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CHAPTER III. 

 BACTERIA. 



BACTERIA occur in every shape, from the smallest specks or 

 spheres to green algae - like filaments ; and they are found in 

 nearly all possible localities, under the most varied conditions. 

 According to their action, a distinction is made between 

 pathogenic, zymogenic, and chromogenic bacteria, or those 

 that produce disease, fermentation, and coloration respectively. 



Our first knowledge of these living forms was obtained by 

 placing small quantities of different substances under the 

 microscope, and examining them with high powers. In 

 putrefying meat minute spherical bodies were found, which 

 clearly multiplied by division ; in sour milk short rods occurred, 

 and in decomposing vegetable matter large spherical bodies 

 and long fine threads ; in the mucus of teeth very fine spiral 

 threads were found. Thus it was convenient provisionally 

 to retain these forms, and to describe them as independent 

 species. Credit is due to Cohn for the first systematic classi- 

 fication of bacteria. 



Bacteria always consist of single cells. In their simplest 

 form they occur as spherical bodies (coccus, Fig. 16, a) of varying 

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

 the strongest powers, and only give evidence of their existence 

 as organisms during propagation by division. If the coccus 

 divides in one plane, Diplo- and Streptococcus are produced 

 (b) (c). By division in two planes, the Micrococcus (b) is ob- 

 tained, and by division in three directions the Sarcina form.* 

 If the cells assume a cylindrical form we have bacteria (e), 



* In the Sarcina forms that occur in beer, the division is incomplete in all 

 three directions, but appears to vary, so that an irregular piling up of spherical 

 bacteria takes place, or else a marked displacement of single cells is found. Two 

 spherical bacteria are often found strung together. All such conglomerates of 

 cells are surrounded by a gelatinous envelope, the development of which is de- 

 pendent upon the nutritive conditions. 



