42 VARIATIONS IN MORPHOLOGY [OH. iv 



B. MORPHOLOGICAL VARIATIONS IN INDIVIDUAL 

 ORGANISMS. 



Morphological variations in individual bacteria may occur 

 as normal phases in its life history, or they may develop in 

 response to changes in their environment. 



I. PLEOMORPHISM IN THE LIFE HISTORY. 



While many bacteria are only known under certain forms 

 and are regarded as a micrococcus, a bacterium, a bacillus or 

 a spirillum, others are known which in the course of their 

 development pass through several such forms and are called 

 "pleomorphic." 



The non-pathogenic Bacterium rubescens already men- 

 tioned (Ray Lankester, 1873) affords a good example of the 

 various phases an individual organism may pass through in its 

 life history forms described as spherical, biscuit-shaped, rod- 

 like, filamentous and acicular succeeding each other in turn 1 . 



Amongst pathogenic organisms that of cholera affords a 

 good example, the characteristic comma-shaped vibrio or 

 spirillum giving place to a coccus or to a straight thread 

 (Haffkine, 1895). 



In old cultures of the meningococcus bacillary forms make 

 their appearance (Arkwright, 1909). Young cultures of B. coli 

 show not only typical bacilli but also small oval rods and tiny 

 coccus-like forms (Gordon, 1897). In very young cultures of 

 B. diphtheriae "solid" types largely predominate, but in a few 

 hours these give place to "granular" types (Denny, 1903). 



The variations in morphology displayed by B. diphtheriae 

 are many of them so characteristic as to be of value in the 

 identification of the organism, and with this object have 

 been classified by Westbrook, Wilson and McDaniel (1900). 

 Similarly Gordon (1900-1) observed that the streptococcus 

 of scarlatina was characterised by a tendency to take the form 



1 Miss M. C. W. Young has recently reported some observations of great 

 interest in this connection, revealing a similar pleomorphism in bacteria which 

 extended, in her experiments, over a cycle of 14 days. (Brit. Med. Journ. 1914, 

 11, p. 710.) 



