Microorganisms in Relation to Man 577 



Bacilli move at different rates of speed, some very slowly and 

 others very rapidly. The swiftly moving forms may travel at 

 a rate equivalent to that of an automobile making thirty miles 

 an hour. It should also be noted that a bacterium has about as 

 much free moving space in a drop of water as a fish in a good- 

 sized lake. The manner of locomotion varies with different 

 species, some traveling in straight courses, others following 

 tortuous routes. 



The bent rod-shaped bacteria are generally called spirilla, for 

 they assume a more or less spiral form. A very slightly bent 

 form is the vibrio of cholera (Fig. 171 /), and a very pronounced 

 spiral form (g) is found in the mucus or tartar of the teeth. Both 

 of the cylindrical forms appear separately or grouped in pairs 

 joined end to end, or in long bead-like strings. 



Size of bacteria. Bacteria are the smallest of plants, but 

 probably not the smallest living organisms. Most bacteria 

 are microscopic in size, but a few varieties are ultra-microscopic, 

 revealed only through their effects or products. Bacteria 

 range in size from T-^-O- of an inch in diameter to -grroinr inch. 



The usual practice is to speak of their size in terms of the 

 metric system, as T Vo f a millimeter (called one micron or 

 I p). This is equivalent to about syfonr f an inch. The 

 size may be appreciated by a concrete comparison. To equal 

 the thickness of ordinary book paper, it would require three to 

 four thousand bacteria of medium diameter, placed side by 

 side. Some bacteria are much smaller, as the influenza bacillus, 

 which is about i-grinnr f an mcn (-2 micron) and has a length- 

 wise diameter of TUTTF mcn (-5 micron). 



Reproduction. The rate and manner of reproduction of 

 bacteria are of practical importance. The usual manner of 

 reproduction is vegetative, or by simple division. Some types 

 also reproduce by spores, as may be determined by the external 

 conditions. Bacteria on the average reach maturity in favor- 

 able conditions in about half an hour. As soon as mature, each 

 begins to divide itself into two sections and quickly there are 

 2p 



