Microbiology: Gram-Positive Bacteria and Microbiomes

Study Notes

A gram-positive bacterium has a thick, multilayered cell wall consisting mainly of peptidoglycan (150 to 500 Å) surrounding the cytoplasmic membrane.
The peptidoglycan is a meshlike exoskeleton that allows diffusion of metabolites to the plasma membrane.
The peptidoglycan is essential for structure, replication, and survival in hostile conditions.
Lysozyme, an enzyme found in human tears and mucus, can degrade peptidoglycan, leading to bacterial lysis.
The gram-positive cell wall may include other components such as proteins, teichoic and lipoteichoic acids, and complex polysaccharides.
Virulence proteins, such as the M protein of streptococci and protein A of S. aureus, are covalently bound to the peptidoglycan.
Teichoic acids are water-soluble anionic polymers of polyol phosphates that are covalently linked to the peptidoglycan and essential to cell viability.
Lipoteichoic acids have a fatty acid and are anchored in the cytoplasmic membrane.

Gram-negative cell walls are more complex than gram-positive cell walls, both structurally and chemically.
The gram-negative cell wall contains a thin peptidoglycan layer (5-10% of the cell wall by weight) and an outer membrane.
There are no teichoic or lipoteichoic acids in the gram-negative cell wall.
The outer membrane contains a periplasmic space with transport systems for iron, proteins, sugars, and other metabolites, as well as hydrolytic enzymes.
The periplasmic space contains virulence factors such as collagenases, hyaluronidases, proteases, and β-lactamase in pathogenic gram-negative species.
The gram-negative cell wall also contains type I to V secretion devices that provide mechanisms for the uptake and release of different metabolites and other compounds.