Gram positive and gram negative

1. Comparison chart

Gram-negative Bacteria Gram-positive Bacteria
Gram reaction
Can be decolourized to accept counter
stain (Safranin or Fuchsine); stain red
or pink, they don't retain the Gram
stain when washed with absolute
alcohol and acetone.
Retain cry stal violet dye and stain dark
violet or purple, they remain coloured
blue or purple with gram stain when
washed with absolute alcohol and
water.
Peptidoglycan layer Thin (single-lay ered) Thick (multilay ered)
T eichoic acids Absent Present in many
Periplasm ic space present Absent
Outer m embrane Present Absent
Lipopolysaccharide
(LPS) content
High Virtually none
Lipid and lipoprotein
content
High (due to presence of outer
membrane)
Low (acid-fast bacteria have lipids
linked to peptidoglycan)
Flagellar structure 4 rings in basal body 2 rings in basal body
T oxins produced Primarily Endotoxins Primarily Exotoxins
Resistance to physical
disruption
High Low
Inhibition by basic
dy es
High Low
Susceptibility to
anionic detergents
High Low
Resistance to sodium
azide
High Low
Resistance to dry ing High Low
Cell wall com position
The cell wall is 7 0-120 Armstrong thick
two lay ered.The lipid content is 20-
30% (High), whereas Murein content
is 10-20% (Low).
The cell wall is 100-120 Armstrong
thick, single lay ered. The Lipid content
of the cell wall is low , whereas Murein
content is 70-80% (Higher).

2. Gram-negative Bacteria Gram-positive Bacteria
Mesosome Mesosome is less prominent. Mesosome is more prominent.
Antibiotic Resistance More Resistant to antibiotics. More Susceptible to antibiotics


GRAM POSITIVE CELL WALL

Carbohydrate backbone is composed of alternating N-acetylmuramic acid (NAM) and N-
acetylglucosamine (NAG) molecules.
Attached to each of the muramic acid molecules are a tetrapeptide consisting of both D- and L- amino
acids, the precise composition of which differs between bacteria.

3. Special Aminoacids found in Peptidoglycan layer:
 Diaminopimelic acid: Unique to bacterial cells.
 D- alanine: Involved in the cross links between tetrapeptides and in the action of penicillin.
Functions of Peptidoglycan Layer
1. It provides rigid support to bacterial cells and maintains the characteristic shape of the cell.
2. Allows bacterial cell to withstand media of low osmotic pressure, such as water.
Medical Importance of Peptidoglycan Layer
Peptidoglycan is a good target for antibacterial drugs. Eg. Penicillins, cephalosporins etc inhibit
transpeptidase reaction which makes cross-links between the two adjacent tetrapeptides.
Lysozyme enzyme present in human tears, mucus, and saliva cleave peptidoglycan backbone breaking
its glycosyl bonds.
plasmid (ˈplæzmɪd)

— n
a small circle of bacterial DNA that is independent of the main bacterial chro
mosome. Plasmids oftencontain genes for drug resistances and can be trans
mitted between bacteria of the same and differentspecies: used in genetic e
ngineering


Difference Between Endotoxin and Exotoxin
Exotoxins are toxic substances secreted by bacteria and released outside the cell.
Endotoxins are bacterial toxins consisting of lipids that are located within a cell.

4. Gram-negative bacteria display the following characteristics:
1. Cell membrane (cytoplasmic).
2. Thin peptidoglycan layer (which is much thicker in gram-positive bacteria)
3. Outer membrane containing lipopolysaccharide (LPS, which consists oflipid A, core
polysaccharide, and O antigen) in its outer leaflet andphospholipids in the inner leaflet
4. Porins exist in the outer membrane, which act like pores for particular molecules
5. There is a space between the peptidoglycan layer and the secondary cell membrane called
the periplasmic space
6. The S-layer is directly attached to the outer membrane rather than the peptidoglycan
7. If present, flagella have four supporting rings instead of two
8. No teichoic acids or lipoteichoic acids are present
9. Lipoproteins are attached to the polysaccharide backbone.

5. 10. Some of them contain Braun's lipoprotein, which serves as a link between the outer
membrane and the peptidoglycan chain by a covalent bond
11. Most, with very few exceptions, do not form spores.
12. Release some endotoxin


Lipopolysaccharide (LPS) of Gram Negative Bacteria,
characteristics and functions











Outer layer of cellwall of gram negative bacteria also called endotoxin. It is a characteristics feature of
Gram Negative Bacteria. As in peptidoglycan biosynthesis, LPS molecules are assembled at the plasma
or inner membrane.
Exception: Only one Gram Positive Bacteria, i.e. Listeria monocyotogenes has been found to
contain an authentic Lipopolysaccharide.

Lipopolysaccharide is Pyrogenic (Responsible for fever), and also causes endotoxic shock etc.The LPS is
composed of three distinct units.

6.  A phospholipid called Lipid A, which is responsible for toxic effects
 A core polysaccharide of fiver sugars linked through ketodeoxyoctulonate (KDO) to lipid A

Structural Unit of Lipopolysaccharide Source: South Carolina School of Medicine
 An outer polysaccharide consisting of up to 25 repeating units of 3-5 sugars, also called somatic or O
antigen. O antigens are used to identify certain organisms in microbiology laboratory.

8. 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.