| STRUCTURE | CHEMICAL COMPOSITION | FUNCTION |
| Appendages | ||
| Flagellum | Proteins. | Motility. |
| Pilus/fimbria | Glycoprotein. | Mediate adherence of bacteria to cell surface; sex pilus forms during conjugation. |
| Specialized structures | ||
| Spore | Keratin-like coat; dipicolinic acid; peptidoglycan, DNA. | Gram ⊕ only. Survival: resist dehydration, heat, chemicals. |
| Cell envelope | ||
| Capsule | Organized, discrete polysaccharide layer (except poly-d-glutamate on B anthracis). | Protects against phagocytosis. |
| Glycocalyx | Loose network of polysaccharides. | Mediates adherence to surfaces, especially foreign surfaces (eg, indwelling catheters). |
| Outer membrane | Outer leaflet: contains endotoxin (LPS/LOS).
Embedded proteins: porins and other outer membrane proteins (OMPs) Inner leaflet: phospholipids. |
Gram ⊝ only.
Endotoxin: lipid A induces TNF and IL-1; antigenic O polysaccharide component. Most OMPs are antigenic. Porins: transport across outer membrane. |
| Periplasm | Space between cytoplasmic membrane and outer membrane in gram ⊝ bacteria. (Peptidoglycan in middle.) | Accumulates components exiting gram ⊝ cells, including hydrolytic enzymes (eg, β-lactamases). |
| Cell wall | Peptidoglycan is a sugar backbone with peptide side chains cross-linked by transpeptidase. | Net-like structure gives rigid support, protects against osmotic pressure damage. |
| Cytoplasmic membrane | Phospholipid bilayer sac with embedded proteins (eg, penicillin-binding proteins [PBPs]) and other enzymes. Lipoteichoic acids (gram ⊕ only) extend from membrane to exterior. | Site of oxidative and transport enzymes; PBPs involved in cell wall synthesis. Lipoteichoic acids induce TNF-α and IL-1. |