CLASSIFICATION AND NOMENCLATURE 655 



Of these, neither habitat nor pathogenicity, though useful adjuncts, can be regarded 

 as satisfactory basal criteria for classification. The separation therefore of the 

 group Erwinia from other coliforin organisms merely on the basis of plant patho- 

 genicity is not sound taxonomical practice. If this property was closely associated 

 with some more valid criterion, there would be something to be said for recog- 

 nizing the Erwinia group ; but though Elrod (1942) maintains that the secretion 

 of an enzyme, protopectinase, which is responsible in plants for the production 

 of soft rot, is characteristic of the Erwinia group, it is clear that this is merely 

 another way of saying that these organisms are pathogenic to plants. Elrod's 

 own observations on the biochemical characters of the plant pathogens support 

 those of Dowson (1939) in showing that these organisms cannot be distinguished 

 satisfactorily from coliform organisms of animal origin. Until some more stable 

 differential character is found, we prefer, therefore, to regard the coliform organisms 

 of plant and animal origin as belonging to a single group. 



The division of the organisms from animal sources into Escherichia, Aerobacter, 

 and Klebsiella is, in our opinion, hardly more fortunate. The first two groups are 

 morphologically and culturally indistinguishable ; and their differences in bio- 

 chemical behaviour, though worthy of specific recognition, are not of sufficient 

 constancy or importance to serve as criteria for generic differentiation. Klebsiella, 

 which comprises capsulated organisms found in the respiratory tract, is perhaps 

 even less justified. Practically all coliform bacilli seem to be capable of forming 

 capsules under favourable conditions ; and in our experience there is no way of 

 distinguishing a non-motile capsulated Gram-negative bacillus in the respiratory 

 tract from a similar organism in the intestinal tract. Neither habitat nor capsule 

 formation is sufficiently constiant or distinctive to justify the recognition of the 

 genus Klebsiella. 



We would plead, therefore, for the maintenance of the Bacterium group as 

 a convenient repository for a wide range of Gram-negative non-sporing bacilli 

 that cannot be classed at present in either the Salmonella or Shigella groups. As 

 knowledge increases, it is probable that further genera will be split off. But 

 international agreement is essential if the new genera are to receive universal 

 recognition ; without it they will merely add to the existing confusion. 



The strict taxonomist is worried by the fact that the type species. Bacterium 

 triloculare Ehrenberg, 1828, is no longer recognizable, and that the genus Bacterium 

 is therefore invalid. On the other hand, the organism that has for years been 

 regarded by bacteriologists as typical of this genus is the common coliform organism 

 of the intestine, Bacterium coli. We see no reason why the genus should not be 

 re-defined, with this organism as the type species, so as to include the numerous 

 Gram-negative non-sporing bacilli that cannot be assigned to other recognized 

 genera. Whether this genus is admitted as valid by the taxonomists, or is accepted 

 temporarily for purposes of convenience (see Breed and Conn 1936), is a matter 

 more of academic than practical importance. Our tentative definition of this genus 

 follows closely that given by the American Committee (Winslow et al. 1917, 1920). 



Morphology. — Neither the shape, size, structure or arrangement of the bacterial 

 cells, nor the appearances presented by cultures on the ordinary solid or liquid 

 media, afford any adequate criteria for the differentiation of species within this group. 



The modal form of the individual cell is that of a bacillus, 2 to 3 /^ in length 

 and 0-6 /i in breadth, with parallel sides and rounded ends (see Fig. 139). By the 

 usual methods of examination the cell appears to be almost devoid of internal 



