EVOLUTIONARY GENETICS OF PASTEURELLA-HAEMOLYTICA ISOLATES RECOVERED FROM CATTLE AND SHEEP

Genetic diversity and relationships among 194 Pasteurella haemolytica isolates, which were recovered predominantly from cattle (39%) and she ep (58%) suffering from pneumonic pasteurellosis in the United Kingdom , Germany, and the United States, were estimated by examination of all elic variation at 18 enzyme-encoding loci detected by multilocus enzym e electrophoresis. The isolates formed two major divisions. One includ ed 178 Pasteurella haemolytica sensu stricto strains representing sero types Al, A2, A5 to A9, A12 to A14, and A16; the other was composed of 16 isolates belonging to the All taxon, P. haemolytica isolates were classified into 22 electrophoretic types (ETs) that formed three prima ry phylogenetic lineages, One Lineage was represented by ovine serotyp e A2 isolates, a second lineage consisted of bovine serotype A2, toget her with serotype A7 and A13 isolates, and the third lineage included isolates representing all of the other serotypes, as well as a second group of serotype A7 strains, Electrophoretic types were nonrandomly a ssociated with specific capsular serotypes, lipopolysaccharide (LPS) t ypes, outer membrane protein (OMP) types, and host species. Bovine iso lates were represented by only three serotypes (Al, A2, and A6) in 5 E Ts, whereas ovine isolates were represented by all of the serotypes in 19 ETs. The majority (76%) of bovine isolates were of serotypes Al or A6 and belonged to a single ET that marked a virulent, cattle-specifi c clonal group. Among the ovine isolates, 40% were of serotype A2 and belonged to two ETs that represented two virulent, sheep-specific clon al groups. Bovine Al and A6 isolates and bovine A2 isolates were phylo genetically distinct from ovine isolates of the same serotypes, indica ting that different subpopulations of these serotypes are associated w ith disease in cattle and sheep. Consistent differences in the OMP pro files of strains of the bovine and ovine lineages of these three serot ypes suggest that certain OMPs are involved in host specificity and vi rulence. Evolutionary relationships among P. haemolytica isolates indi cate that the ancestral host is the sheep and that several distinct cl onal lineages have crossed the species barrier into cattle. The All ta xon is a heterogeneous group of opportunistic pathogens of sheep that represents a separate species.