CHAPTER III. 

 BACTERIA. 



more our knowledge of these micro-organisms is 

 extended, the more difficult it is to give a general defini- 

 tion of them. They are known in all forms, from the smallest 

 specks or spheres to green, alga-like filaments ; and they occur 

 in nearly all possible localities, under the most varied conditions. 

 According to their action a distinction is made between patho- 

 genic, zymogenic, and chromogenic bacteria, or such as produce 

 disease, fermentation, or coloration respectively. 



The first knowledge of these forms was obtained by 

 placing small quantities of the different substances under the 

 microscope and examining them with high powers. In putre- 

 fying meat very small spherical bodies were found, which 

 clearly multiplied by successive divisions ; in sour milk short, 

 rod-like bodies occurred; and in putrefying vegetable matter 

 large spherical bodies and long, fine, thread-like forms ; in 

 saliva, on the contrary, very fine, spirally-twisted threads were 

 found. On this account it was convenient to provisionally 

 retain these forms, and to describe them as so many distinct 

 species. COHN especially has earned credit in this respect, 

 since to him is due the first systematic classification of 

 bacteria. 



We will first consider the various forms and individuals 

 somewhat more closely. As stated above, the bacteria in 

 their simplest form occur as spherical bodies of different 

 sizes, often so small that they can only just be seen even with 

 the strongest powers, and only give evidence of their existence 



