Figure 1:

The Gram-negative cell envelope, consisting of an OM, an IM, and a thin peptidoglycan layer within the periplasmic space between the two membranes. The OM is an asymmetric phospholipid bilayer, with the outer leaflet modified by lipopolysaccharides (LPSs) that form the core structure for the permeability barrier and with the inner leaflet made up of phospholipids. Multiple porin proteins are embedded in the OM, functioning as channels for transport of nutrients and metabolites. The IM is a symmetric phospholipid bilayer that anchors proteins of electron transport chain complexes (e.g., NADH dehydrogenase complex, cytochrome bc₁ complex, and cytochrome oxidase complex), and F_OF₁ ATP synthase for generation of ATP. The major drug efflux AcrAB-TolC complex consists of the proton-dependent AcrB pump located at the IM, the channel protein TolC at the OM, and the bridging AcrA protein.