The various characteristic features of Gram-positive and Gram-negative bacteria shall be discussed
at length in this particular section.
For Gram-negative bacteria.
the cell envelopes of
Gram-negative bacteria, namely :
(
a) An uniform inner layer approximately 2–3 mm wide, and
(
b) A thicker outer layer nearly 8–10 nm wide.
Importantly, the
peptidoglycan is prominently confined to the inner layer ; whereas, the outer
layer (membrane) essentially comprises of
proteins, lipoproteins, and lipopolysaccharides.
The principal chemical differences that predominently occur between the cell walls of Grampositive
bacteria and the inner rigid wall layer and outer wall layer(s) of Gram-negative bacteria have
been duly summarized
Cardinal characteristic features of component variants in Gram +ve and Gram –ve microorganisms
:
The various important characteristic features of component variants in Gram +ve and Gram
–ve microbes are as stated under :
(1) Peptidoglycans belonging to the Gram –ve microorganisms exhibits a rather low extent of
cross linkages within the glycan strands.
(2)
Outer-membrane. The fine structure of the outer membrane, very much akin to cell membrane,
essentially comprises of a lipid bilayer wherein both phospholipids and
lipopolysaccharides are definitely present. Besides, the lipopolysaccharide generates the major
component of the outer membrane, and represents an extremely complex molecule varying
in chemical composition within/between the Gram –ve bacteria.
(3)
Outer surface. The peptidoglycan of the wall has particular kinds of lipoproteins residing
on its
outer surface, that are strategically linked by peptide bonds to certain diaminopimelic
acid residues present in the peptidoglycan.
(4) Lipoproteins evidently serve as a sort of bridge right from the peptidoglycan upto the outerwall-
layer.
(5) The total number of proteins definitely present, unlike in the inner membrane, are quite a
few in number (approx. 10) ; and, therefore, these are markedly distinct from those invariably
found in the inner membrane.
Explanations :
The proper explanations for the various transformations occurring in Figure : 2.13
are as given below :
(
i) The various subunits in lipopolysaccharide are duly linked via pyrophosphates with the ‘lipid
zone
’.
(
ii) The ‘lipid zone’ comprises of a phosphorylated glucosamine disaccharide esterified adequately
with long chain fatty acids.
(
iii) The ‘core region’ comprises of a short-chain of carbohydrates, and the O-side chain consists
of different carbohydrates and is much longer in comparison to the R-core region.
(
iv) Lipopolysaccharides represent the major antigenic determinants, and also the receptors
for the active adsorption of several bacteriophages.
Comparative Activities of Gram-negative and Gram-positive Bacteria
The various glaring comparative activities of both Gram-negative and Gram-positive bacteria are
enumerated below :
(1) It has been duly demonstrated that the outer membrane of Gram-negative bacteria prominently
behaves as a solid barrier to the smooth passage of certain critical substances, for
instance : antibiotics, bile salts*, and dyes into the cell. Hence, the Gram-negative organisms
are comparatively much less sensitive to these substances than the Gram-positive ones.
(2) Adequate treatment of Gram-negative bacteria with an appropriate chelating agent, such as :
ethylenediaminetetra acetic acid (EDTA), that eventually affords the release of a substantial
amount of
lipopolysaccharides renders ultimately the cells more sensitive to the drugs and
chemical entities. Thus, the presence of
lipopolysaccharide on the surface of the cell alsohelps the bacteria to become resistant to the phagocytes** of the host
(3) The resistance acquired in (2) above is almost lost only if the host enables to synthesize the
antibodies
that are particularly directed against the O-side chain (Figure 2.13). There exists
a vast diversification in the specific structure of the O-side chain ; and, therefore, gives rise
to the
somatic*** antigenic specificity very much within the natural bacterial populations.
Evidently, the ensuing
antigenic diversity exhibits a distinct selective advantage specifically
for a pathogenic bacterial species, because the animal host is not in a position to possess
higher antibody levels strategically directed against a relatively large number of varieties
of O-side chains.
(4) In general, the prevailing lipids are invariably found to be
phosphatidylethanolamine, and
apparently to a much smaller extent
phosphatidylserine and phosphatidylcholine, presentduly in Gram-negative and Gram-positive bacteria.