BACTERIA 17 



example Streptococcus pyogenes (Fig. 7), one of the pus-forming bac- 

 teria ; others form plates or masses of cells. Another class includes 

 the rod-shaped bacteria, many of which form chain-like colonies ; 

 Bacillus subtilis is a type of this group. Those of the third class are 

 spiral or corkscrew-shaped. The spiral may be very short, as in the 

 " comma bacillus " that causes Asiatic cholera (Fig. 8, A) • or it may 

 be composed of several coils, as in the species of Spirillum, one of which 

 (Fig. 8, D) is' the germ of recurrent fever. Bacterial cells differ in size 

 as well as in form, although they are all very small ; in general they are 

 the smallest known living organisms. One of the smallest is xTiVinr 

 inch long and j2 juTns ™ch wide. One of the largest is yjsj to jjj inch 

 long and ^-i^-g to ^ Ac ^<^^ wide. It is estimated that it would require 

 16,800,000,000,000 baciUi of average size to weigh an ounce. 



28. The Activities of Bacteria. — We have seen that in the 

 process of obtaining food for their own use, bacteria by means 

 of enzyms change the nature of organic substances such as 

 meat and hay and make them soluble. One of the most 

 striking, and from our point of view one of the most important, 

 characteristics of bacteria is their power of thus changing 

 the chemical nature of substances with which they come in 

 contact. All such changes are, of course, of direct benefit to 

 the bacteria ; but they are also in many cases of great conse- 

 quence to other plants and to animals. If, as is very com- 

 monly the case, the change is a breaking down of organic 

 substances — that is, of materials which have once been 

 parts of the living bodies of plants or animals — the process 

 is called decay. Not all forms of decay are due to bacteria ; 

 but bacteria are more largely concerned in decay and produce 

 more kinds than all other sorts of organisms taken together. 

 If in the process of decay bubbles of gas are formed, we often 

 speak of the process as fermentation ; if an unpleasant odor 

 arises, we call it putrefaction. 



Bacteria are often thought of as a group of useless and 

 harmful creatures ; as a matter of fact, only a small number 

 of species are injurious to man, and a much greater propor- 

 tion are useful in one way or another. Processes of decay, 

 due largely to bacteria, are constantly destroying the dead 



