2-Bactirial Structures

Bacterial Structures

Overview

• Bacterial structures play essential roles in survival, pathogenicity, and cellular function.

• Major structures include the cell wall, capsule, plasmid, ribosomes, cell membrane, and flagella.

Key Bacterial Structures

• Chromosome: Carries genetic information, typically found in a single, circular form.

• Pilus (Fimbriae): Hair-like appendages used for attachment and conjugation.

• Ribosomes: Sites of protein synthesis, smaller than eukaryotic ribosomes (70S vs 80S).

• Inclusion Bodies: Storage sites for nutrients and other material in the cytoplasm.

• Flagellum: Structures used for motility, capable of rotation and movement towards stimuli.

• Capsule or Slime Layer: A gelatinous layer that protects bacteria and aids in adherence.

• Cell Wall: Provides structure and protection, composed mainly of peptidoglycan.

• Plasmid: Small, circular DNA molecules that can confer antibiotic resistance.

• Cell Membrane: A phospholipid bilayer that regulates material entering and exiting the cell.

Bacterial Cell Wall

Characteristics

• Outer layer composed primarily of peptidoglycan, providing shape and rigidity.

• Contains both D- and L-amino acids, critical for its structure.

Peptidoglycan Structure

• Composed of alternating units of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM).

• Cross-linked by oligopeptides, contributing to overall strength.

Gram Staining

• Bacteria categorized as Gram-positive or Gram-negative based on their ability to retain the dye during the Gram stain process.

  • Gram-positive: Thick peptidoglycan layer, retains crystal violet dye (appears purple).

  • Gram-negative: Thin peptidoglycan layer, cannot retain dye (appears red after counterstaining).

Capsule and Slime Layer

Definitions

• Capsule: A well-defined, dense layer that protects against phagocytosis and aids in adherence.

• Slime Layer: A loose, unorganized layer that helps prevent desiccation and aids in biofilm formation.

Functions

• Capsules are determinants of virulence, enhancing bacterial survival against immune responses.

• Example: Streptococcus mutans uses its capsule for adherence to tooth enamel, contributing to caries formation.

Flagella

Structure and Function

• Flagella are long, whiplike appendages used for locomotion via chemotaxis.

• Comprised of the protein flagellin, enabling rotary motion powered by ion movement across the membrane.

Arrangement of Flagella

• Monotrichous: A single flagellum at one end.

• Lophotrichous: Multiple flagella at one end.

• Amphitrichous: One flagellum at each end.

• Peritrichous: Flagella distributed all around the bacterium.

Endospores

Characteristics

• Dormant structures formed under stress conditions.

• Highly resistant to heat, radiation, and chemical disinfectants.

Formation Process (Sporulation)

1. Nutrient depletion triggers spore formation.

2. DNA and cytoplasm are isolated and surrounded by a thick protective coat.

Germination

• Endospores can remain dormant for years, activating upon nutrient availability.

Pili

Structure

• Thin appendages made of pilin protein, longer than fimbriae but shorter than flagella.

• Primarily found on Gram-negative bacteria.

Importance in Pathogenicity

• Mediates bacterial adherence to host tissues.

• Involved in the process of conjugation for DNA transfer.

Lipopolysaccharides (LPS) in Gram-negative Bacteria

Structure

• Composed of a lipid and polysaccharide; major component of the outer membrane.

  1. O-antigen: Polysaccharide side chains varying among strains.

  2. Core polysaccharide: Attached to lipid A, necessary for structure and function.

  3. Lipid A: Anchors LPS in the membrane and is responsible for toxicity.

Functions

• LPS acts as an endotoxin, triggering strong immune responses in hosts.

• Important in bacterial identification and pathogenic mechanisms.

Acid-Fast Bacteria

Characteristics

• Genera like Mycobacterium have an unusual, lipid-rich cell wall that cannot be Gram stained.

• Acid-fast due to resistance to decolorization after staining, commonly tested using Ziehl-Neelsen stain.

Composition

• High lipid content including mycolic acids, which contribute to the bacterium's virulence.

Mycoplasma

• A genus that lacks a cell wall; resistant to antibiotics targeting cell wall synthesis.

• Can cause various human diseases, including atypical pneumonia.