BACTERIA 267 



many species by the condensation of the protoplasm into a small 

 shining body within the cell which is able to resist greater extremes 

 of heat, cold, dryness, etc. 



They usually multiply by division, each cell dividing into two 

 equal parts as in the blue-green algae. This is called multiplica- 

 tion by fission, which means a cutting in two. For this reason 

 they are sometimes called fission fungi. In reproduction many 

 of the bacteria form spores, the protoplasm condensing inside 

 the cell into a small, rounded, shining body which is much more 

 resistant to dessication, heat, cold, and the action of poisonous 

 substances than are the vegetative cells, some being able to resist 

 the heat of boiling water for several minutes. 



The bacteria live within, or upon the surface of, the substance 

 upon which they feed. Their method of nutrition is similar to 

 that of the fungi. They absorb solutions of food substances 

 through their cell walls. The bacteria are "omnipresent," and 

 being very minute and capable of rapid multiplication they exist 

 in marvellous numbers. Some species multiply so rapidly as to 

 produce new individuals in a half hour. They and their spores 

 are easily carried about on floating particles of dust, on the cloth- 

 ing, the hands and other parts of the body, and by insects and 

 other animals. They exist in the mouth, in the stomach and 

 throughout the intestinal canal. Fortunately the larger number 

 of bacteria are harmless and many are beneficial, as we have seen 

 in the study of decay, nitrification, fixation of nitrogen, etc. 

 (Chapter XV). Acid fermentation caused by bacteria. Alcohol 

 is fermented by the acetic acid bacteria into acetic acid. This 

 process takes place in the making of vinegar from cider. The 

 lactic acid bacteria (Bacillus acidi lacti) cause the souring of milk. 

 Sugar and cellulose can also be fermented into butyric acid by 

 certain bacteria. 



406. The principal forms of bacteria and their methods 

 of multiplication. Many of the rod-like bacteria belong to the 

 genus Bacillus. In multiplication this rod divides into two short 

 rods which increase in length to the size of the parent rod. In 

 some species these daughter rods separate very readily, while in 



