The O-specific polysaccharide is a heteropolysaccharide consisting of a chain of repeating oligosaccharide units (from 3 to 8 monosaccharides each) which are strain specific and determining for the serological identity of the respective bacteria .O-polysaccharides are located on the external surface exposed to the external environment of the bacteria. 1–8 glycosyl residues can be observed in the O-polysaccharide region as repetitive units among various Gram-negative strains. These sugars vary in their types, sequences, substitutions, chemical bonds, ring shapes, substitutions, presence or absence of non-carbohydrate moieties, etc., giving a heterogeneous nature to O-polysaccharides from different Gram negative stains (Erridge et al ., 2002). diverse and specific arrangements using various sugar monomers in the O-polysaccharide of LPS generate hundreds of distinct patterns or serotypes for each Gram-negative bacterial strain in nature. There are approximately 1 to 50 well-represented subunits (repeating units) to complete the O-polysaccharide chain (Erridge et al., 2002). Each subunit consists of three to eight sugar units and there can be up to fifty identical subunits in an O chain. Some bacteria have shorter O chains on average than others. Smooth-type Salmonella species fall into this category. In some cases, the last sugar unit at the non-reducing end of the O chain carries a substituent that blocks other addition subunits acting as a terminal signal. Sugar monomers in repeating subunits of the O-polysaccharide chain can be linear or branched homopolymers (with a single monosaccharide component) or heteropolymers in which it is frequently observed. A particular Gram negative strain can produce multiple O-polysaccharide chain lengths...... middle of paper......viability of the microorganism nor the biological potency of LPS. Deoxysugars are frequent components in O-chain structures, but sugars more characteristic of the inner core region such as heptose are rarely present. The most common substituents are O- and N-acetylphosphate and phosphorylethanolamine. Amino acids in amide bonds, acetamidino groups as well as formyl groups and glyceric acid are often found as non-stoichiometric substituents (Brade et al., 1999). The extended O-polysaccharide chains extending from the bacterial outer membrane act as a shield that allows bacteria to move away from the lytic activity provided by the complement cascade. Many Gram-negative strains require O-polysaccharide as an essential component for survival in the host system because it prevents attack by the complement membrane attack complex (Joiner et al.., 1984).