Bacterial cell structure and function

July 22, 2017 Gaurab Karki Bacteriology, Microbiology 0




Bacterial cell Structure and Function

Bacterial are unicellular prokaryotic organism. Bacterial cell have simpler internal structure. It lacks all membrane bound cell organelles such as mitochondria, lysosome, golgi, endoplasmic reticulum, chloroplast, peroxisome, glyoxysome, and true vacuole. Bacteria also lacks true membrane bound nucleus and nucleolus. The bacterial nucleus is known as nucleoid.

A typical bacterial cell have following structure.

A. Structure Outside cell wall

  1. Capsule
  2. Flagella
  3. Pili
  4. Sheath
  5. Prostheca
  6. Stalks

B. Structure Inside cell wall

  1. Cell wall
  2. Cytoplasmic membrane
  3. Nucleoid
  4. Mesosome
  5. Ribosome
  6. cytoplasm
  7. Spore

1. Capsule:

  • Capsule is 0.2µm thick viscus layer outer layer to the cell wall.
  • Capsule is 98% water and 2% polysaccharide or glycoprotein/ polypeptide or both.
  • There are two types of capsule.

i. Macro-capsule: thickness of 0.2µm or more, visible under light microscope

ii. Microcapsule: thickness less than 0.2µm, visible under Electron microscope

  • Capsule is very delicate structure. It can be removed by vigorous washing.Capsule is most important virulence factor of bacteria.

Function:

  • It helps in attachments as well as it prevent the cell from desiccation and drying.
  • Capsule resist phagocytosis by WBCs

 2. Flagella:

  • It is 15-20 nm hair like helical structure emerges from cell wall.
  • Flagella is not straight but is helical. It is composed of flagellin protein (globular protein) and known as H antigen.
  • Flagella has three parts. Basal body, Hook and filament

Function:

  • It helps motility of the bacteria

3. Pili or fimbriae:

  • Pili are hollow filamentous and non-helical structure.
  • They are numerous and shorter than flagella
  • Pili is the characteristic feature of gram –ve bacteria.
  • Pili is composed of pilin protein.
  • Bacteria containing pili: Shigella, Proteus, Neisseria gonorrhoae, E. coli

Function:

  • Attachment: pili helps the bacteria to attach the host cell surface. Most of the human pathogens of respiratory tract, urinary tract are attached with the help of pili.
  • Pili (fimbrae) possess antigenic property
  • Specialized function: some pili are modified for specialized function. Eg. Sex pilus (F-pili) help in transfer of DNA from donor to recipient cell during conjugation.
  • F-pili also act as receptor for bacteriophage.

4. Sheath:

  • Some bacteria forming chain or trichome are enclosed by a hollow tube like structure known as Sheath.
  • Aquatic bacteria mostly form sheath
  • Some sheathed bacteria are; Sphaerotilus, leptothrix, clonothrix etc

Function:

  • Mechanical support
  • Sometime sheath is impregnated with ferric or manganese hydroxide which provide strength to sheath.

5. Prosthecae:

  • Prosthecae are semi-rigid extension of cell wall and cell membrane
  • One bacteria may contains one or many prosthecae.
  • Some prosthecae develop bud at the tip and hence helps in reproduction.
  • Some prosthecate bacteria are: Caulibacter, Stella, Prosthecobacter, Hyphomicrobium

Function:

  • Prosthecae increase surface area for nutrition absorption. It is usually formed in bacteria living in very dilute environment where concentration of nutrition is low.
  • Helps in adhesion
  • Asexual reproduction by budding

6. Stalk:

  • It is non-living ribbon like tubular structure.
  • It is formed by excretory product of bacteria.
  • Some stalked bacteria are: Gallionella, Planctomyces

Function:

  • Helps in attachment to solid surface.

7. Cell wall:

  • It is an important structure of a bacteria.
  • It give shape to the organism.
  • On the basis of cell wall composition, bacteria are classified into two major group ie. Gram Positive and gram negative.

Gram positive cell wall

Cell wall composition of gram positive bacteria.

  1. Peptidoglycan
  2. Lipid
  • Teichoic acid

Gram negative cell wall

Cell wall composition of gram negative bacteria

  1. Peptidoglycan
  2. Outermembrane:
    • Lipid
    • Protein
    • Lipopopysaccharide (LPS)

Peptidoglycan:

  • It consists of glycan backbone formed by repeated unit of NAG (n-acetyl Glucosamine) and NAM (N-acetyl muramic acid) and the glycan backbone is cross linked by peptide bond.
  • Peptidoglycan layer is present in cell wall of both gram positive as well as gram negative bacteria. However, gram positive have thick layer of peptidoglycan.

Teichoic acid:

  • Teichoic acid is water soluble polymer of glycerol or ribitol phosphate present in gram positive bacteria.
  • It constitutes about 50% of dry weight of cell wall.
  • It is the major surface antigen of gram positive bacteria

Outer membrane:

  • It is an additional layer present in gram negative bacteria.
  • It is composed of lipid bilayer, protein and lipo-polysaccharide(LPS)

LPS:

  • LPS is composed of lipid-A and polysaccharide.
  • Lipid-A: it is phosphorylated glucosamine disaccharide. It is antigenic
  • Polysaccharide: it consists of core-polysaccharide and O-polysaccharide.

8. Cell membrane:

  • Cell membrane is the inner layer that lies inside the cell wall and encloses the cytoplasm.
  • It is also known as cytoplasmic membrane or plasma membrane.
  • It is about 80nm thick.
  • Cell membrane of bacteria is composed of phospholipid and proteins.

Function:

  • It is selectively permeable as it allows to pass selective substances such as sugar, aminoacids across it.

9. Nucleus:

  • Nucleus is the most important part of the cell.
  • It controls and directs all the cellular activities and stores hereditary information of cell
  • Bacterial nucleus is known as nucleoid; it lacks nuclear membrane, nuceloplasm and nucleolus.
  • Bacterial DNA is naked (lacked histone protein)

Function:

  • It contains and stores hereditary information of the cell.
  • It controls all cell activities.

10. Ribosome:

  • Bacterial ribosome is of 70s type.
  • Ribosomes are rounded granules found freely floating in the cytoplasm
  • Ribosomes are known as universal cell organelle because it is found in both bacterial cell and eukaryotic cell.
  • Chemically the ribosomes are made up of nucleic acids (particularly RNA and proteins).

 Function:

  • It helps in protein synthesis

 11. Mesosome:

  • Mesosome is a spherical or round sac like structure found commonly in gram positive bacteria.
  • Function: It is the site for respiration in bacterial cell

12. Cytoplasm:

  • It is colorless, viscus fluid present inside cell membrane.
  • All the cell organelles and inclusions are found floating in cytoplasmic fluid.
  • It contains proteins, lipid, minerals, nucleic acids, glycogen, water etc.

Function:

  • It helps to distribute water, oxygen as other substances throughout the cell.
  • Literally, all the cellular content including nucleus, and other cell organelle are floating in cytoplasm.

13. Spores (endospore):

  • Spore is metabolically dormant structure produced during unfavourable condition by the process called sporulation
  • Sporulation occur during late log phase or early stationary phase
  • Under favourable condition spores germinate to give vegetative cell.

To visualize these internal structure of bacteria, we use electron microscope 

Bacterial cell Structure and Function