SURFACi; STRUCTURES 27 



cocci to branched tilanicnts, a single bacterium may contain from two to a 

 dozen relatively tiny cells. The partitions between these cells may break 

 down in the course ot the autogamous processes which accompany sporulation, 

 and at other times, but are usually found in the vegetative cells. 



The cell wall is permeable, it does not grow, but is secreted, in certain 

 well-marked areas, by the semi-permeable cell membrane which underlies 

 it. Measurements of electron micrographs of sectioned bacteria suggest 

 that its thickness varies from lOO to 250 Angstrom units. 



The cell wall is difficult to demonstrate and is seldom observed in pre- 

 parations stained by the usual methods of routine bacteriology. Under such 

 conditions, the existence of the cross-walls is liable to pass unsuspected. The 

 cross-walls are laid down, in the dividing cell, by cytoplasmic septa which 

 stain well with basic dyes, and when the multicellular structure of: such 

 bacteria goes unrecognised these basophilic septa are liable to be confused 

 with nuclear structures and other cytoplasmic inclusions. 



The complex cellular structure of many bacteria has long been known but 

 seldom adequately appreciated until Robinow (1945, </.('. also for earlier 

 literature) demonstrated beyond any reasonable doubt the structure and 

 type of cell division in both unicellular and multicellular bacteria. The 

 former divide by constriction of the cell wall, the latter by the formation of 

 complete cross-walls, which subsequently split. 



Fig. 8 



CELL DIVISION IN BACTERIA 



Cell division, in various bacterial tvpes ; stained bv Hale's method. All at ■ .3000 except 

 (1), X 4,500, and (6) < 1,700. 



(1) Typical multicellular coccus. Each unit, which appears by routine staining method? 

 as a single, spherical cell, contains two, four or more cells, separated by cross-walls, each of 

 which is formed at right-angles to the preceding. 



(2) Streptococcus sp. Although each coccus may contain two or more cells, the cross-walls 

 are all in the same plane. 



(3) Typical unicellular bacterium {Pseiidoinonas sp.). Dividing cells are separated by a 

 short-lived septum, lacking the rigidity of a true cross-wall. Frequently one pole, the growing 

 point, is marked by a concentration of stainable material. 



(4) Spirillum sp. (modified stain by Mr R. A. Fox). The cross-walls of spirilla are not 

 easy to demonstrate, nor, in view of their differences of plane, to photograph, but they appear 

 to be true cross-walls, comparable with those of the Gram-positive genera. 



(5) Nocardia rhodnii. Irregular septation with transient branching of the filaments. 



(6) Caryophanon laluiii. The highest degree of multicellularity is seen in these giant, 

 intestinal bacteria, where each cell is reduced to a disc. 



(7) An exceptional degree of irregular multicellularity is seen in aberrant .stains of Bacillus 

 tereus, and this appearance is accentuated by their high lipid content. 



