Contribution of Porphyromonas gingivalis lipopolysaccharide to periodontitis

Lipopolysaccharide is an essential macromolecule that comprises the outer surface of gramnegative bacteria. Lipopolysaccharide, recognized by the human host as a foreign molecule, elicits an immune response that is designed to eliminate the bacterial intruder. Lipopolysaccharide from enteric bacteria such as Escherichia coli provokes a strong immune response. On the molecular level this is because E. coli lipid A, the bio-active component of lipopolysaccharide, is a strong agonist of the innate immune receptor, Tolllike receptor 4 (TLR4). However, lipopolysaccharide from the periodontopathogen, Porphyromonas gingivalis, elicits a strikingly weak immune response. Additionally, P. gingivalis is capable of synthesizing a heterogeneous population of lipid A molecules, which are subtly different from each other in structure. One of these lipid A structures has the capacity to dampen the immune response elicited by lipid A agonists at TLR4. The weak agonistic and antagonistic lipid A structures of P. gingivalis are proposed to play an important role in disrupting the oral immune homeostasis required for maintaining oral health, and thereby promote periodontal disease.