CHAPTER VII. 



VITAL PHENOMENA OF BACTERIA MOTION, HEAT, AND LIGHT 

 PRODUCTION CHEMICAL EFFECTS. 



Motility. Many bacteria when examined under the microscope are 

 seen to exhibit active movements in fluids. The movements are of a 

 varying character, being described as rotary, undulatory, sinuous, etc. 

 At one time they may be slow and sluggish, at another so rapid that 

 any detailed observation is impossible. Some bacteria are very active 

 in their movements, different individuals progressing rapidly in differ- 

 ent directions, while with many it is difficult to say positively whether 

 there is any actual motility or whether the organism shows only molec- 

 ular movements so-called "Brownian" movements a dancing, trem- 

 bling motion possessed by all finely divided organic particles. Very 

 young cultures in neutral nutrient bouillon should be examined at a 

 temperature suitable for their best growth. Not all species of bacteria 

 which have flagella exhibit at all times spontaneous movements; such 

 movements may be absent in certain culture media and at too low or 

 too high temperatures, or of either an insufficient or excessive supply 

 of oxygen. 



Chemotaxis. Some chemical substances seem to exert a peculiar 

 attraction for bacteria, known as positive chemotaxis, while others repel 

 them negative chemotaxis. Moreover, all varieties are not affected 

 alike, for the same substances may exert on some bacteria an attraction 

 and on others a repulsion. Oxygen, for example, attracts aerobic and 

 repels anaerobic bacteria, and for each variety there is a definite propor- 

 tion of oxygen, which most strongly attracts. The chemotactic proper- 

 ties of substances are tested by pushing the open end of a fine capillary 

 tube, filled with the substance to be tested, into the edge of a drop of 

 culture fluid containing bacteria and examining the hanging drop 

 under the microscope. We are able thus to watch the action of the 

 bacteria and note whether they crowd about the tube opening or are 

 repelled from it. Among substances showing positive chemotaxis for 

 nearly all bacteria are peptone and urea, while among those showing 

 negative chemotaxis are alcohol and many of the metallic salts. 



Production of Light. Bacteria which have the property of emitting 

 light are quite widely distributed in nature, particularly in media rich 

 in salt, as in sea-water, salt fish, etc. Many of these, chiefly bacilli and 

 spirilla, have been accurately studied. The emission of light is a prop- 

 erty of the living protoplasm of the bacteria, and is not usually due to 

 the oxidation of any photogenic substance given off by them; at least 

 only in two instances has such substance been claimed to have been 



