Structure and Function of Bacteria

Overview

• Main Topics: Structure of bacteria, function of bacterial components, gram staining procedure.
• Important Structures: Appendages (flagella, fimbre, pilous), cell envelope, ribosomes, DNA structures (chromosome, plasmid), specialized structures (endospores).

Bacterial Structure

Appendages

• Flagella: Major role in motility. Powered by ATP and composed of:

  • Basal body
  • Hook
  • Filament

• Configurations:

  • Monotrichous: Single flagella (e.g., Vibrio cholerae)
  • Lophotrichous: Multiple flagella at one end
  • Amphitrichous: Flagella at both ends
  • Peritrichous: Flagella around entire surface (e.g., E. coli)

• Fimbre vs. Pilous:

  • Fimbre: Shorter, thinner, more numerous, formed from bacterial chromosome, used for attachment.
  • Pilous: Fewer, longer, thicker, formed from plasmid, involved in bacterial conjugation.

Specialized Structures

• Endospores: Found in Clostridium and Bacillus species. Allow bacteria to withstand harsh environments (high temp, UV radiation, chemicals).

Cell Envelope

• Components:
  • Capsule/Slime layer (glycocalyx)
  • Outer membrane (only in gram-negative bacteria)
  • Cell wall
  • Periplasmic space (only in gram-negative bacteria)
  • Inner membrane

Capsule vs. Slime Layer

• Capsule: Organized polysaccharide layer acting as a virulence factor (reduces phagocytosis).
• Slime Layer: Looser polysaccharide layer aiding in adherence to surfaces.

Outer Membrane

• Only present in gram-negative bacteria.
• Contains endotoxin (lipopolysaccharide - LPS): Lipid A, core polysaccharide, O antigen.

Cell Wall

• Made of peptidoglycans (N-acetylmuramic acid and N-acetylglucosamine).
• Function: Provides shape and integrity, found in gram-positive and gram-negative bacteria.
• Thickness: Thicker in gram-positive bacteria.
• Contains lipoteichoic acid in gram-positive bacteria.

Periplasmic Space

• Only in gram-negative bacteria.
• Contains beta-lactamase, aiding antibiotic resistance.

Inner Membrane

• Present in both gram-positive and gram-negative bacteria.
• Contains penicillin-binding proteins (important for antibiotic action).

Gram Staining Procedure

Steps

1. Apply bacteria to slide and heat fix.
2. Stain with crystal violet.
3. Apply iodine (mordant).
4. Wash with ethanol (decolorizer).
5. Counterstain with safranin.

Results

• Gram-positive bacteria: Retain crystal violet (purple)
• Gram-negative bacteria: Stain with safranin (pink)
• Atypical bacteria: Do not stain clearly (e.g., Mycoplasma, Chlamydia)

Atypical Bacteria

• Mnemonic: These Atypical Microbes Usually Lack Color Because Microbes Barely Eat Ramen.
• Includes: Trypanema, Anaplasmosis, Mycoplasma, Ureaplasma, Leptospira, Legionella, Chlamydia, Bartonella, Mycobacteria, Borrelia, Ehrlichia, and Rickettsia.

This guide covers the critical elements of bacterial structure, their functional components, and the gram staining process, providing a comprehensive understanding of the subject.