4: Bacterial Cell Structure and Type

Basic Cell Types and Shapes

Basic Cell Types

• Prokaryote: Single-celled organisms.

• Eukaryote: Single-celled or multi-cellular organisms.

• Etymology:

  • Pro = before

  • Eu = true

  • Karyon = nucleus

Similarities Between Cell Types

• DNA

• Plasma membrane

• Cell wall (in plant and fungal cells)

Common Bacterial Shapes

• Coccus

• Coccobacillus

• Vibrio

• Bacillus

• Spirillum

• Spirochete

Coccus Arrangements

• Diplococcus: Pair of cocci.

• Streptococci: Chains of cocci.

• Neisseria: Example of diplococcus arrangement.

• Streptococcus: Example of streptococci arrangement.

• Tetrad: Group of four cocci.

• Merismopedia: Example of tetrad arrangement.

• Sarcinae: Cuboidal arrangement of eight cocci.

• Staphylococci: Grape-like clusters of cocci.

Overview of Bacterial Structure

• 3 main parts:

  • Cell membrane (usually with a cell wall)

  • Cytoplasm with inclusions

  • External structures

    • Capsules

    • Flagella

    • Pili

Typical Prokaryotic Cell Structures

• Pilus (fimbria)

• Ribosomes

• Chromosome

• Inclusion

• Flagellum

• Plasmid

• Cytoplasm

• Capsule or slime layer

• Cell wall

• Cell membrane

The Cell Wall

• Located outside the cell membrane in most bacteria.

• Functions:

  • Maintains cell shape.

  • Prevents cell bursting due to osmotic pressure.

Components of Bacterial Cell Walls

• Peptidoglycan (murein):

  • Sugars:

    • N-acetylglucosamine (NAG)

    • N-acetylmuramic acid (NAM)

  • Peptide chains:

    • 4 amino acids (tetrapeptides)

Peptidoglycan Structure

• Sugar backbone consists of alternating NAG and NAM.

• Tetrapeptides are attached to NAM.

• Peptide bonds cross-link the tetrapeptides for strength.

Peptidoglycan 3D View

• Basic structural unit includes N-acetylmuramic acid, N-acetylglucosamine, and tetrapeptide.

• Cross-linking occurs between tetrapeptides.

Teichoic Acid

• Found in Gram-positive bacteria.

• Composed of glycerol, phosphates, and ribitol (sugar alcohol).

• Functions:

  • Movement of ions in/out of cell.

  • Attachment site for bacteriophages.

Outer Membrane (OM)

• Found in Gram-negative bacteria.

• Located outside the peptidoglycan layer.

• Proteins called porins form channels through the OM.

• OM has surface antigens and receptors.

Periplasmic Space

• Found in Gram-negative bacteria.

• Contains digestive enzymes.

• Gram-positive bacteria lack both an OM and a periplasmic space.

Gram-Positive Bacteria

• Cell wall structure includes:

  • Cell membrane

  • Peptidoglycan layer

  • Teichoic acid

  • Lipoteichoic acid

  • Wall-associated protein

Gram-Negative Bacteria

• Cell wall structure includes:

  • Cell membrane

  • Peptidoglycan layer

  • Periplasmic space

  • Outer membrane

  • Lipopolysaccharide

  • Porin proteins

Cell Membrane

• Separates cytoplasm from exterior.

• Composed of phospholipid molecules and proteins.

• Contains transporters.

Phospholipid Molecule

• Has a hydrophilic head and hydrophobic tails.

Fluid-Mosaic Model of the Membrane

• Lipid bilayer with proteins embedded.

• Components include:

  • Phospholipids with polar heads (hydrophilic) and fatty acid tails (hydrophobic)

  • Glycolipids (Lipid + Carbohydrate)

  • Glycoproteins (Protein + Carbohydrate)

  • Peripheral proteins

  • Integral (transmembrane) proteins

  • Channel proteins forming pores

Internal Structures

• Pilus (fimbria)

• Ribosomes

• Chromosome

• Inclusion

• Plasmid

• Cytoplasm

• Cell membrane

• Cell wall

• Capsule or slime layer

Nuclear Region (Nucleoid)

• Contains DNA (+ RNA and protein).

• Usually one circular chromosome.

• Vibrio cholerae: Two chromosomes, one large and one small.

• Plasmids: Extrachromosomal pieces of smaller, circular DNA.

Bacterial Nuclear Region

• The nucleoid is the region containing the bacterial chromosome.

Ribosomes

• Composed of RNA + protein.

• Function in protein synthesis.

• Polyribosomes: clusters of ribosomes.

• 70S in bacteria; 80S in eukaryotes.

• Streptomycin & Erythromycin bind specifically to 70S ribosomes and disrupt bacterial protein synthesis.

Inclusions

• Storage granules: glycogen or polyphosphate.

• Vesicles: gas or lipid.

  • PHB granules

  • Parasporal BT crystal in Bacillus thuringiensis

  • Carboxysomes in Anabaena viriabilis

  • Sulphur globules in Beggiatoa

Endospores

• Resistant resting structure found in bacteria such as Bacillus sp. and Clostridium sp.

• Composed of:

  • Core

  • Cortex

  • Spore coat

External Structures

• Pilus (fimbria)

• Ribosomes

• Chromosome

• Inclusion

• Flagellum

• Plasmid

• Cytoplasm

• Cell membrane

• Cell wall

• Capsule or slime layer

Flagella

• For movement.

• Types:

  • Monotrichous: single polar flagellum

  • Lophotrichous

  • Amphitrichous: two flagella, one at each end

  • Peritrichous: flagella all over the surface

  • Atrichous: Bacteria without flagella

Examples of Flagella Arrangements

• Pseudomonas

• Proteus

• Spirillum

Pili

• Short, hollow projections.

• Function in attachment.

• Conjugation pilus involved in conjugation.

• Fimbriae: type of pili.

Cell Structures Listing

• Pilus (fimbria)

• Ribosomes

• Chromosome

• Inclusion

• Flagellum

• Plasmid

• Cytoplasm

• Cell membrane

• Cell wall

• Capsule or slime layer