38 MORPHOLOGY OF BACTERIA 



Spirilla. The third group is the spirilla (sing, spirillum) and may 

 be likened unto a corkscrew. The spiral may be loosely or tightly 

 coiled or there may be one, two, or many coils. At times the curve 

 may be so slight that the organism viewed under the microscope 

 appears " comma-shaped. ' ' 



More bacilli are known than cocci and more cocci than spirilla. 

 Migula enumerates 833 bacilli, 343 cocci, and 96 spirilla, a total of 

 1272. Other workers have tabulated more with a similar propor- 

 tional distribution among the various groups. 



Gradations. The difference between these fundamental types is 

 at times very slight. In fact the cocci often merge into the bacilli 

 and the bacilli into the spirilla. It is often difficult accurately to 

 distinguish between the various groups, as is exemplified by the 

 fact that at times B. prodigious has been described by one investi- 

 gator as a coccus and at another time by a different worker as a 

 bacillus. This same condition holds for the pneumonia germ and the 

 one causing pear blight, whereas the cholera organism has been 

 described both as a bacillus and a spirillum. 



Pleiomorphism. By pleiomorphism is meant a permanent or 

 semipermanent change in the normal form of the organism. The 

 organism may at one time represent a coccus, at another a bacillus, 

 and at still another a spirillum. This led the early writers to believe 

 that there was a mutability of species. The condition is especially 

 likely to occur among some soil organism and much light has been 

 thrown on the subject by Lohnis who finds the life history of bacteria 

 to be only slightly less complex than that of other organisms. 



FIG. 7. Involution forms from bacilli. (From Fliigge.) 



Involution Forms. Although the form of bacteria is quite constant 

 under normal conditions, yet there is a tendency with many organ- 

 isms, especially when grown for some time on artificial media, to 

 show abnormal or bizarre forms. Such organisms are known as 

 involution forms. Some of the rod-shaped organisms may appear 



