Gram-negative bacteria are widely used for the production of gene-based products such as DNA vaccines and bio-drugs, where endotoxin contamination can occur at any point within the process and its removal is of great concern. In this article, we review the structures of endotoxin and the effects that it causes in vivo. The endotoxin removal strategies are also discussed in the light of the different interaction mechanisms involved between endotoxins and bioproducts particularly plasmid DNA and proteins. For most cases, endotoxin removal is favoured at a highly ionic or acidic condition. Various removal methods particularly chromatography-based techniques are covered in this article according to the relevant applications. 1. Introduction Gram-negative bacteria are widely used in the biotechnology industry for recombinant DNA production, where endotoxin contamination can occur at any point within the processes [1]. Endotoxins must be removed from proteins prepared from Gram-negative bacteria prior to its administration into the human and animal bodies to avoid any adverse side effect [1]. Many purification methods have been developed for endotoxin removal, including LPS affinity interactions, two-phase extractions, ultrafiltration, affinity chromatography and anion exchange chromatography [2]. The use of tailor-made endotoxin-selective adsorbent matrices for endotoxin removal is reported elsewhere [3]. The selection of a suitable endotoxin removal system is based on the properties of the bioproducts being purified. The interaction between the anionic phosphate in LPS and the cationic ligands on the sorbents are mostly utilised as the mechanism of endotoxin removal [4]. Anion exchange and affinity chromatography are based on cationic functional ligands such as diethylaminoethanol, histidine, polymyxin B, poly (ε-lysine), and poly (ethyleneimine) [4]. Hydrophobic interactions between the lipid A portion and sorbent are also considered to be important attributes that removal techniques can take advantage of [4]. Endotoxin molecules tend to form micelles or vesicles in aqueous solution [5]. Due to the difference in sizes of endotoxins and water as well as salt and other small molecules in protein-free solutions, ultrafiltration can be employed. In the presence of proteins, affinity chromatography and two-phase extraction methods can take advantage of the physical-chemical interaction between endotoxin and protein to completely remove endotoxin [6]. Detergents can be used to separate endotoxin from a protein surface, however an additional step is required to
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