1) Bacteria have different cell surface structures including flagella, pili, and fimbriae that vary in thickness, structure, and function. 2) The cell wall lies outside the cell membrane and is composed of peptidoglycan. Peptidoglycan molecules have a backbone, tail, and bridges that crosslink chains. 3) Bacteria are classified as Gram positive or Gram negative based on differences in cell wall structure identified by the Gram stain. Gram positive walls are thicker while Gram negative walls have an outer membrane and periplasmic space.

1. BACTERIA STRUCTURE
FLAGELLA, FIMBRIAE, PILI AND CELL WALL

2. Differential characters of flagellum, pilus, and
fimbria
racter Flagellum pilus fimbria
Very long Ahort but longer
than fimbria
Short
kness Thick Thin Thin
cture Wavy, helical Straight, nonhelical Straight, nonhelical
mical
positon
Flagellin Pilin Pilin
ence In some bacteria In male bacteria In all bacteria
mber One to many One to two Few to several
hundred
ction Locomotion Conjugation,
receptor for viruses
adherence

3. CELL WALL
• The cell wall is the outer most layer of the cell. In many
cases the cell wall comes in direct contact with the
environment.
• All bacteria can be classified on the basis of their cell wall
into :
1) Gram positive
2) Gram negative.
• The two different cell wall types can be identified in the lab
by a differential stain known as the Gram stain.

4. CHEMICAL COMPOSITION OF CELL WALL:
• Composed of peptidoglycans, also called murein.
• Peptidoglycan is found only in procaryotes.
• Peptidoglycan molecule consists of three parts:
1) the backbone,
2) the tail, and
3) the bridge.
• It differs in composition and structure from one specie to
another.

5. COMPOSITION OF PEPTIDOGLYCAN
• The backbone of peptidoglycan molecule is a long chain composed
of two amino sugars,
1. N-acetylglucosamine (NAG),
2. N-acetylmuramic acid (NAM).
• These two amino sugars are linked by a β-1, 4 linkage to form long
polysaccharide chains consisting of alternating units of these sugars.

6. COMPOSITION OF PEPTIDOGLYCAN
• The tail of peptidoglycan molecule is a short chain
composed of four amino acids, tetrapeptide.
• It is attached to the carboxyl group of NAM. The amino
acids in tetrapeptide chain include
• D- glutamic acid
• D-alanine
• L-alanine
• Either diaminopimelic acid or lysine.

7. COMPOSITION OF PEPTIDOGLYCAN
• The bridge provides rigidity and strength to the peptidoglycan by
cross linking the tails of neighboring chains. Each bridge is a peptide
bond between the third amino acid of one tail and fourth amino
acid of an adjacent tail.

8. GRAM POSITIVE CELL WALL:
• Usually thick, homogenous, composed mainly of peptidoglycan.
• 20-80 nm
• It accounts for 50-90% of the dry weight of the cell wall.
• Contain large amount of teichoic acids (polymers of glycerol or
ribitol joined by phosphate group).

9. TEICHOIC ACID
• There are two types of teichoic acid:
1) lipoteichoic acid which acts a as a link between peptidoglycan and
cytoplasmic membrane
2) wall techoic acid which is linked to peptidoglycan layers
Function of Teichoic Acid:
1. It provides negative charge to the bacterial cell.
2. It acts as an antigenic determinants and used in serologic
identification of many species.

13. GRAM NEGATIVE CELL WALL
• The unique feature of the gram-negative cell wall has two main
components:
1. an outer membrane, and
2. the inner rigid layer of peptidoglycans.
• The outer layer is phospholipid bilayer which is attached to the
peptidoglycan by the lipoprotein molecules.

14. GRAM NEGATIVE CELL WALL
• The outer half is composed primarily of lipopolysaccharide (LPS),
lipoproteins and phosphilopids.
• LPS consists of three regions,
• O-polysaccharides
• Core polysaccharides and,
• Lipid A.
• The space between cytoplasmic membrane and the outer
membrane including peptidoglycan is termed periplasmic
membrane. This space is the site for enzyme activities that result in
the degradation of large molecules to be used as nutrients.

16. FUNCTION OF CELL WALL
• To maintain shape of the cell.
• Protects the internal components of the cell from the harmful
changes in the external environment.
• Provides site for the anchoring of the pili and flagella.
• LPS of gram negative cell wall plays a major role in diseases caused
by Gram negative pathogens as it ascts as an endotoxin.it helps in
the serologic identification of the bacterial species.