Pathophysiology
Clinical meaning
Bacterial cell structure is foundational to understanding antibiotic mechanisms and resistance. The cell envelope differs dramatically between gram-positive and gram-negative organisms, directly impacting treatment strategies. Gram-positive bacteria possess a thick peptidoglycan layer (20-80 nm) composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues cross-linked by short peptide bridges. This thick mesh provides structural rigidity but is porous to large molecules, making gram-positive organisms accessible to many antibiotics. Teichoic acids (wall teichoic acids and lipoteichoic acids) thread through the peptidoglycan and anchor to the cytoplasmic membrane, contributing to cell wall charge, cation homeostasis, and phage receptor function. Teichoic acids also serve as adhesins for host tissue colonization and stimulate inflammatory cytokine release. Gram-negative bacteria have a thin peptidoglycan layer (1-3 nm) sandwiched between an inner (cytoplasmic) membrane and an outer membrane. The outer membrane is an asymmetric lipid bilayer: the inner leaflet contains phospholipids while the outer leaflet is composed primarily of lipopolysaccharide (LPS). LPS consists of three domains: Lipid A (the endotoxin component embedded in the outer membrane that triggers TLR4-mediated septic shock signaling via the NF-kB pathway), the...