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A phenotypic characterisation of 150 isolates of bacteria previously identified as Pasteurella multocida was performed. All the isolates had been obtained from Australian pigs in the three eastern States of Queensland (110 isolates), New South Wales (21 isolates) and Victoria (19 isolates). Seven different biochemical biovars were recognised amongst the isolates. A total of 100 isolates (67%) were assigned to biovar 3, previously shown to be the most common biovar in isolates of P. multocida from Australian poultry [Fegan, N., Blackall, P.J., Pahoff, J.L., 1995. Phenotypic characterisation of Pasteurella multocida isolates from Australian poultry. Vet. Microbiol., 47, 281-286.]. Six of the seven biovars, including biovar 3, were identified as P. multocida subsp. multocida, 124 isolates in total. One other biovar, consisting of thirteen isolates, was identified as P. multocida subsp. gallicida. Within the six biovars that were identified as P. multocida subsp. multocida, biovars 12, 13 and 14 represented unusual biochemical variants. The nine isolates assigned to biovar 12 appeared to be lactose positive variants of P. multocida subsp. multocida. The three isolates in biovar 13 appeared to be ornithine decarboxylase (ODC) negative variants of P. multocida subsp. multocida. The single isolate in biovar 14 appeared to be an ODC negative, lactose positive variant of P. multocida subsp. multocida.

Biochemical profiles, restriction endonuclease analysis (REA) and ribotyping were used to investigate Pasteurella multocida isolates from outbreaks of fowl cholera on 7 turkey farms in New South Wales. While only a single isolate was available from 5 of the farms, multiple isolates, 4 and 12 respectively, were available from the other 2 farms. The available field evidence suggested that 8 outbreaks had occurred with one farm suffering 2 outbreaks. The isolates obtained were all confirmed as Pasteurella multocida. Biochemical profiles allocated the isolates to 4 groups, 3 being variants of P multocida subsp multocida and the fourth being P multocida subsp septica. REA performed with HpaII established 7 groups. Ribotyping using the HpaII digests probed with the 16S rRNA operon of Haemophilus paragallinarum recognised the same 7 groups as REA. Unlike the biochemical profiles, both REA and ribotyping provided a fine subdivision that identified outbreaks as either related or unrelated. The REA and ribotyping patterns as well as biochemical profiles were stable for all isolates from the outbreaks in which multiple isolates were obtained from either the same bird or from different birds. REA and ribotyping were found to be superior to biotyping methods for the investigation of fowl cholera outbreaks.

Objective To use the technique of ribotyping to investigate the genetic diversity of Australian isolates of Pasteurella multocida associated with outbreaks of clinical disease in Australian pigs. Design One hundred and seven porcine P multocida isolates were analysed by ribotyping using the restriction enzymes HpaII and HindIII. The genetic population structure of the Australian porcine P multocida isolates was determined through statistical analysis of the joint ribotype patterns, and this was then compared with biochemical and epidemiological data available for the population. Results A total of 25 combined ribotypes were recognised, which were grouped into five ribotype clusters. Despite the deliberate selection of diverse isolates, the study revealed only a limited degree of genetic diversity. Fourteen of the ribotypes contained multiple isolates, and 12 of these ribotypes were present on more than one farm. Three of the seven biovars analysed in the study showed very limited diversity. All fifteen biovar 2 isolates (subsp multocida) were found in a single cluster (III), while all four biovar 8 isolates, which correspond to P multocida subsp gallicida, were allocated by themselves to a single cluster (IV). All nine of the biovar 12 isolates (lactose‐positive subsp multocida) were assigned to a single cluster (I), together with the single biovar 14 isolate, which was the only other lactose‐positive isolate in the population (ODC‐negative). Conclusion A limited number of ribotypes of P multocida are associated with Australian pigs. The majority of these ribotypes are widely distributed across multiple farms, and across multiple states. Individual farms can possess multiple ribotypes of P multocida. Some of the unusual biochemical variants of P multocida present in Australian pigs have a very limited genetic diversity. The nature of pig production in Australia, primarily involving continuous flow systems with few closed herds, has possibly contributed to the widespread distribution of a limited number ribotypes.

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