656 BACTERIUM 



structure. It stains evenly ; it forms no spores ; and it shows no granules. It is 

 Gram-negative, and non-acid-fast. This modal form is, however, widely departed 

 from as regards the shape and size of the individual cells. Some strains are almost 

 coccal in form, others sh ow long, sometimes filamentous bacilli. There is a tendency 

 for the cocco-bacillary or the elongated, form to predominate in any single strain, 

 but some cultures show a wide diversity in this respect. Cell length is, indeed, a 

 highly variable character in this group ; and it is possible, as Barber (1907) has 

 shown, to obtain long-celled strains of Bad. coli by simple selection of individual 

 cells for successive subculture. 



Rudimentary branching, with the formation of Y forms, followed by division 

 at each of the three points of the Y, has been described by Hort (1920), and by 

 Gardner (1925), as an occasional happening in some species of Bacterium. 



Many species are motile ; other species are non-motile. By the usual methods 

 of staining, the flagella of motile species appear to be numerous and to have a 



peritrichous arrangement. How far 

 J • / this appearance corresponds to reality 



is doubtful. Pijper (1938), for ex- 



^ . » • ample, using solar dark-ground illu- 



y ^' ^ 1 mination, finds that the typhoid 



^ ' ^ , bacillus — an organism morphologi- 



^ '■' • I ■ cally indistinguishable from J5aci. co/'i 



, -• '/ .^ ' — possesses only two flagella, at- 



^V^"" \^ - ' tached one on each side of the body 



I f ^ •<< "'^ "/ near the middle. They are broadly 



' ' coiled spiral structures, which, when 



A» -^ in action, become entwined to form 



^ tm ^\ . ** ^ ^^^E ^^i^ ^J ■^tich the organism 



* . ^ - ' propels itself through the surround- 



^ \J ' ^ y ing medium. There is evidence that 



' each of these flagella is made up of 



'" a number of extremely fine threads ; 



Fig. l39.~Bact. coli. but, according to Pijper, there are 



From 24-hours' culture on agar ( X 1000). q^\j two flagella to each organism, 



with single opposite points of attach- 

 ment. The peritrichous appearance disclosed by flagella stains is regarded as an 

 artefact. How far Pijper's conclusions will be confirmed, it is impossible to say. 

 Whether there are two flagella or several flagella is not likely to give rise to 

 confusion, so long as it is recognized that they are attached to the sides of the 

 rod and not, as in Pseudomonas, to the end (see Fig. 13, p. 31). 



Before the Salmonella and the Shigella groups were separated from the Bacterium 

 group, motility was of some importance in distinguishing, for instance, the typhoid 

 and paratyphoid bacilli, which are nearly always motile, from the dysentery bacilli, 

 which are consistently non-motile. In the present Bacterium group, however, 

 motility is of far less differential value. It is true that some members, like Bact. 

 coli, are usually motile, and that other members, like Bact. aerogenes, are usually 

 non-motile ; but the property of motility or non-motility is a characteristic of 

 the individual strain, not of the species as a whole. Moreover, motile organisms 

 may give rise to non-motile variants ; though the reverse phenomenon, of normally 

 non-motile species giving rise to motile variants, does not appear to occur. Motility 



