Date post: | 17-Sep-2015 |
Category: |
Documents |
Upload: | muhammad-usama |
View: | 34 times |
Download: | 1 times |
Gram Positive & Gram Negative Bacteria
Morphology
4/10/2015
Muhammad Usama (11-Ch-98)
Submitted To : Mam Masooma
Summary of Morphology of Gram Positive & Gram Negative Bacteria
Introduction:
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.
Shape & Size
Bacteria have three shapes: cocci (spheres),
bacilli (rods), and spirochetes (spirals).
Cocci are arranged in three patterns: pairs
(diplococcic), chains (streptococci) and clusters (staphylococci).
The size of most bacteria ranges from 1 to 3 micrometer. Mycoplasma, the
smallest bacteria (and therefore the smallest cells) are 0.2 micrometer.
Some bacteria, such as Borrelia, are as long as 10 micrometers; that is, they are
longer than a human red blood cell, which is 7.4 micrometers in diameter.
Comparison chart
Properties 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 crystal 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-layered) Thick (multilayered)
Teichoic acids Absent Present in many
Periplasmic space Present Absent
Outer membrane Present Absent
Lipopolysaccharide
(LPS) content
High Virtually none
Lipid & 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
Toxins produced Primarily Endotoxins Primarily Exotoxins
Resistance to
physical disruption
Low High
Inhibition by basic
dyes
Low High
Susceptibility to
anionic detergents
Low High
Cell wall composition The lipid content is 20-30%
(High), whereas Murein content
is 10-20% (Low).
The Lipid content of the cell wall is
low , whereas Murein content is
70-80% (Higher).
Antibiotic Resistance More resistant to antibiotics. More susceptible to antibiotics
Gram-Positive Bacteria
In general, the following characteristics are present in gram-positive bacteria:
1. Cytoplasmic cell membrane is composed of proteins containing usual amino acids,
phospholipid and polysaccharide. It has enzymatic activity.
2. Thick peptidoglycan layer (cell wall).
3. Teichoic acids and lipids are present, forming lipoteichoic acids, which serve
as chelating agents, and also for certain types of adherence.
4. Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial
enzyme DD-transpeptidase.
5. A much smaller volume of periplasm than that in gram-negative bacteria.
Only some species have a capsule usually consisting of polysaccharides (e.g., hyaluronic
acid). Also only some species are flagellates, and when they do have flagella they only have
two basal body rings to support them (gram-negative have four). Both gram-positive and
gram-negative bacteria commonly have a surface layer called an S-layer.
In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer (in gram-
negative bacteria, the S-layer is attached directly to the outer membrane). Specific to gram-
positive bacteria is the presence of teichoic acids in the cell wall.
Some of these are lipoteichoic acids, which have a lipid component in the cell membrane
that can assist in anchoring the peptidoglycan. Cell wall contain mucocomplex substances
i.e. glutamic acid, lysine, diaminopimalic acid, alanine, amino sugars, acetyl glucosamine,
galactosamine, acetlymuramic acid etc. Their cell wall does not take part in biochemical
activity as it has no enzymes.
Gram-Negative Bacteria
Gram-negative bacteria display the following characteristics:
1. Cell membrane (cytoplasmic). Its not clearly distinct from cell wall and has number
of biochemical functions.
2. Thin peptidoglycan layer (which is much thicker in gram-positive bacteria) which is
composed of lipid up to 20%, protein containing 21 amino acids and
polysaccharides. This cell wall has enzymatic activity.
3. Outer membrane i.e. capsule or slime containing lipopolysaccharide (LPS, which
consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet
and phospholipids in the inner leaflet. So, it has protein-polysaccharide-lipid
complexes.
4. Porins exist in the outer membrane, which act like pores for particular molecules
5. Between the outer membrane and the cytoplasmic membrane there is a spacethe
periplasmic space filled with a concentrated gel-like substance called periplasm
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
10. Some 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. Some release liposaccharide
Gram negative bacteria deploy their periplasm to secrete bacterial outer membrane
vesicles (OMVs) for trafficking bacterial biochemicals to target cells in their environment.
OMVs also carry the lipopolysaccharide initiating disease process in their host.