Untitled Quiz

Bacterial Structure

• Bacteria are classified into three basic groups: cocci (round), bacilli (rods), and spirochetes (spiral).
• Some bacteria are pleomorphic, meaning they have variable shapes.
• Bacterial morphology includes cocci in clusters, chains, pairs, or various shapes.
• Rods demonstrate square ends, round ends, club-shaped, fusiform, or comma shapes.
• Spirochetes can be tightly coiled or relaxed.

Learning Objectives

• Students should be able to define typical bacteria.
• Bacterial morphology (appearance).
• Cell wall structure (peptidoglycans).
• Cell wall variations in gram-positive and gram-negative bacteria.
• Cell membrane, mesosomes, ribosomes, and genetic material.
• Specialized structures outside the cell wall (capsule, pili, flagella, glycocalyx).

Structure of Bacterial Cells

• Bacteria are classified by shape into cocci, bacilli, and spirochetes.
• Cocci are round, bacilli are rod-shaped, and spirochetes are spiral-shaped.
• Some bacteria are variable in shape and are called pleomorphic.

Bacterial Morphology

• Cocci:
  • Clusters (e.g., Staphylococcus).
  • Chains (e.g., Streptococcus).
  • Pairs (e.g., Streptococcus pneumoniae; Neisseria).
• Rods (bacilli):
  • Square ends (e.g., Bacillus).
  • Rounded ends (e.g., Salmonella).
  • Club-shaped (e.g., Corynebacterium).
  • Fusiform (e.g., Fusobacterium).
  • Comma-shaped (e.g., Vibrio).
• Spirochetes:
  • Relaxed coil (e.g., Borrelia).
  • Tightly coiled (e.g., Treponema).

Essential Cell Structures

• Cell wall: The outermost layer (except in Mycoplasma).

  • Composed of peptidoglycan and an outer membrane.
  • Varying thickness and chemical composition between bacterial types.
  • Provides structural support and maintains cell shape.
  • Protects against osmotic pressure.

• Peptidoglycan: Also called murein or mucopeptide.

  • Found only in bacterial cell walls.
  • Provides rigid support to the cell.
  • Maintains cell shape.
  • Site of action of penicillin and related antibiotics.
  • Composed of glycan chains connected by peptide bonds.
  • Repeating sugars NAG and NAM in the glycans.
  • The glycan chains, connected by peptide bonds, are the target for lysozyme.

• Gram-positive and Gram-negative walls:

  • Gram-positive cells have a thick peptidoglycan layer (50%–80% of dry weight); gram-negative cells have a thin peptidoglycan layer (5%–10% of dry weight).

• Cytoplasmic membrane: Located inside the peptidoglycan.

  • Similar to eukaryotic membranes, but without sterols - except in Mycoplasma.
  • Contains phospholipids and transmembrane proteins.
  • Involved in molecule transport, energy generation, and secretion of enzymes and toxins

• Mesosome: Invagination of the cytoplasmic membrane.

  • Important in cell division
  • Site of DNA replication.

• Cytoplasm: The interior of the cell.

  • Contains ribosomes, nutrient granules, metabolites, and plasmids, which are in an amorphous matrix.

Ribosomes

• Bacterial ribosomes are 70S (50S and 30S subunits). Eukaryotic ribosomes are 80S (60S and 40S subunits).

Granules

• Cytoplasm contains granules that function in nutrient storage.
• The granules stain uniquely with certain dyes.

Nucleoid

• The nucleoid is the area of the cytoplasm where DNA is located.
• Prokaryotic DNA is a single, circular molecule.
• Bacterial DNA lacks a nuclear membrane, nucleolus, mitotic spindle, and histones.
• Bacterial DNA contains no introns, which differs from eukaryotic DNA.

Plasmids

• Plasmids are extrachromosomal, double-stranded, circular DNA molecules.
• They replicate independently of the bacterial chromosome.
• Some plasmids are transmissible (e.g., conjugation).
• Some are non-transmissible.
• Carry important genes, for example antibiotic resistance.

Specialized Structures

• Capsule:
  • Gelatinous layer covering the entire cell (except anthrax, which has a protein capsule).
  • Composed of polysaccharide (except in anthrax bacillus).
  • Sugar components vary between bacterial species; these variations are used in bacterial identification.
  • The capsule can protect bacteria from phagocytosis.
• Flagella:
  • Long, whip-like appendages involved in chemotaxis.
  • Location and number vary between species.
  • Useful in bacterial identification (H antigens).
• Pili (fimbriae):
  • Hairlike filaments.
  • Shorter and straighter than flagella.
  • Primarily found in gram-negative bacteria.
  • Essential in bacterial attachment to surfaces and conjugation.
• Glycocalyx (slime layer):
  • Polysaccharide coating.
  • Allows bacteria to attach firmly to surfaces.
  • Mediates adherence to surfaces
  • Plays a role in biofilm formation.

Spores

• Highly resistant structures forming in adverse conditions in certain genera. -e.g., Bacillus and Clostridium species
• Enable survival during harsh conditions.
• Thick, protective coat.

Teichoic Acid

• Found only in gram-positive bacteria.
• Involved in pathogenicity (induces septic shock).
• Site of attachment to mucosal surfaces.

Porins

• Protein structures in gram-negative bacteria cell walls.
• Form channels, allowing the passage of various substances.
• Phage receptors.

Acid-Fast and Associated Cell Wall

• Example genera include Mycobacteria and Nocardia.
• Rich in lipid, especially mycolic acids.
• Cell wall structure resists Gram staining.
• Resist decolorization with acid-alcohol; that is, they are "acid-fast."

L-form Bacteria

• Strains lack a cell wall.
• Can survive exposure to lysozyme or antibiotics.

Lipopolysaccharide (LPS)

• Part of the outer membrane of gram-negative bacteria.
• Endotoxin
• Responsible for several symptoms of disease.

Exotoxin

• Secreted by bacteria.
• Causes various effects and diseases.

Important Note

• Differences between prokaryotic and eukaryotic ribosomes are important for the development of antibiotics.