Three genes, IgtF, Iic2C and IpsA, have a primary role in determining the pattern of oligosaccharide extension from the inner core of Haemophilus influenzae LPS

Lipopolysaccharide (LPS) is a virulence determinant of Haemophilus influenzae and exhibits substantial heterogeneity in structure within and between strains. Key factors contributing to this heterogeneity are the genes required to add the first glycose to each of the three heptose residues of the LPS inner core. In each case this addition can facilitate further oligosaccharide extension. IgtF is invariably present in strains and the product has a function in adding the glucose to the first heptose. Iic2C is present in half the strains and was found to add a glucose to the second heptose. Insertion of Iic2C into a strain that does not naturally contain it resulted in hexose incorporation from the second heptose of the LPS. The product of the IpsA gene can add a glucose or galactose to the third heptose. By allelic replacement of IpsA between strains it is shown that the sequence of the gene can be the sole determinant of this specificity. Thus, IgtF, Iic2C and IpsA make significant but very distinct contributions to the conservation and variable patterns of oligosaccharide extensions seen in H. influenzae LPS.