Bacterial structures

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.

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