TLR4 and bacterial infectious disease: As the major lipid species coating the outer surface of Gram-negative bacteria, lipid A is the predominant signaling molecule that is detected by the mammalian Toll-like receptor 4 (TLR4)/myeloid differentiation factor 2 (MD-2) innate immune receptor (1) and caspase-4/-5/-11 (2) to trigger the host innate immune response to bacterial infection.