CHAPTER IV 



SPIROCHAETES 



Forms Closely Related to Bacteria 



In his great work on Die Infusionsthierschen als vollkommne 

 Organismen {Infusoria as Complete Organisms) , published 

 in 1838, a famous German microscopist, Christian Gottfried 

 Ehrenberg, described the genus of bacteria known as spirillum 

 as a rigid, corkscrew-like organism, and the genus known as 

 spirochaeta as a flexible, undulating, spirally wound organism. 

 His type species of the latter — Spirochaeta plicatilis — is a sinu- 

 ous, somewhat snake-like form found among decomposing matter 

 in pools and ditches. The difference between these two genera — 

 spirillum and spirochaeta — is slight at best according to Ehren- 

 berg's characterizations. In many of the species discovered since, 

 it is difficult to decide whether an individual is rigid or not. In 

 any case, the spirochaetes, as the group is called, are closely 

 related to spirillum and through this genus to the bacteria, but 

 are usually treated as a distinct group of minute organisms. Some 

 authorities regard them as related to the flagellated protozoa,* 

 but the majority of observers consider them as intermediate 

 forms between bacteria and protozoa. 



Their Importance 



No matter how scientists may differ as to their classification, 

 spirochaetes are organisms which cannot be neglected, for they 

 are responsible for some of the most loathsome diseases known 

 to man. The majority of. species, however, are probably harm- 

 less and live as commensals 1 " — harmless or perhaps even useful 

 inhabitants — in the digestive tracts of all animals, or as free 

 organisms in natural waters. They differ sufficiently from each 

 other to make it possible to classify them to some extent as be- 



* See "The Animal World," page 39, in this Scries. 



t Commensal (from two Latin words, meaning "together at table") ; in biology 

 an organism, not truly parasitic, which lives in, with, or on, another, partaking 

 usually of the same food ; both species may be benefited by the association. 



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