io6 



BACTERIA AND FOOD 



like shot, some oval like an egg, others cylindrical like a cigar, 

 others spiral like a corkscrew. Some have long hairlike pro- 

 jections, others have short hairlike projections, and some have 

 no projections at all. 



Because bacteria are too small to be seen with the unaided eye, 

 they are not easily controlled. The housewife can see the mold 

 which starts on her jelly and can remove it and protect the 

 remainder of the jelly; but she cannot see the bacteria in her 

 canned vegetables and milk, and she does not know that harm 

 is being done by them until it is too late to save the food. Bac- 

 teria increase much more rapidly than molds. Jelly does not 

 become badly molded over night, but milk may sour in a few 

 hours. 



The abundance of bacteria. Bacteria are everywhere, in 

 the air, in water, in milk, in dust, in soil, in the mouth, and 



on the hands. We can 

 prove that bacteria are 

 in the air by removing 

 the cover from a dish 

 containing especially 

 prepared agar or gelatin 

 and exposing the agar 

 to the air for a few 

 minutes (Fig. 44). If 

 the vessel is put in a 

 warm, dark place, slimy 

 spots appear in a few 

 days. Each spot is a 

 colony of bacteria, and 

 each colony developed 

 from a single bacterium! 

 The bacteria from which the various colonies grew drifted to the 

 agar from the air. If the dish contains numerous colonies, the 



FIG. 44. Colonies of bacteria. 



