Answer Posted / r.sathish kumar

Gram-positive bacteria are those that are stained dark blue
or violet by Gram staining. This is in contrast to
Gram-negative bacteria, which cannot retain the crystal
violet stain, instead taking up the counterstain (safranin
or fuchsine) and appearing red or pink. Gram-positive
organisms are able to retain the crystal violet stain
because of the high amount of peptidoglycan in the cell
wall. Gram-positive cell walls typically lack the outer
membrane found in Gram-negative bacteria.

When treated as a clade, the term "posibacteria" is
sometimes used.
Characteristics
Gram-negative bacteria
Gram-negative bacteria (or negibacteria) are bacteria that
do not retain crystal violet dye in the Gram staining
protocol.[1] In a Gram stain test, a counterstain (commonly
safranin) is added after the crystal violet, coloring all
Gram-negative bacteria with a red or pink color. The test
itself is useful in classifying two distinct types of
bacteria based on the structural differences of their
bacterial cell walls. Gram-positive bacteria will retain the
crystal violet dye when washed in a decolorizing solution.

The pathogenic capability of Gram-negative bacteria is often
associated with certain components of Gram-negative cell
walls, in particular, the lipopolysaccharide layer (also
known as LPS or endotoxin layer).[1] In humans, LPS triggers
an innate immune response characterized by cytokine
production and immune system activation. Inflammation is a
common result of cytokine (from the Greek cyto, cell and
kinesis, movement) production, which can also produce host
toxicity.

When treated as a clade, the term "negibacteria" is
sometimes used.[2]
Gram-positive and -negative cell wall structure
Structure of Gram-positive cell wall

The following characteristics are generally present in a
Gram-positive bacterium:[2]

cytoplasmic lipid membrane
thick peptidoglycan layer
teichoic acids and lipoids are present, forming
lipoteichoic acids, which serve to act as chelating agents,
and also for certain types of adherence.
capsule polysaccharides (only in some species)
flagellum (only in some species)
if present, it contains two rings for support as
opposed to four in Gram-negative bacteria because
Gram-positive bacteria have only one membrane layer.
The individual peptidoglycan molecules are cross-linked
by pentaglycine chains by a DD-transpeptidase enzyme. In
gram-negative bacteria, the transpeptidase creates a
covalent bond directly between peptidoglycan molecules, with
no intervening bridge.
Classification

In the original bacterial phyla, the Gram-positive organisms
made up the phylum Firmicutes, a name now used for the
largest group. It includes many well-known genera such as
Staphylococcus, Streptococcus, Enterococcus, (which are
cocci) and Bacillus, Corynebacterium, Nocardia, Clostridium,
Actinobacteria, and Listeria (which are rods and can be
remembered by the mnemonic obconical). It has also been
expanded to include the Mollicutes, bacteria-like Mycoplasma
that lack cell walls and cannot be Gram stained, but are
derived from such forms. Actinobacteria are the other major
group of Gram-positive bacteria, which have a high guanine
and cytosine content in their genomes (high G+C group). This
contrasts with the Firmicutes, which have a low G+C content.

Both Gram-positive and Gram-negative bacteria may have a
membrane called an S-layer. In Gram-negative bacteria, the
S-layer is directly attached to the outer membrane. In
Gram-positive bacteria, the S-layer is attached to the
peptidoglycan layer. Unique to Gram-positive bacteria is the
presence of teichoic acids in the cell wall. Some particular
teichoic acids, lipoteichoic acids, have a lipid component
and can assist in anchoring peptidoglycan, as the lipid
component is embedded in the membrane.


Gram Positive Classification.svg
Characteristics
Structure of gram-negative cell wall
Gram-positive- and negative bacteria are chiefly
differentiated by their cell wall structure.

The following characteristics are displayed by Gram-negative
bacteria:

Cytoplasmic membrane
Thin peptidoglycan layer (which is much thinner than in
Gram-positive bacteria)
Outer membrane containing lipopolysaccharide (LPS, which
consists of lipid A, core polysaccharide, and O antigen)
outside the peptidoglycan layer
Porins exist in the outer membrane, which act like pores
for particular molecules
There is a space between the layers of peptidoglycan and
the secondary cell membrane called the periplasmic space
The S-layer is directly attached to the outer membrane,
rather than the peptidoglycan
If present, flagella have four supporting rings instead
of two
No teichoic acids or lipoteichoic acids are present
Lipoproteins are attached to the polysaccharide backbone.
Most of them contain Braun's lipoprotein, which serves
as a link between the outer membrane and the peptidoglycan
chain by a covalent bound
Most do not sporulate (Coxiella burnetii, which produces
spore-like structures, is a notable exception)

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