CAPSULES 29 



According to Knaysi, the frequently reported constriction of the cell wall prior 

 to division is an optical illusion, depending on a constriction of the cytoplasm. 

 The double cell wall is well shown in the electron micrograph of B. anthracis made 

 by Mudd and others (1941). 



Kobinow (1944) also obtained evidence that the transverse divisions of the 

 cytoplasm of dividing bacillary cells are laid down before the bacilli divide in the 

 accepted sense of the term. In some of his preparations (Fig. 9) the transverse 

 walls between daughter cells appear to be growing inwards as annular diaphragms 

 along the plane of the cytoplasmic cell boundary (personal communication). 



Electron micrographs show a cell wall that retains its form even after the 

 cells have been fractured, and their contents dispersed, by ultrasonic vibration 

 (Mudd et al. 1941). A rigid cell wall extending along the length of chains of cocci, 

 and connecting adjacent cells in chains of bacilli, was observed by similar means 

 (Figs. 10a, b ; 11b) (Mudd and Lackman 1941 ; see also Friihbrodt and Ruska 

 1940, Mudd et al. 1942, Johnson et al. 1943). 



The surface of the inner protoplasm takes up stain with greater avidity than 

 the cell wall or the inner protoplasm itself. This, according to Knaysi (1938), 

 is the cytoplasmic membrane, made up chiefly of surface active materials, lipoids 

 and lipoproteins, which must be clearly distinguished from the rigid semipermeable 

 cell wall. 



Capsules.- — Many species of bacteria, such as the pneumococcus, the pneumo- 

 bacillus and the anthrax bacillus, are characterized by the ability to develop a 

 well-marked capsule, which may be observable in stained or unstained preparations 

 as a clear zone surrounding the bacterial cell or may, after suitable fixing and 

 mordanting, be stained by various methods. The degree to which the capsule 

 is developed is largely determined by the environmental conditions. Thus, the 

 pathogenic capsulated bacteria show their maximal capsule formation when grow- 

 ing in the animal tissues (Babes 1895, Gruber and Futaki 1907, Preisz 1907, Bail 

 1908, Sauerbeck 1909 a, b, Eisenberg 1903, 1908, 1909). When such bacteria 

 are grown in artificial culture there is usually a high correlation between the degree 

 of capsulation and the content of the medium in unaltered, or slightly altered, 

 animal protein. 



It is possible that the response to a medium containing animal protein is a 

 reaction to an unfavourable environment. For instance, the pneumobacillus 

 forms capsules in artificial culture when the phase of maximum proliferation has 

 ceased (Hoogerheide 1938). On the other hand, the capsules of a strain of Str. 

 ■pyogenes developed only during its active growth phase in a serum-enriched medium 

 (Morison 1941). 



The capsule is usually regarded as being formed by a thickening and alteration 

 in consistency of the outer layer of the bacterial cell. It was, however, held 

 (Meyer 1912, Zettnow 1918) that the capsular substance was an active secretion. 

 ( 'ertain bacteria secrete a mucilaginous material in which large numbers of neigh- 

 bouring cells are embedded ; and some organisms, such as the Type III pneumo- 

 coccus, form this extracellular mucilaginous material in addition to possessing 

 a well-marked capsule (Fig. 11). It seems, however, more convenient to differ- 

 entiate between the capsulated and the so-called " mucoid " types of bacteria 

 (see Etinger-Tulczynska 1933), and the fact that capsular material may frequently 

 become separated from the cell can hardly be taken as a proof that it is a secretion 

 in the ordinary sense of that term. But the sharp delimitation of the opaque. 



