Lipopolysaccharide (LPS)

Schematic image of a lipopolysaccharide (LPS). Variations in structure occur in different bacterial species. The core can be divided into an inner and an outer part. The inner portion consists of 1-3 KDO residues (keto deoxy octulosonate) and 2-3 heptose residues. The outer part consists of several different hexose residues (e.g. D-glucose, D-mannose and D-galactose).

Bild: Karl-Erik Johansson (BVF, SLU) - Click on the image to enlarge it.

Introduction

Lipopolysaccharides (LPS), which are also known as lipoglycans, are only present in the outer membrane of gram negative bacteria. LPS is a so-called endotoxin (se även toxin nedan) and the toxicity is associated to lipid A (see below).

Structure

LPS is an amphipathic molecule, i.e. it has both hydrophilic (water loving) and hydrophobic (water repellent = lipid loving) regions. The hydrophobic portion (hydrocarbon chains) anchors LPS in the outer lipid layer of the bacterial outer membrane and the hydrophilic region (charged groups) points outward against the bacterial environment. Chemically, LPS consists of a lipid moiety and a polysaccharide moiety. The lipid moiety comprises lipid A, which is a phosphorylated glucosamine disaccharide with several (4-6) linked hydrocarbon chains, which constitute the hydrophobic portion of the molecule. The carbohydrate moiety consists of a so-called core oligosackaride (core antigen or R antigen), which is directly bound to lipid A. The polysaccharide (O-polysaccharide, O-antigen or somatic antigen) is bound to the R antigen.

Specificity

Thanks to the diversity, found in LPS from various gram-negative bacteria, its antigenic properties can be used for typing and subtyping of bacteria. The antigenic properties will also vary in different parts of LPS:

Lipid A is often family-specific. In for instance the family Enterobacteriaceae, there is almost no variation in the structure of lipid A.
The R antigen is often specific to the genus, i.e. no significant variation occurs in this part within a single bacterial genus.
The O-antigen is not even species specific, i.e. considerable variation occurs even within a species and this provides opportunities for subtyping by serological methods. Thus, one can distinguish between different strains of the same species, which makes it possible to use serology for epidemiological studies.

Effects on the host animal

Animals (including humans) are constantly exposed to small amounts of LPS in the blood circulation because of the turnover of intestinal gram negative bacteria and thus the innate immune system is contiuously stimulated. Exposure to large amounts of LPS, as during for instance sepsis, will cause release of cytokines, leading to fever and inflammation. At toxic concentrations of LPS, blood clots form in the capillary system, which in turn can lead to life-threatening conditions.