Bacterial endotoxin structure, mechanism of action, and testing methods

Bacterial Endotoxins

Endotoxins are nothing but complex polysaccharide molecules present in the outer cell membrane of gram-negative bacteria. Endotoxins are a structural component of the bacteria and they are only released when the bacteria are lysed. This differs from exotoxins that are secreted by live bacteria. From organic dust in the poultry, cotton processing, sewage processing, grain processing and vegetable dust to home dust and air conditioning, endotoxins are everywhere. Endotoxins are found in high concentrations in damp areas and thus stagnant water and sewage are high-risk zones for endotoxin exposure. Unlike exotoxins which can be destroyed with heat endotoxins are found to be heat stable. Boiling them does not destroy their structure. Endotoxins are harmful for humans and animals inducing pyretic and severe inflammatory responses. Respiratory distress due to severe inflammation of the lungs is a commonly observed reaction to exposure to high concentrations of endotoxins in the air. Asthma, Toxic pneumonitis and organic dust toxic syndrome are some of the adverse health conditions caused by acute endotoxin exposure. Occupational exposure to endotoxins is therefore a serious issue. [Eliot Epstein]

Endotoxin structure and Mechanism of Action

The LPS contains a polysaccharide and a lipid Moitey that is commonly known as Lipid a. The Lipid a consists of phosphorylated N-acetylglucosamine (NAG) with 5 or 6 saturated fatty acids. The structure of the Lipid a component is preserved in all species of gram-negative bacteria. It is found that the virulence and of the endotoxins vary vastly based on the changes in the arrangement of the polysaccharides (changes in the oligosaccharide subunits.). The variations in these polysaccharide chains also result in a variety of antigenic types. Therefore, even though humans produce antibodies, they are only restricted to the particular polysaccharide chain, leaving them unprotected to a wide variety of endotoxins. The mechanism of action of endotoxins in humans is complex. The LPS binds to the LBP (lipid binding protein) in the serum, which transfers it to the CD14, which further transfers it to the MD2 protein and finally the Toll Like receptor 4 (TLR4). This association with TLR4 triggers the inflammatory response by the secretion of cytokines and nitrous oxide that further result in the secretion of prostaglandins and leukotrienes. This is followed by the activation of the coagulation cascade resulting in the person experiencing inflammation, intravascular coagulation, hemorrhage and shock. [Kenneth Todar]

Tests for endotoxins are done to ascertain the concentration of endotoxins in the samples. The Limulus Amebocyte Lysate test is one of the standard tests for endotoxins. It was observed that Limulus polyphemus, the horseshoe crab, developed intravascular coagulation upon infection by gram-negative bacteria. Later it was discovered by researchers Levin and Bang that this reaction was triggered by the activation of several enzymes located in the blood cells (amebocytes) of the crab by the endotoxins in the gram-negative bacteria. Since then, this test using the LAL reagent extracted from the crab, is found to be the most accurate and sensitive test for endotoxins. Modifications and variations such as the gel clot technique; turbidimetric assay and the chromogenic assays are available. [ACC]

Conclusion

Endotoxins are ubiquitous. There is certainly the possibility of a major health risk due to endotoxic contamination and exposure. Particularly, for people living in flooded areas and damp zones and for those who are occupationally exposed to higher concentrations of endotoxins (grain processing units, sewage farms, poultry, etc.) the risk of developing adverse health conditions is very high. As the Conference of Governmental Industrial Hygienists (ACGIH) recommends, any concentration that is more than ten times that of the background levels is considered hazardous [NRDC]and it is incumbent upon industrialists and corporates to ensure that the working conditions do not exceed exposure levels that can be seriously harmful.