CHAPTER XXVI 

 BACILLI OF THE COLON-TYPHOID-DYSENTERY GROUP 



The bacilli belonging to this group of microorganisms, while present- 

 ing great differences in their pathogenic characteristics, possess many 

 points of morphological and biological similarity which have made their 

 differentiation extremely difHcult. Among pathogenic bacilli, they are 

 probably the ones most commonly encountered and because of the fact 

 that some of them are specifically pathogenic, while others are essen- 

 tially saprophytic and are pathogenic only under exceptional conditions, 

 the necessity of accurate differentiation is a daily occurrence in bacteri- 

 ological laboratories. It has been through the study of this group par- 

 ticularly that many of the modern differential methods of bacteriology 

 have been developed. 



The group includes the colon bacillus and its allies, the typhoid 

 bacillus, the paratyphoid organisms, the several varieties of dysentery 

 bacillus and numerous closely related species, and Bacillus fecalis alka- 

 ligenes. Closely related to the group though not properly within it, 

 are Bacillus lactis aerogenes, bacilli of the Friedlander or mucosus 

 capsulatus group, and a number of less important subdivisions of this 

 last group. 



All bacilli of the group possess morphological characteristics which, 

 although exhibiting slight differences, are insufBcient to permit accurate 

 morphological diagnosis. They are none of them spore-bearing. Stained 

 by Gram's method they are decolorized. 



Cultivated upon artificial media, they grow readily both at room and 

 at incubator temperatures. None of them liquefies gelatin. Though 

 showing, often, distinct differences in the speed and luxuriance of growth 

 upon ordinary media, these differences are, nevertheless, too slight to 

 become the basis of differentiation. 



In order to distinguish between the individual members of this 

 group, therefore, we are forced to a careful biological and cultural 

 study. This is carried out by the observation of the cultural character- 

 istics upon special media and by the study of serum reactions in speci- 

 fic immune sera. Our mainstays in the accurate differentiation of these 



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