Which antibiotic is most effective against Gram-positive bacteria?
Gram-positive bacteria possess a uniquely thick, exposed mesh of peptidoglycan often 20 to 80 nm composed of many layers of glycan chains highly cross-linked, lacking an outer membrane that would otherwise act as permeability barrier. Vancomycin is a large hydrophilic glycopeptide of about 1449 Da with multiple aromatic rings and sugars, physicochemical properties that prevent efficient passage through porin channels and lipopolysaccharide outer membrane of Gram-negative bacteria but allow ready diffusion through porous peptidoglycan of Gram-positives. It binds non-covalently via five specific
Ref: Prescott's Microbiology, 11th ed., Chapter 8: Gram-Positive Cell Wall and Vancomycin Activity.
