CHAPTER 28 

 BACTERIUM 



Definition. — Bacterium. 



Gram-negative, non-sporing rods : often motile, with peritrichate flagella. 

 Some species capsulated. Easily cultivable on ordinary laboratory media. Aerobic 

 and facultatively anaerobic. All species ferment dextrose with the formation of 

 acid, or acid and gas. Many species are active fermenters of a wide range of 

 carbohydrates and aUied substrates. Typically intestinal parasites of man and 

 animals, though some species may occur in other parts of the body, on plants, or 

 in the soil. Many species are pathogenic. 



Tjrpe species. Bacterium coli. 



Classification and Nomenclature. 



In the past the generic term Bacterium has been used to comprise a broad 

 group of Gram-negative, non-sporing rods occurring in the intestinal canal of 

 man and animals and on plants, and living either a saprophytic, commensal, or 

 pathogenic existence. It was early realized that the members which were patho- 

 genic to man and animals differed from most of the non-pathogenic forms in failing 

 to ferment lactose. Of recent years the non-lactose-fermenting group has been 

 subdivided, mainly on the basis of biochemical and antigenic characters, into a 

 number of sub-groups, which have been given the generic names of Eberthella, 

 Salmonella and Shigella. Of these, Eberthella, which comprises the typhoid bacillus, 

 is so closely related to the Salmonella group, that we can see no useful purpose 

 in maintaining its separate identity. We shall therefore deal in the following 

 chapters with the Salmonella group, which comprises the typhoid, paratyphoid, 

 and food-poisoning organisms, and the Shigella group containing the dysentery 

 bacilli. A residue of strains fermenting lactose late, weakly, or not at all, and 

 usually regarded as non-pathogenic, is classified in the relatively indeterminate 

 group of paracolon bacilli and will be considered in the j^resent chapter. 



The classification and nomenclature of the lactose-fermenting organisms are 

 subject to wide variation in opinion. On the one hand, there is a school, led 

 by American workers (see Bergey et al. 1939), who would do away completely 

 with the genus Bacterium, and substitute for it a number of genera — Escherichia, 

 Aerobacter, and Klebsiella — to include organisms of animal origin, and a single 

 genus — Erwinia — to comprise the plant pathogens. On the other hand, there is 

 a school, represented largely by workers in Great Britain but not without con- 

 siderable support in the United States, which adopts for the present a conservative 

 attitude and prefers to include all these organisms in a single genus — Bacterium. 

 With this latter school we would identify ourselves. 



In the definition of the genera mentioned above, reliance is placed for differ- 

 ential purposes largely on habitat, biochemical characters, and pathogenicity. 



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