24 THE MORPHOLOGY OF BACTERIA 
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. It is difficult to determine 
with exactitude whether this organism is a slightly oval coccus or an 
unusually short bacillus. 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 
INVOLUTION, PLEOMORPHISM 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 
temperature 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 organisms in every culture are somewhat larger or smaller than 
their fellows, appearing as occasional giants or dwarfs. These occa- 
sional giant and dwarf forms represent normal variations in size from 
the average or mean. Available evidence indicates that permanent 
giant or dwarf strains do not develop from such normal variants. 
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 alkalinity, 
by the presence of harmful chemicals, or by specific antagonistic 
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, wholly unlike the 
typical short rod-shaped bacillus. If, on the contrary, the organisms 
permanently lose some morphological or chemical characteristic, they 
are spoken of as degeneration forms. Thus, anthrax bacilli heated for 
several hours at 43° to 44° C. may lose their ability to form mature 
spores. Bacillus prodigiosus cultivated at 40° C. loses its ability to 
produce its characteristic red pigment. Many bacteria which liquefy 
gelatin gradually lose this property upon prolonged cultivation in 
artificial media. 
Pleomorphism. — By pleomorphism is meant a permanent or semi- 
permanent change in the normal form of the organism. A pleo- 
morphic organism would be one, for example, which might at one time 
resemble a bacillus, again a coccus, or even a spirillum, depending upon 
the age and growth of the organism or the fitness of the culture medium. 
This phenomenon is rarely met with among the pathogenic bacteria. 
