BACTERIA 



17 



path, either alone — a bacillus — or in chains of bacilli somewhat 

 like chains of miniature sausages. A curious spiral form — spiril- 

 lum — like an animated corkscrew, darts about, now forward, 

 now back, and turns on its long axis at the same time (Fig. 7). 

 These small living things — cocci, bacilli, spirilla — represent 



Fig. 7— SPIRAL BACTERIA 

 (SPIRILLUM UNDULANS) 



Two individuals from stagnant water. 

 A so-called flagellum may be seen at 

 each end, and granules at intervals 

 down the body; note the absence of 

 a nucleus 



From a photomicrograph by the author 

 Magnification, 755 



the living world of bacteria, creatures which can not be regarded 

 as animals nor as plants, but which play an amazing role in 

 nature. The science of bacteriology, developed in the last third 

 of the nineteenth century on the foundations laid by Louis 

 Pasteur (1822-1895) in France and by Robert Koch (1843- 

 1910) in Germany, deals exclusively with these organisms. The 

 structure and form of the hundreds of types that are now known 

 differ little from the cocci, bacilli, or spirilla. All bacteria re- 

 semble one, or some slight variation of one, of these three types. 

 The individual unit counts for very little to the student of bac- 

 teriology. The bacteriologist is concerned rather with the mode 

 of its growth, the substances on which it can flourish, and with the 

 effects of its activities. It is by these distinguishing character- 

 istics that he describes bacteria. For the most part a single 

 bacterium is too small to be studied effectively even with the 

 highest power of a microscope. Culture methods, or means of 

 rearing bacteria in mass, are, therefore, absolutely essential for 

 the study of bacteria. 



