Bacterial endotoxins: structure, function, and biological effects

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]

Occupational Exposure

Environmental concentrations of endotoxins are measured in Endotoxin Units (EU) per m3 (based on biological activity) or ng/m3 (based on chemical activity). There are no strict regulations pertaining to permissible levels of endotoxins in the U.S. As well as the Europe. However, the international commission on occupational health has prescribed levels of endotoxins at which the toxicity is acute. Based on the report released by the commission, at concentrations of 20-50 ng/m3 endotoxin exposure leads to irritation of mucous membrane, at 100-200 ng/m3 acute brocho constriction is reported while very high levels such as 1000-2000 ng/m3 leads to organic dust toxic syndrome. In view of the high risks of occupational exposure several studies have focused on occupational contamination. The results from these studies show that the typical endotoxin concentrations at sewage processing plants, cattle sheds and grain processing units are 0 to 330 ng/m3, 1000 to 10,000 ng/m3 and 0 to 70,000 ng/m3 respectively. [Eliot Epstein]

Some studies focused on occupational health have reported increased prevalence of neurological and gastrointestinal disorders among sewage workers. Symptoms of endotoxin exposure are high among sewage workers as the human waste contains very high concentrations of endotoxins. One recent study by Smit et.al (2005) reported endotoxin exposure as one of the main causes for most of the symptoms. The study found that exposures to endotoxins at levels higher than 50 EU/m (3) was associated with more severe symptoms. 'Lower respiratory symptoms', 'skin symptoms', 'flu like and systemic symptoms' are commonly reported conditions at such levels of exposure. [Smit et.al, (2005)]