Pasteurella multocida Heddleston serovars 1 and 14 express different lipopolysaccharide structures but share the same lipopolysaccharide biosynthesis outer core locus

Pasteurella multocida strains are classified using the Heddleston lipopolysaccharide (LPS) serotyping scheme into 16 serovars. Understanding the structural and genetic basis for this LPS typing scheme is important because protection against infections caused by P. multocida is generally considered to be serovar specific. Here we show that the serovar 14 type strain P2225 and the serovar 1 strains X73 and VP161 express similar LPS structures. However, the serovar 14 LPS lacks the terminal phosphocholine (PCho) residues present on the serovar 1 LPS and contains the 1,4-linked β-galactose but not the 1,6-linked β-galactose. Sequencing analysis of the LPS biosynthesis outer core loci of P2225 and the serovar 1 type strain X73 showed that they were nearly identical. However, the phosphocholine biosynthesis gene, pcgA of P2225 contained a 19 bp nucleotide deletion. Complementation of P2225 with an intact pcgA resulted in an LPS structure identical to that expressed by serovar 1 strain VP161 and highly similar to that expressed by strain X73, with a 1,6-linked β-galactose and both terminal PCho residues. This study has shown unequivocally that strains belonging to serovar 1 and 14 share a common LPS outer core locus and that minor changes within this locus can dramatically alter the LPS structure expressed on the surface of P. multocida, and thus has implications into our understanding of the potential to generate cross-protective vaccines.