Bacterial Cell Structures & Functions

Bacterial Cell Structure

Bacterial Cell Structure

• Surface layers -Cell wall, cell membrane,

capsule

• Appendages - Flagella, pili or fimbriae

• Cytoplasm - Nuclear material, ribosome,

mesosome, inclusions etc.

• Special structure - Endospore

Cell envelope• Various layers-Collectively cell envelope

• Gram positive- plasma memebrane, cell wall sometimes capsule

• Gram negative- plasma memebrane, cell wall , outer membrane sometimes capsule

• Plasma membrane in gram negative bacteria is sometimes called inner membrane

• Space between inner membrane and outer membrane is called Periplasmic space.

Bacterial Cell Wall:

•10-25nm in thickness, Neg 10-15nm, Pos 20-25nm

Functions

•Accounts shape of the cells

•Provides protection of the cells against Osmotic damage

•Confers rigidity

•Takes part in cell division

•Target site for antibiotic

•Carries bacterial antigens- virulence & immunity

• General structure:

• Chemically made up of Peptidoglycan.

• It is made by two hexose sugars

N- acetylglucosamine [NAG] and

N- acetylmuramic acid [NAM]

in alternating chains interconnected by tri, tetra or penta pedtide chains.

Gram positive cell walls:

a. Peptidoglycan-Thicker in gram positive

b.Polysachharides –Teichoic acids- polymer of glycerol and ribitol phosphates

• Some gram positive bacteria eg Mycobacteria contain lipid- Mycolic acids

Gram negative cell walls• Complex structure

A. Lipoprotein layer- connects the peptidoglycan to outer membrane.

B. Outer membrane- Outer membrane proteins-target site for antibiotics.

C. Lipopolysachharides- This layer consists of lipid A to which is attached a polysachharide .

D. Periplasmic space- Inner and outer membrane.

E. Peptidoglycan

Cytoplasmic membrane• 5-10nm thick, elastic semipermeable layer

which lies beneath cell wall

• Chemically consists of phospholipids and protein molecules

• Acts as osmotic barrier

• Consists of enzymes permease, oxidase and polymerase

• Contains enzymes of tricarboxylic acid cycle and enzyme necessary for cell wall synthesis.

• Bacterial electron transport system

Cytoplasm

• Organic and inorganic solutes, water

• Lacks mitochondria and endoplasmic reticulum etc

• Contains ribosomes, mesosomes, vacuoles and inclusions.

Ribosomes• Centre of protein synthesis

• Composed of ribosomal RNA and robosomal proteins

• Two subunits 50s and 30s - 70s

Mesosomes• Centre for respiratory enzymes

• Septal and lateral

• Septal attached to bacterial chromosome involved in DNA segregation and formation of cross wall during binary fission.

Inclusions• Sources of stored energy.

• May be present as polymetaphosphate,lipids and polysachharides and granules of sulphur.

Nucleus• No nuclear membrane and nucleolus.

• Dna doesn’t contain any basic proteins.

• Genomic DNA is double stranded in the form of circle.

Plasmids• Small circular covalently closed double

stranded DNA molecules found in cytoplasm.

• Not essential for life , confer on certain properties like drug resistance and toxigenecity.

• Can be transmitted from one bacteria to another by conjugation or by bacteriophage.

CAPSULE AND SLIME LAYER

• Amorphous viscid bacterial secretion surrounding the bacteria

• Loose undemarcated secretion-slime layer• Sharply defined structure – capsule• Very thin- microcapsules• Protects bacteria against

phagocytes,adherence promote virulence, reservoir of food,

• Demonstrated by negative staining and capsule swelling reaction [ Quellung reaction].

Flagella• Cytoplasmic appendages protruding through cell wall.• Thread or hair like structure- protein flagellin• Organ of locomotion• All motile bacteria except spirochaetes

• Parts:1.Basal body: Embedded in cell envelope & consists of

small,central rod surrounded by a series of rings2. Hook :Connects basal body with the filaments3.Filament or shaft: External to cell surface Composed of protein molecule flagellin

Organ of bacterial locomotion

• Structure

• Gram negative- 2 pair of rings-

• M -Plasma membrane

• S -periplasmic space

• P- peptidoglycan

• L- lps

• Gram positive- 1 pair-

• M- Plasma membrane

• S - peptidoglycan

Structure of the flagellum

• Parts:

1. Basal body: Embedded in cell envelope & consists of small , central rod surrounded by a series of rings

2. Hook :Connects basal body with the filaments

3. Filament or shaft:

External to cell surface

Composed of protein molecule - flagellin

• Arrangements/Types

Demonstration:

• Electron microscopy

• Silver impregnation methods

• Dark field microscopy

• Special stains eg Leifsons stain

Fimbriae• Hair like appendages projecting from cell

surface as straight filaments.

• Also called pili

• 0.1-1um length and 10nm thick

• Gram negative bacteria

• Protein pilin

• Best seen in liquid cultures

• Antigenic

• E. coli fimbriae

• Types

1. Common pili- Adhesion to host cells

2. Sex pili or F fertility pili-

Found on male or donor or + strains help in attachment to female or recipient or – strains through conjugation tubes and aid in gene transfer.

Functions

Adhesion, Transfer of genetic materials

Demonstration

Electron microscopy, Haemagglutination

Bacterial spores• Highly resistant resting stages formed in

unfavourable condition

• Formed inside the cells so called endospores

• Each form one spore, which on germination form a single vegetative cell

• Non metabolising and non reproducing

• Highly resistant to heat, UV radiation, mechanical disruption, chemical disinfectants etc

• Structure

• The core of the fully developed spore has homogenous protoplasm, containing chromosome, enzymes of glycolysis and protein synthesis.

• Core is surrounded by spore walls or inner membrane.

• Outside this spore wall is thick layer the cortex enclosed by outer membrane.

• Spore coat surrounds this spore wall.

• Some bacteria has additional loose outer covering Exosporium.

• Demonstration

• Gram stain- unstained.

• AFB stain- 0.25-0.5% H2So4- Red colour.

• Use

• Spores of bacillus stearothermophilus are employed as indicator of proper sterilisation.