deoxyhexoses and keto-sugars with different structures depending on the species and on the culture conditions. Figure 2.4 shows the LPS structures found in Salmonella typhimurium and Escherichia coli. O-antigen mutants of pathogens are less virulent and show a different phage susceptibility, suggesting that the O-antigen is involved in pathogenesis and phage recognition processes. The structure of O-antigen varies with changes in growth conditions such as temperature, and this polysaccharide may therefore protect the cell during such changes. Lipid A consists of 6-7 molecules of 3-hydroxy fatty acids bound to phosphorylated glucosarnine (Figure 2.5).