A total of 110 isolates of Pasteurella multocida from Australian poultry and reference strains for the 16 somatic serovars plus the three subspecies (gallicida, multocida, septica) were analysed to examine their population structure and diversity. The 81 field isolates examined by multilocus enzyme electrophoresis (MLEE) were diverse, being divided into 56 electrophoretic types (ETs), with the 19 reference strains in another 15 ETs. The population was clonal and somatic serotyping was not particularly useful in establishing relationships between isolates. The 71 ETs formed three distinct subclusters (A, B and C) at a genetic distance of 036. Biovars tended to be associated with these subclusters: A with biovars 1,3,4, 5 and 8 and B with biovars 2,6,7,9 and 10. Ribotyping, performed on all 110 isolates using Hpall, recognized 21 ribotypes forming nine clusters (Rl-Rg). The isolates in ribotype cluster R 1 were almost identical to those in MLEE cluster B. Using both MLEE and ribotyping, the 19 non-Australian reference strains were found to be distributed over the full diversity of the Australian isolates of P. multocida. This study has shown that a range of P. multocida clones are associated with fowl cholera in Australia and that many of the Australian isolates are similar to non-Australian reference strains. Both the MLEE results and the ribotyping data identified a previously unrecognized subset of P. multocida strains.

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