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.