har- 

 bor bacteria, and thus keep them from spoiling. The industry 

 of canning is built upon this method of sterilization. 



Pasteurization. — Milk is one of the most important food sup- 

 plies of mankind. It is also one of the most difficult things to 

 get in good condition. This is due in part to the fact that milk is 

 often produced at long distances from the place where it is used 

 and must be brought first from farms to the railroads, then shipped 



