CHAPTER II 



GENERAL MORPHOLOGY, REPRODUCTION, AND CHEMICAL 

 AND PHYSICAL PROPERTIES OF THE BACTERIA 



BACTERIA are exceedingly minute unicellular organisms which may 

 occur perfectly free and singular, or in larger or smaller aggregations, 

 thus forming multicellular groups or colonies, the individuals of which 

 are, however, physiologically independent. 



The cells themselves have a number of basic or ground shapes which 

 may be roughly considered in three main classes : The cocci or spheres, 

 the bacilli or straight rods, and the spirilla or curved rod forms. 



The cocci are, when fully developed and free, perfectly spherical. 

 When two or more are in apposition, they may be slightly flattened 

 along the tangential surface, giving an oval appearance. 



The bacilli, or rod-shaped forms, consist of elongated cells whose 

 long diameter may be from two to ten times as great as their width, 

 with ends squarely cut off, as in the case of bacillus anthracis, or gently 

 rounded as in the case of the typhoid bacillus. 



The spirilla may vary from small comma-shaped microorganisms, 

 containing but a single curve, to longer or more sinuous forms which 

 may roughly be compared to a corkscrew, being made up of five, six 

 or more curves. The turns in the typical microorganisms of this class 

 are always in three planes and are spiral rather than simply curved. 



Among the known microorganisms, the bacilli by far outnumber 

 other forms, and are probably the most common variety of bacteria in 

 existence. Many variations from these fundamental types may occur 

 even under normal conditions, but contrary to earlier opinions it is 

 now positively known that cocci regularly reproduce cocci, bacilli 

 bacilli, and spirilla spirilla, there being, as far as we know, no. mutation 

 from one form into another. 



The size of bacteria is subject to considerable variation. Cocci may 

 vary from .15 ;u to 2. ju in diameter. The average size of the ordinary 

 pus coccus varies from .8 p. to 1.2 /x in diameter. Fischer has given a 

 graphic illustration of the size of a staphyloccocus by calculating that 

 one billion micrococci could easily be contained in a drop of water hav- 



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