ABNORMAL FORMS 23 



to the slightly curved rods which are the characteristic morphologic 

 forms. There are a few bacteria in which the morphology is still a 

 subject of controversy. For example, Micrococcus melitensis is called 

 Bacillus melitensis by some observers. The vast majority of bacteria, 

 however, are easily referable to their proper morphological type by 

 simple inspection under the microscope. 



B. ABNORMAL FORMS: VARIATION, DEGENERATION AND IN- 

 VOLUTION, PLEIOMORPHISM AND BRANCHING. 



Variation. The composition of the medium in which bacteria are 

 growing, the age of the culture, and to a limited degree even the tem- 

 perature of incubation influence somewhat the average size of bacteria. 

 Given constant conditions, however, bacteria growing in a favorable 

 environment exhibit constancy of form and size, although a few organ- 

 isms in every culture are somewhat larger or smaller than their fellows, 

 appearing as occasional giants or dwarfs. These occasional giant and 

 dwarf forms represent normal variations in size from the average or 

 mean. 



Degeneration and Involution. Bacteria growing in an unfavorable 

 environment, brought about by the accumulation of waste products, 

 by undue changes in reaction resulting in excessive acidity or alka- 

 linity, by the presence of harmful chemicals, or by specific antago- 

 nistic substances, may gradually assume atypical shapes, probably the 

 direct result of these harmful influences. These atypical organisms 

 may exhibit little or no resemblance to the normal organism, either in 

 form or size; they may or may not develop into normal organisms when 

 they are placed again in a favorable environment. If the change is 

 a morphological one, the atypical organisms are designated involution 

 forms : thus, plague bacilli grown in nutrient agar containing 3 per cent, 

 common salt appear as swollen, balloon-like bodies, notably unlike 

 the typical short rod-shaped bacillus. If, on the contrary, the 

 organisms permanently lose some morphological or chemical charac- 

 teristic, they are spoken of as degeneration forms. Thus, anthrax 

 bacilli heated for several hours at 43 to 44 C. lose their ability to 

 form mature spores. 



Pleiomorphism. By pleiomorphism is meant a permanent or semi- 

 permanent change in the normal form of the organism. A pleio- 

 morphic organism would be one which might at one time resemble 

 a bacillus, again a coccus, or even a spirillum, depending upon the age 



