Structure and function of Salmonella inner membrane.

The Salmonella inner membrane is not just a selectively permeable barrier. It is also a working platform where transport, energy metabolism, protein export, signal transduction, envelope biogenesis, and host adaptation are brought together. In this review, inner membrane proteins are therefore not discussed only as isolated structural units. We use representative transporters, respiratory enzymes, secretion systems, sensor kinases, envelope assembly factors, and phospholipids to show how these modules can work with one another under infection-related stress. Protein secretion covers Sec/Tat translocation as well as type III secretion systems (T3SSs), because these routes together link protein export with envelope assembly, motility, invasion, and intracellular survival. ATP-binding cassette (ABC) importers and secondary transporters help Salmonella take up nutrients, metals, ions, and osmoprotectants, whereas efflux systems and lipid transporters help Salmonella cope with antibiotics, bile, antimicrobial peptides, and small molecules from the host. Respiratory complexes and F₀F₁-ATPase translate changing oxygen and electron-acceptor conditions into proton motive force and ATP, which then feed back into transport, secretion, and stress tolerance. At the same time, lipid A/lipopolysaccharide (LPS), phospholipids, peptidoglycan, and cell division pathways shape envelope integrity, immune recognition, antimicrobial peptide resistance, and intracellular survival. These effects are often condition-dependent, so drug resistance and virulence phenotypes need to be read in the context of host niche, stress conditions, growth state, strain background, and compensatory regulation. Viewed in this way, the Salmonella inner membrane is a coordinated adaptive network, and this view can help identify antibacterial targets that weaken bacterial adaptation rather than simply blocking one isolated protein.