The bacterial cell is a typical prokaryotic cell.
Bacterial cell types on the basis of shape:
- Coccus: spherical. E.g., Monococcus, Diplococcus, Streptococcus (chain like), Staphylococcus (in bunches) & Sarcina (cube).
- Bacillus: rod shape.
- Vibrio: comma shaped.
Gram staining of bacteria:
- Bacterial cells are stained with Gram stain (crystal violet & 5% iodine).
- Cells become blue/purple.
- Washed with 95% alcohol/acetone.
-Bacteria that retains color is Gram +ve.
-Bacteria that loses color is Gram -ve.
Prokaryotic cell structure:
- Primitive nucleus. A single circular DNA associated with polyamines is present at the center. Also, referred as genophore or incipient nucleus.
- Outer protective covering that includes cell wall, cell membrane & Glycocalyx.
- Cell wall: made of peptidoglycan (murein). It is made of alternating units of N-acetyl glucosamine & N-acetyl muramic acid. Provide rigidity to the cell & provides shape.
- Cell membrane: made of phospholipid bilayer & proteins. They act as permeability barriers. Sterol like Hopanoids present.
- Glycocalyx: made of polysaccharides.
- Capsule is thick & tough. It is impermeable & prevents bacteria from drying and phagocytosis.
- Slime layer occurs as a loose sheath. It helps in attachment of bacteria to other cells.
- In Gram +ve bacteria, the cell wall is very thick & peptidoglycan constitutes 95% of the cell wall. It lies outer to the cell membrane.
- In Gram -ve bacteria, the cell wall is thin & peptidoglycan constitutes 5 – 10% of the cell wall. Cell wall is present between an outer & an inner membrane. Periplasmic space is present between cell membrane & cell wall. Porins are present in outer membrane that help in passive transport of ions, sugars & amino acids into the periplasm.
- A gel-like matrix composed of water, enzymes, nutrients, wastes, and gases and contains cell structures such as ribosomes and plasmids.
- Plasmids: are small circular DNA present in the cytoplasm. They are self-replicative & have an independent existence.
- Ribosomes: are of 70S type.
- Inclusions: are glycogen granules, lipid droplets, pigments & crystals. Volutin granules are complex inorganic polyphosphate and are called metachromatic granules.
- Gas vacuoles are present in planktonic bacteria & Cyanobacteria. They provide buoyancy that helps in floating.
- Mesosome: are invaginations of the plasma membrane in the shape of vesicles, tubules, or lamellae. They increase the surface area of the cell membrane. They are rich in enzymes that help in cellular respiration, DNA replication, secretion of glycocalyx and cell division.
- Whip like structures that provide motility.
Distribution of flagella:
- Atrichous: flagella absent.
- Monotrichous: one flagellum.
- Amphitrichous: one flagellum at both the ends.
- Cephalotrichous: 2 or more flagella at one end.
- Lophotrichous: 2 or more flagella at both the ends.
- Peritrichous: flagella over the whole body.
- The flagellum consists of 3 components: basal body, hook & filament.
Basal body is anchored in cell wall & cell membrane. It is made of 2 rings in Gram +ve bacteria & 4 rings in Gram -ve bacteria.
Hook is a curved tubular structure. It connects filament with the basal body.
Filament is a helical structure made of protein flagellin.
- Thin bristle like structures present in the entire surface of the bacterial cell. They help in attachment.
- Hollow tube like structure made of protein pilin. They help in conjugation. Mostly found in Gram -ve bacteria.
Difference between Gram +ve & Gram -ve bacteria:
|Gram +ve||Gram -ve|
|Cell wall/ peptidoglycan layer||Thick 20 – 80 nm||Thin 8 – 12 nm|
|Lipid content||Low||20 – 30 %|
|Porins||Absent||Present in outer membrane|
|Flagella||Have 2 rings||Have 4 rings|