30 



MORPHOLOGY 





rigid cell-membrane from the space presumably occupied by the capsule in electron 

 micrographs, and the low density to the electron beam of the capsules themselves 

 (Friihbrodt and Ruska 1940, Mudd, Heinmets and Anderson 1943), together with 

 the known delimitation of the cytoplasmic membrane inside the cell wall, suggest 

 that the hypothesis of a " secreted " capsule is the more likely to be vaUd. We 

 may note that we have, within recent years, acquired a considerable knowledge 

 of the chemical constitution of bacterial capsules. The capsules of the different 

 types of pneumococci are for instance composed of complex polysaccharides (see 

 p. 279), while the capsule of the anthrax bacillus appears to be composed of a 

 polypeptide material (see p. 281). 



Whether the capacity to form a capsule is really confined to those species 



that are typically capsulated is very 

 doubtful. Many observers have 

 described capsule-formation by nor- 

 mally non-capsulated forms under 

 particular environmental conditions ; 

 and mucoid variants of normally non- 

 mucoid species are of relatively com- 

 mon occurrence (see Chapter 9). 

 The absence of a detectable capsule 

 does not exclude the existence of 

 thin capsules distinct from the cell 

 wall. For example, in a cocco- 

 bacillus as small as Br. melitensis, 

 the lipopolysaccharide material com- 

 prising 10 per cent, of the whole 

 bacillus, if confined exclusively to 

 an external covering, would form a 

 layer less than 20 m/^ thick (Miles 

 and Pirie 1939). 



Flagella. — A large number of 

 bacteria, including a few coccal forms, many bacilli, and most known spirilla and 

 vibrios, are more or less actively motile by means of flagella. These flagella are 

 long, thread-like processes, arranged in various ways on the bacterial cell. They 

 vary greatly in length, but are often longer than the organism to which they are 

 attached. They are extremely slender (Fig. 14a); recent measurements of the 

 flagella of Proteus vulgaris and Salm. paratyphi B under the electron microscope 

 indicate a thickness of 20 to 50 nifi, at any rate of the electron-opaque part of 

 the structure (Piekarski and Ruska 1939). Flagella were first effectively demon- 

 strated by Loeffler (1890). 



According to the observations of Trenkmann (1890), Ellis (1902-03), Fuhrmann 

 (1910) and Meyer (1912), the central portion of the flagellar thread passes through 

 the cell-membrane and is in direct connection with the cytoplasm, and Fuhrmann 

 believes that the proximal extremity is connected with a granule of chromatin, 

 which would then be analogous to the blepharoplast of protozoan flagellates. 

 This conclusion is adversely criticized by Zettnow (1918). Whether the optical 

 methods at present available allow any decision on such points as these seems 

 doubtful. 



The arrangement of the flagella on the bacterial cell may take one of four forms. 



Fig. 12. — Zopfius zenkeri. 

 Stained to show flagella. ( x 1 : 1200). 



