surfac:e structures 31 



Pcrlinps surprisingly, tlic lipid clement is an important factor in the 

 maintenance of the ct>nsicierable structural rigidity <-^t the cell wall. If it is 

 removed the strength of the structure is greatly reduced. 



Diftcrences exist between the nature and composition of die cell walls 

 ot Gram-positive and Gram-negative bacteria. Those from Gram-negative 

 genera have a higher lipid content and a much fuller range of amino-acids 

 in the protein traction ; and their cytochemical behaviour more closely 

 resembles that ot intact wool proteins, whereas Gram-pc^sitive bacteria 

 resemble degraded wool proteins in this respect. These observations may 

 serve partially to explain the greater apparent rigidity of the cell walls of 

 Gram-negative bacteria, which can form relatively long filaments without 

 the necessity ot the cross-walls which Gram-positive genera produce under 

 the same circumstances. They are also very interesting from the immuno- 

 logical viewpoint ; the great complexity of the cell wall proteins of Gram- 

 negative bacteria is almost certainly correlated with their more satisfactory 

 behaviour as antigens, whereas in the rough variants of Bactcriaccae the loss 

 oi antigenic specificity is associated with a change to the septate morphology 

 and mode of cell division characteristic of Gram-positive bacilli. In addition, 



Fig. 11 

 CELL ENVELOPES IN BACILLUS 

 In " rough " septate bacilli the bacillus is usually divided centrally by a complete cross cell 

 wall and subdivided by developing cross-walls at varying stages. The latter may stain as 

 cell walls or cell membranes {i.e. basophilic) according to the stage of development. 



(1) B. cereus, cell walls by Hale's method. Demonstrating only mature cross-walls x 3000. 



(2) B. megaterium, cell walls by tannic-acid-violet. Cross-walls at various stages can be 

 seen ; the most mature appear double, possibly because of tannic-acid complex deposited on 

 their faces. < 4,500. 



(3) B. megaterium stained by haematoxylin. Developing cross-walls show as dark bars ; 

 the cell wall does not stain. Such basophilic areas have frequently been mistaken for nuclei 

 or portions of nuclear structures, x 4,500. 



(4) B. megaterium stained by acid-thionin. This dye is less specific for the nuclear structures 

 than Giemsa and stains also the basophilic areas of developing cross-walls, which appear 

 either as readily recognisable bars or else as dots between the nuclei. In the latter form they 

 have been confused with mitotic centrioles. x 4,500. 



(5), (6) Cell walls and cell membranes stained simultaneously by the Victoria blue method 

 of Robinow and Murray (1953). The bacilli {B. megaterium) are slightly plasmolysed and the 

 cell membrane is retracted from the mature cross-walls. The developing cross-walls show as 

 dark bars, apparently composed of, or surrounded bv, the same material as the cell membrane. 

 *x 3,000. 



