Practice question

Gram-negative and Gram-positive cell envelope architectures diverge sharply. Gram-positive envelope comprises thick peptidoglycan multilayer with interspersed wall teichoic acids and lipoteichoic acids anchored to underlying plasma membrane, without outer membrane. Gram-negative envelope contains thin peptidoglycan layer sandwiched in periplasm plus additional outer membrane asymmetric membrane whose outer leaflet is almost exclusively lipopolysaccharide and inner leaflet phospholipid, harboring porins, Bam complex, and Lpt transport machinery. Peptidoglycan itself, plasma membrane composed of ester-linked phospholipids, and lipoteichoic acids are not exclusive; peptidoglycan occurs in both but differs in thickness, plasma membrane is universal among cellular life, lipoteichoic acids are Gram-positive signature. Outer membrane is absent from Gram-positives, archaea, and eukaryotes, representing evolutionary innovation of Gram-negative lineage that confers intrinsic resistance to lysozyme, detergents, and many antibiotics, while also providing additional barrier requiring specialized protein folding chaperones like SurA and Skp in periplasm for outer membrane protein biogenesis and transport. Cryo-electron tomography reveals outer membrane is densely packed with lipopolysaccharide stabilized by ionic crosslinks, and its biogenesis demands coordinated transport of proteins and lipids from inner membrane, making outer membrane assembly a target for novel Gram-negative specific antibiotics like darobactin that inhibits Bam complex folding.