Chemical structure and immunobiological activity of lipid A from Serratia marcescens LPS

The chemical structure and immunobiological activities of Serratia marcescens lipid A, an active centre of LPS, were investigated. LPS preparations of S. marcescens were extracted using a hot phenol/water method, after which purified lipid A specimens were prepared by weak acid hydrolysis, followed by normal phase and gel filtration chromatographic separation. The lipid A structure was determined by MS to be a diglucosamine backbone with diphosphates and five C(14) normal chain acyl groups, including two acyloxyacyl groups at the 2 and 3 positions of the non-reducing side. S. marcescens lipid A and Escherichia coli-type synthetic lipid A (compound 506) exhibited definite reactivity in Limulus amoebocyte lysate assays. The lethal toxicity of S. marcescens lipid A was nearly comparable to that of compound 506, and both induced nuclear factor-kappaB activation in murine cells via Toll-like receptor (TLR)4/MD-2 but not TLR2, as well as various inflammatory cytokines in peritoneal macrophages of C3H/HeN mice but not C3H/HeJ mice. Furthermore, S. marcescens lipid A induced nearly the same amounts of tumour necrosis factor alpha, interleukin-6, and nitric oxide production by the murine alveolar macrophage cell line MH-S as compared with compound 506. These results indicate that S. marcescens possesses a penta-acylated lipid A, which is nearly identical to E. coli lipid A in regard to biological activities, while it also may be a crucial virulence factor of the bacterium.