ISOLATION AND PROPERTIES OF WALLS 



used with considerable advantage in the removal of cyto- 

 plasmic materials from crude cell-wall fractions. Thus 

 ribonuclease, trypsin, and lipase can be used without de- 

 stroying the rigidity of or apparently degrading the wall 

 structure. 



Owing to the small dimensions of microbial structures, 

 the only satisfactory method of establishing their morpho- 

 logical homogeneity has been by electron microscopic ex- 

 amination. Cell walls can thus be differentiated from other 

 structures such as flagella, fimbriae, ribosomes, and intra- 

 cellular particles. 



Electron Microscopy of Isolated Cell Walls 



Microbial walls isolated by the foregoing procedures 

 generally retain the shape and outline of the organism 

 from which they had been derived. This fact, together with 

 the morphological changes accompanying enzymic removal 

 of walls with protoplast formation (Weibull ~^), makes it 

 certain that it is the wall that confers the shape on a par- 

 ticular organism. Walls of rod-shaped organisms are typi- 

 cally cylindrical in shape on examination in the electron 

 microscope and those of Streptococcus faecalis are ellip- 

 soidal. ^^ 



Some of the first microbial walls isolated by mechanical 

 methods showed no evidence of fine structure. The wall of 

 baker's yeast isolated by Northcote and Home ^^ appeared 

 as a thick amorphous structure on examination in the 

 electron microscope. However, by treatment with alkali 

 and acid successively, Houwink and Kreger ^^ removed some 

 of the matrix from the walls of Candida tropicalis and 

 showed a microfibrillar structure in the walls of this yeast 

 (Fig. 1). By using more selective methods of extracting 

 wall compounds, Nickerson and his colleagues ^6. 27 were 



