MORPHOLOGY 859 



organism is found in virgin soil taken far from human or animal habitations renders 

 this view improbable. It woiild seem more likely that the primary habitat of the 

 majority of the anaerobes is the soil ; that they are ingested frequently with 

 vegetables and fruit ; and that some of them are able to adapt themselves tem- 

 porarily or permanently to a life in the intestinal canal. 



Their presence in soil and faeces accounts for their frequent appearance in dust, 

 milk and sewage. In spite of the fact that they lead a saprophytic existence, 

 several of these species are causally related to well-recognized diseases in man and 

 animals. 



Morphology.- — The anaerobes are endowed with a pleomorphism that renders 

 their identification on a morphological basis very difficult, and often frankly im- 

 possible. Not only may an organism assume different shapes under changing 

 environmental conditions, but under one and the same set of conditions it may 

 present very difierent forms. 



Like the aerobic spore-bearing bacilli they are large, rod-shaped organisms. 

 In length they vary from about 3 /x to 7 or 8 //, but long filamentous forms are 

 quite common. Their breadth varies from about 0'4 to 1*2 [x. The vegetative 

 bacilli are straight or curved, their sides are parallel, and their ends rounded or 

 somewhat truncated. Most are arranged singly, but some occur in pairs or in 

 chains, others in bundles the members of which are arranged parallel to each other. 

 Irregular forms include navicular or boat-shaped organisms ; citron forms shaped 

 like a lemon with a small knob at each end ; large, swollen, non-sporing rods or 

 " orgonts " ; snake-like filaments ; deeply stained bulb-like types ; and a great 

 variety of so-called involution forms varying both in shape and in depth of staining. 

 Autolysis frequently sets in with the commencement of sporulation so that shadow 

 forms are numerous, particularly in certain species. 



Sporulation is common to all members, but there is considerable variation in 

 the readiness with which it occurs. CI. sporogenes, for example, spores readily 

 on all media ; CI. welchii only in media free from a fermentable carbohydrate, and 

 then inconstantly. All the pathogenic members are able to form spores in the 

 animal body, though CI. welchii does so rarely. 



It has been customary to classify the anaerobes according to the shape of the 

 spore and the position in the rod at which it appears. Thus we have (1) those 

 with an equatorial or subterminal spore ; (2) those with an oval terminal spore ; 

 and (3) those with a spherical terminal spore. This division is useful for certain 

 purposes, but it must not be used too rigidly. It is common, for instance, to find 

 organisms that usually form subterminal spores giving rise to spores that are 

 strictly terminal. The distinction between a spherical and an oval terminal 

 spore may also be a matter of the utmost nicety. 



The spores of most members are wider than the vegetative bacilli ; they there- 

 fore confer on the organism a distinctive appearance according to the position in 

 which they arise. If they are formed at the equator the Clostridium is spindle- 

 shaped ; if subterminally club-shaped ; with an oval terminal spore the organism 

 may look like a tennis racket ; with a spherical terminal spore like a drum-stick. 



With the exception of CI. welchii, all the members are motile, by peritrichate 

 flagella. Motility, however, is often difficult to demonstrate, especially in artificial 

 cultures and in strains that have been subcultured for some time. Young cultures 

 in broth or cooked meat medium, not more than 6 to 24 hours old, are the most 

 suitable for examination. If these are negative, the organisms should be examined 



