Genetic diversity and relationships among 194 Pasteurella haemolytica isolates, which were recovered predominantly from cattle (39%) and sheep (58%) suffering from pneumonic pasteurellosis in the United Kingdom, Germany, and the United States, were estimated by examination of allelic variation at 18 enzymeencoding loci detected by multilocus enzyme electrophoresis. The isolates formed two major divisions. One included 178 Pasteurella haemolytica sensu stricto strains representing serotypes A1, A2, A5 to A9, A12 to A14, and A16; the other was composed of 16 isolates belonging to the A11 taxon. P. haemolytica isolates were classified into 22 electrophoretic types (ETs) that formed three primary phylogenetic lineages. One lineage was represented by ovine serotype A2 isolates, a second lineage consisted of bovine serotype A2, together 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 associated with specific capsular serotypes, lipopolysaccharide (LPS) types, outer membrane protein (OMP) types, and host species. Bovine isolates were represented by only three serotypes (A1, A2, and A6) in 5 ETs, whereas ovine isolates were represented by all of the serotypes in 19 ETs. The majority (76%) of bovine isolates were of serotypes A1 or A6 and belonged to a single ET that marked a virulent, cattle-specific clonal group. Among the ovine isolates, 40% were of serotype A2 and belonged to two ETs that represented two virulent, sheep-specific clonal groups. Bovine A1 and A6 isolates and bovine A2 isolates were phylogenetically distinct from ovine isolates of the same serotypes, indicating that different subpopulations of these serotypes are associated with disease in cattle and sheep. Consistent differences in the OMP profiles of strains of the bovine and ovine lineages of these three serotypes suggest that certain OMPs are involved in host specificity and virulence. Evolutionary relationships among P. haemolytica isolates indicate that the ancestral host is the sheep and that several distinct clonal lineages have crossed the species barrier into cattle. The A11 taxon is a heterogeneous group of opportunistic pathogens of sheep that represents a separate species. Pasteurella haemolytica is the etiological agent of pneumonic pasteurellosis of cattle and sheep, which are infections that cause considerable economic loss to the cattle and sheep industries (19, 24). P. haemolytica consists of 13 capsular serotypes, designated A1, A2, A5 to A9, A11 to A14, A16, and A17 (4, 55). In addition to isolates that express these serotypes, approximately 10% of disease isolates recovered from cattle and sheep are untypeable (21, 45). Whereas some untypeable isolates are closely related to serotype A1, A2, or A11 isolates and probably represent either new capsular types or capsuledeficient strains, others represent distinct species (18, 39, 40). Serotype A1 isolates are responsible for most cas...

Genetic diversity among 60 British Pasteurella trehalosi isolates representing the four recognized capsular serotypes, T3, T4, T I 0 and T15, and recovered predominantly from sheep suffering from systemic pasteurellosis, was estimated by analysing electrophoretically demonstrable allelic variation a t structural genes encoding 19 enzymes. Thirteen of the loci were polymorphic and 20 distinctive multilocus genotypes (electrophoretic types, ETs) were identified. The population structure of P. trehalosi is clonal and its genetic diversity is limited compared with most other pathogenic bacteria. ETs represent clones, and isolates of the same ET were generally associated with the same combination of serotype, LPS type and outer-membrane protein (OMP) type. The genetic diversity of isolates within each of the capsular serotypes varied. Serotype TI0 was represented b y 18 isolates in two related ETs and exhibited little diversity. By contrast, serotype TI5 was represented by 18 isolates in nine ETs and was almost as diverse as the species as a whole. Serotype T4 was represented by 18 isolates in five ETs and was less diverse than serotype T15. Although serotype T3 was more diverse than serotype TI5 it was represented by only three isolates. With the exception of the TI0 isolates and those recovered from healthy sheep, 35 disease isolates belonged to 16 ETs, each of which was represented by only one to four isolates. The fact that a high proportion of disease is caused by a relatively large number of clones suggests that P. trehalosi is essentially an opportunistic pathogen. In addition to having the same capsular structure, isolates belonging to the two TI0 clones were characterized by possession of similar, if not identical, O-antigens (LPS types 2 and 4). The occurrence of 18 serotype T I 0 isolates in only two ETs suggests that the TI0 capsule and type 2/4 O-antigen confer enhanced virulence on members of these two clones. Multilocus enzyme electrophoresis (MLEE) had greater resolving power than did capsule/LPS/OMP analysis, being able to distinguish 20 rather than 14 sub-divisions within P. trehalosi. The technique demonstrated genetic identity or non-identity among strains of the same or different serotypes from different geographic localities within the UK and was a useful epidemiological tool.