Learning Objective: Understand the structural components of bacteria, their chemical composition, and their functions, with emphasis on clinically relevant features (eg, capsule-mediated phagocytosis resistance, LPS endotoxin effects, β-lactamases in periplasm).
Appendages
| Structure | Chemical Composition | Function |
|---|---|---|
| Flagellum | Protein | Provides motility |
| Pilus / Fimbria | Glycoprotein | Mediates adherence of bacteria to host cell surfaces; sex pilus forms during conjugation |
Specialized Structures
| Structure | Chemical Composition | Function |
|---|---|---|
| Spore | Keratin-like coat, dipicolinic acid, peptidoglycan, DNA | Ensures survival: resists dehydration, heat, chemicals |
Cell Envelope
| Structure | Chemical Composition | Function |
|---|---|---|
| Capsule | Polysaccharide (rarely protein) | Protects against phagocytosis |
| Slime Layer (S layer) | Loose network of polysaccharides | Mediates adherence to surfaces, important in biofilm formation (eg, catheters) |
| Outer Membrane (Gram– only) | – Outer leaflet: LPS/LOS (endotoxin) – Proteins: porins, OMPs – Inner leaflet: phospholipids |
– Endotoxin (lipid A): activates TNF, IL-1 – O polysaccharide: antigenic – Porins: transport across the membrane – OMPs: antigenic |
| Periplasm (Gram– only) | Space between the cytoplasmic and outer membranes, with peptidoglycan in the middle | Stores enzymes (eg, β-lactamases) and other components exiting the cell |
| Cell Wall | Peptidoglycan sugar backbone cross-linked by peptide side chains (via transpeptidase) | Provides rigid support and protection from osmotic pressure |
Cytoplasmic Membrane
| Structure | Chemical Composition | Function |
|---|---|---|
| Cytoplasmic Membrane | Phospholipid bilayer with embedded proteins (eg, PBPs) and enzymes; lipoteichoic acids (Gram+ only) | – Site of oxidative/transport enzymes – PBPs: cell wall synthesis – Lipoteichoic acids: induce TNF-α, IL-1 |
Activity
