Danish scientist Hans Christian Gram devised a method
to differentiate two
types of bacteria based
on the structural differences in their cell walls.
In his test, bacteria that retain the crystal violet
dye do so because of
a thick layer of
peptidoglycan and are called Gram-positive bacteria.
In contrast, Gram-negative bacteria do not retain the
violet dye and are colored red or pink.
Compared with Gram-positive bacteria, Gram-negative bacteria are more resistant against antibodies because of
their impenetrable cell wall. These bacteria have
a wide variety of applications ranging from medical treatment to industrial use and Swiss cheese production.
Cell
well
A cell wall, not just of bacteria but for all organisms, is
found outside of the cell membrane. It’s an additional layer that typically provides some strength that
the cell membrane lacks, by having a
semi-rigid structure.The bacterial cell walls contain an
ingredient known as peptidoglycan. This particular substance hasn’t been found anywhere else on Earth,
other than the cell walls of bacteria. The bacterial cell wall performs several functions as well, in
addition to providing overall strength to the cell. It also helps maintain the cell shape, which is important
for how the cell will grow, reproduce, obtain nutrients,and move. It protects the cell from
osmotic lysis, as the cell moves from one environment to another or transports in nutrients from its surroundings.The bacterial cell
wall can contribute to the pathogenicity or disease –causing ability of the cell for certain bacterial pathogens.Peptidoglycan is a polysaccharide made of two glucose derivatives, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), alternated in long chains. The chains are
cross-linked to one another by a tetrapeptide that extends off the NAM sugar unit,
allowing a lattice-like structure to form. The four amino acids that compose
the tetrapeptide are: L-alanine,
D-glutamine, L-lysine or meso-diaminopimelic acid (DPA), and D-alanine
.
Gram posItive
The cell walls of
gram positive bacteria are composed predominantly of peptidoglycan. In fact,
peptidoglycan can represent up to 90% of the cell wall, with layer after layer forming around the
cell membrane. The NAM tetrapeptides
teichoic acids can either be covalently linked
to peptidoglycan (wall teichoic acids or WTA) or connected to the cell membrane via a lipid anchor, in
which case it is referred to as lipoteichoic acid. Since peptidoglycan is relatively porous, most substances can pass through the gram
positive cell wall with little difficulty. But some nutrients are too
large, requiring the cell to rely on the use of exoenzymes. These extracellular enzymes are made within the cell’s
cytoplasm and then secreted past the cell membrane, through the cell wall, where they function outside of the cell
to break down large macromolecules into smaller components.
Gram negative
The cell walls of gram negative bacteria are more
complex than that of gram positive bacteria, with more ingredients overall. They do contain
peptidoglycan
In addition, LPS plays a role in the host response to
pathogenic gram negative bacteria. The O-antigen triggers an immune response in an infected
host, causing the generation of antibiotics
specific to that part of LPS . Lipid A acts as a toxin,
specifically an endotoxin, causing general symptoms of illness such as fever and diarrhea. A large
amount of lipid A released into the bloodstream can
trigger endotoxic shock, a body-wide inflammatory response which can be
life-threatening The
outer membrane does present an obstacle for the cell. While there are certain
molecules it
would like to keep out, such as antibiotics and toxic chemicals, there are
nutrients that it
would like to let in and the additional lipid bilayer presents a formidable
barrier.
Large
molecules are broken down by enzymes, in order to allow them to get past the
LPS. Instead of
exoenzymes
(like the gram positive bacteria), the gram negative bacteria utilize periplasmic enzymes
that are stored in the periplasm. It is the space located
between the outer surface of the cell membrane and the inner surface of the
outer membrane, and
it contains the gram negative peptidoglycan. Once the periplasmic enzymes have
broken nutrients down
to smaller molecules that can get past the LPS, they still need to be
transported across the outer membrane, specifically
the lipid bilayer. Gram negative cells utilize porins, which are transmembrane proteins
composed of a
trimer of three subunits, which form a pore across the membrane. The peptidoglycan layers are linked to the outer membrane by the use of a
lipoprotein known as Braun’s lipoprotein. At one end the lipoprotein is
covalently bound to the peptidoglycan while the other end is embedded into the
outer membrane via its polar head. This linkage between the two layers provides
additional structural integrity and strength.
Conclution
The glycans backbone of the
peptidoglycans molecule can be cleaved by an enzyme called lysozyme that is present in
animal serum, tissues and secretions. some Gram-positive are very sensitive to lysozymeand the enzyme is quite
active at low concentrations. Gram-negative bacteria are less vulnerable to attack
by lysozyme because their peptidoglycan is shielded by the outer membrane.
The exact site of lysozymal cleavage is the beta
1,4 bond between N-acetylmuramic acid and N- acetylglucosamine.
Gram-positive bacteria are also more sensitive to penicillin than Gram- negative bacteria because the peptidoglycan is not protected by an outer membrane and it is a more abundent molecule in Gram- positive ones whice make it more vulnerable while gram negative ones more resistant.