CHEMISTRY OF CELL WALLS 35 



Occurrence of D-isomers of Amino Acids. Snell and his 

 colleagues -^' -^ were the first to discover that the D-alanine 

 found in bacterial cells was localized in the wall. A high 

 proportion of the cell-wall alanine was present as the o-iso- 

 mer. Glutamic acid was subsequently found in the wall as 

 the D-isomer.2^ Salton *- also showed that o-alanine occurred 

 quite widely in the walls of various bacterial species. The 

 list of D-amino acids in bacterial walls was extended to 

 aspartic acid when Toennies, Bakay, and Shockman ^^ found 

 that this amino acid occurred partly as the D-isomer in the 

 wall of Streptococcus faecalis. Ikawa and Snell *^ have 

 made an extensive investigation of the proportions of d- 

 and L-isomers of alanine, glutamic, and aspartic acids in 

 the walls of many lactic acid bacteria, and some of the re- 

 sults are summarized in Table 14. Thus about half of the 

 cell-wall alanine occurs as the D-isomer and virtually all of 

 the glutamic acid is in the D-form, whereas D-aspartic acid 

 residues constitute roughly three quarters of the total as- 

 partic acid contents. Park ** has observed that many walls 

 have 1 : 1 ratios of D-glutamic acid to muramic acid. 



Evidence so far available suggests that only L-lysine is 

 present in walls.*^ However, DAP can occur in bacteria as 

 the LL-, meso(DL)-, or DD-isomers, and occasionally the ll- 

 and meso-isomers together (Hoare and Work ^^). The meso- 

 isomer is most widely distributed in bacteria and the iso- 

 lated walls, being found in members of the Bacillus, 

 Corynebacterium, Mycobacterium, Nocardia, and, less fre- 

 quently, in certain species of Lactobacillus and Micrococcus 

 groups.^^'20.24,37,45 LL-DAP has been detected in members 

 of the Propionibacterium, Streptomyces, and some Clos- 

 tridium species.2*'45 Hoare and Work ^^ found some DD-iso- 

 mer of DAP in hydrolysates of Micromonospora, the pres- 

 ence of this isomer in isolated walls being confirmed later.-* 



