12 SULPHUR BACTERIA 



Pleomorphism. 



By pleomorphism is meant the capacity of an organism to 

 assume diverse forms. A culture of bacteria may contain 

 several variant forms simultaneously, or these may be formed 

 in succession. Bacteria in general are noteworthy for the 

 ease with which changes in form occur in them. The ease 

 with which certain physiological changes bring about changes 

 in the external appearance of bacterial cells is a matter of 

 common knowledge. Probably no other organisms possess 

 greater plasticity in this respect. Unless, for example, a 

 wide range is given to the dimensions of a bacillus, a state- 

 ment of its length or its thickness has little value for 

 purposes of identification. It is a matter of common ob- 

 servation, too, that, under certain circumstances the rate of 

 growth in size and the rate of division of bacteria do not 

 run parallel. The rate of each is dependent on circumstances 

 of which at present we have very little knowledge. If the 

 rate of division is accelerated the individuals of successive 

 generations become progressively smaller. Under certain cir- 

 cumstances the rate of division of Crenothrix polyspora, one 

 of the iron bacteria, may so far exceed the rate of its growth 

 that the individuals may be so reduced in size that they are 

 barely visible with the highest powers of the microscope. 

 Again, the change that takes place in the size of the individuals 

 of a bacterial culture after frequent subculture is well known. 

 Further, every living bacterial cell is at all times covered by 

 a delicate covering of slime formed by transformation of the 

 outermost layer of the membrane. Given certain conditions, 

 this slime formation increases in intensity, and the motility of the 

 cell is retarded, and may even cease altogether. Conversely, 

 by the elimination of excessive slime formation non-motile 

 forms may be made to assume motility.* Other instances 

 could be adduced in illustration of the morphological plas- 

 ticity of bacteria. Over 2000 species of the genus bacillus 



* By subculturing at the very earliest sign of growth on the surface of 

 agar-slope cultures, the motility of organisms may be considerably en- 

 hanced, and even forms regarded as non-motile may be made to assume 

 motility (Ellis (2-3)). 



