CHAPTEE 35 

 BACILLUS 



AEROBIC SPORE-BEARING BACILLI 



Definition.^ — Bncillus. 



Aerobic, spore-bearing rods, usually Gram-positive. Often occur in long 

 threads, and form rhizoid colonies. Form of rod, as a rule, not greatly changed 

 at sporulation. Liquefy gelatin. Mostly saprophytes. 



Type species. Bacillus subtilis. 



Introductory 



The aerobic spore-bearing Bacillus forms one division of the i3imi\y Bacillacece 

 while the anaerobic spore-bearing Clostridmn forms the other. As many of the 

 organisms in the former division are widely distributed, being found in air, soil, 

 water, milk, dust, fish- meal, wool, faeces, and other situations, it is not unnatural 

 that they were among the first micro-organisms to be studied ; but as there are 

 large numbers of different species, almost all of which are devoid of pathogenic 

 action, it follows from the way in which bacteriological investigation has been 

 directed mainly along medical, veterinary, or agricultural lines, that our know- 

 ledge of these organisms is far from complete. 



Some authors have separated the group into two divisions, the one containing 

 B. anthracis and the closely allied pseudoanthrax bacillus, the other containing 

 B. subtilis and other saprophytic forms. As, however, B. subtilis may itself be 

 confused with B. anthracis, this division is of no real value. We shall treat all 

 the members as belonging to one single group — the group Bacillus — reserving 

 the term " pseudoanthrax bacillus " as an inexact but convenient designation 

 for any organism of the aerobic spore-bearing group that is liable to be confused 

 with the true B. anthracis. 



There is a large group of thermophilic bacilli found in milk, manure, and other 

 situations which grow best at temperatures round about 60° C. We do not propose 

 to describe these organisms, but references to them will be found in a paper by 

 Wilson and his colleagues (1935). Not all of them belong to the Bacillus group ; 

 many species appear to be streptococci or non-sporing rods. 



Group Characteristics 



Morphology and Staining. — Members of this group are rod-shaped organisms 

 varying in size from about 3 /t X 04 /< to 9 // X 2 fi. The sides are parallel, the 

 axis straight or slightly curved, the ends either truncated, as in B. anthracis, or 

 more usually convex. Their arrangement varies considerably ; though single and 

 diplobacillary forms predominate, they may be arranged in chains, often of con- 



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