ISOLATION AND PROPERTIES OF WALLS 15 



quently in the Gram-negative group of bacteria, a difference 

 in the level of complexity that could have been predicted 

 from the early studies of chemical constitution. 



General Physical Properties 



The majority of microbial cell walls are fairly robust 

 structures, and in many instances they must obviously be 

 strong enough to withstand high pressures exerted upon 

 them by those organisms capable of achieving a high con- 

 centration gradient across the wall-membrane (envelope). 

 Mitchell and Moyle ^^ found that the solute concentration 

 in Micrococcus lysodeikticus and Sarcina lutea corresponded 

 to an osmotic pressure of 20 atmospheres. The wall must 

 therefore possess sufficient tensile strength to protect the cell 

 against osmotic explosion. However, the walls of certain 

 halophilic organisms are apparently not strong enough to 

 prevent osmotic lysis when these bacteria encounter envi- 

 ronments of low solute concentrations. *- 



The thickness of microbial walls has been reported by a 

 number of investigators, either from thin sectioning of the 

 cells or isolated walls or from direct measurement of the 

 height of the shadows cast in specimens examined by elec- 

 tron microscopy. Some typical examples for various micro- 

 bial walls are given in Table 2, together with data on the 

 contribution of the wall to cell mass. There would seem 

 to be some anomalies in the data for wall thickness, cell 

 size, and weight contribution for the yeasts and Chlorella 

 in particular, and the final assessment of the accuracy of 

 these measurements will have to await further determina- 

 tions. 



It is evident that the wall accounts for a considerable 

 proportion of the cell weight, the actual contribution de- 

 pending on the phase of growth in the case of a bacterium 

 such as Streptococcus faecalis.^^ Toennies and Shockman ^^ 



