ABSTRACT
I. Introduction 305
II. Envelope of the Gram-Negative Bacteria 311
III. Chemical Structure of Lipopolysaccharide 313 A. Polysaccharide component 314 B. Lipid component 315
IV. Lipopolysaccharide Biosynthesis 318 A. Lipid A 319 B. Core oligosaccharide 320 c. a-specific chain 320
V. Extraction of Lipopolysaccharide 322
VI. Physicochemical Properties of Lipopolysaccharides 323 VII. Biology of Lipopolysaccharides 324
I. INTRODUCTION Because of the variety of unusual lipids found in bacteria, there is a great interest in studying these organisms as a potential source of microbial fats and oils. Bacterial lipids differ both quantitatively and qualitatively from the lipids of higher forms of life. Bacteria generally do not contain sterols because of their inability to form the steroid ring. They are, however, capable of synthesizing
polyisoprenoids, which are found in quinone coenzymes, carotenoids, and undecaphenol phosphates (needed for peptidoglycan and lipopolysaccharide synthesis). Unlike plants and oleaginous species of yeasts and molds, bacteria do not accumulate triacylglycerols but instead produce specialized lipids like poly-8-hydroxybutyrate (PHB) or wax esters. Wax esters, like their animal and plant counterparts sperm whale oil and jojoba oil, are of interest in health care products and in the production of high-temperature and high-pressure lubricants [1]. Although high contents of triacylglycerol are found in some species of Actinomycetales (probably 80% of total lipids), bacteria still remain an unpopular source of edible lipids because of the presence of other complex lipids (some of which are potentially toxic) that make it difficult to extract and purify the desirable lipid component [2]. However, they are currently being studied as producers of biosurfactants [3]. As an example, the glycolipid, dimycolyltrehalose, isolated from these organisms has been studied for its potential application as a surfactant [4]. This lipid material contains complex C88 fatty acids (mycolic acids) attached to the disaccharide trehalose. Emulsan (an extracellular protein-associated lipopolysaccharide), produced by the bacterium Acinetobacter calcoaceticus, has been used for cleaning large oil tanks and tankers [2]. Another commercial application involving bacterial lipids is the intracellular biodegradable homopolymer of D-3-hydroxybutyrate, or PHB, produced principally by species ofAzotobacter, Pseudomonas, and Alcaligenes. PHB offers an advantage over the chemical polymers, such as polyethylene and polypropylene, because it is biodegradable and has the ability to deemulsify both oil-in-water and water-in-oil emulsions.
