GENETIC-RELATIONSHIPS AMONG PASTEURELLA-TREHALOSI ISOLATES BASED ON MULTILOCUS ENZYME ELECTROPHORESIS

Genetic diversity among 60 British Pasteurella trehalosi isolates repr esenting the four recognized capsular serotypes, T3, T4, T10 and T15, and recovered predominantly from sheep suffering from systemic pasteur ellosis, was estimated by analysing electrophoretically demonstrable a llelic variation at structural genes encoding 19 enzymes. Thirteen of the loci were polymorphic and 20 distinctive multilocus genotypes (ele ctrophoretic types, ETs) were identified. The population structure of P, trehalosi is clonal and its genetic diversity is limited compared w ith 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. S erotype T10 was represented by 18 isolates in two related ETs and exhi bited little diversity. By contrast, serotype T15 was represented by 1 8 isolates in nine ETs and was almost as diverse as the species as a w hole. Serotype T4 was represented by 18 isolates in five ETs and was l ess diverse than serotype T15, Although serotype T3 was more diverse t han serotype T15 it was represented by only three isolates. With the e xception of the T10 isolates and those recovered from healthy sheep, 3 5 disease isolates belonged to 16 ETs, each of which was represented b y only one to four isolates. The fact that a high proportion of diseas e is caused by a relatively large number of clones suggests that P. tr ehalosi is essentially an opportunistic pathogen, In addition to havin g the same capsular structure, isolates belonging to the two T10 clone s were characterized by possession of similar, if not identical, O-ant igens (LPS types 2 and 4). The occurrence of 18 serotype T10 isolates in only two ETs suggests that the TIO capsule and type 2/4 O-antigen c onfer enhanced Virulence on members of these two clones. Multilocus en zyme electrophoresis (MLEE) had greater resolving power than did capsu le/LPS/OMP analysis, being able to distinguish 20 rather than 14 sub-d ivisions within P. trehalosi. The technique demonstrated genetic ident ity or non-identity among strains of the same or different serotypes f rom different geographic localities within the UK and was a useful epi demiological tool.