Extensive morphological investigations have adequately revealed that the
cell envelope
Gram-positive bacteria*
is much more simpler with regard to the structure in comparison to that of the
Gram-negative bacteria.**
For Gram-positive Bacteria :
In this instance the cell envelope contains chiefly the peptidoglycan
and the
teichoic acids.
Interestingly, the peptidoglycan represents a substituted carbohydrate polymer found exclusively
in the
prokaryotic microorganisms.
It essentially comprises of
two major chemical entities namely :
(
a) Two acetylated aminosugars e.g., n-acetyl glucosamine ; and n-acetylmuramic acid ; and
(
b) Amino acids e.g., D-glutamic acid ; D- and L-alanine ;
In fact, the
long peptide chains containing the two amino sugars that essentially constitute the
‘glycan strands’
comprise of alternating units of n-acetyl glucosamine and n-acetyl muramic acid in
β
-1, 4-linkage ; besides, each strand predominently contains disaccharide residues ranging from 10 to65 units
Salient Features :
The various important and noteworthy salient features with regard to the
formation of peptide chains are as enumerated under :
(1) The 3rd amino acid
i.e., meso-diamino pimelic acid (Fig. 2.12) has been observed to vary
with different organisms (bacteria) by any one of the
three such amino acids as : lysine,
diamino pimelic acid, or threonine.
(2) Besides, the adjacent peptide chains occurring in a
peptidoglycan could be duly cross-linked
by short peptide chains essentially comprising of a varying number of amino acids.
(3) An important characteristic feature
viz., the variations in the structure of the peptidoglycan
constituents usually take place ; and, therefore, it has been exploited and utilized judiciously
as a
wonderful taxonomic tool.
(4) The exact number of amino acids that eventually form the cross link prevailing between the
two n
-acetyl muramic acid residue i.e., the interpeptide bridge variation, may also vary from
2 to 5, as given in Table 2.7, depending upon the various species of microorganisms. Variations
in the n-acetyl muramic acid are also known and these alterations ultimately do affect
the compactness of the peptidoglycan to an appreciable degree.
.