Ribotype diversity of porcine Pasteurella multocida from Australia

Objective To use the technique of ribotyping to investigate the genetic div ersity of Australian isolates of Pasteurella multocida associated with outb reaks 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 pop ulation structure of the Australian porcine P multocida isolates was determ ined through statistical analysis of the joint ribotype patterns, and this was then compared with biochemical and epidemiological data available for t he population. Results A total of 25 combined ribotypes were recognised, which were groupe d into five ribotype clusters. Despite the deliberate selection of diverse isolates, the study revealed only a limited degree of genetic diversity. Fo urteen of the ribotypes contained multiple isolates, and 12 of these riboty pes 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 biov ar 8 isolates, which correspond to P multocida subsp gallicida, were alloca ted by themselves to a single cluster (IV). All nine of the biovar 12 isola tes (lactose-positive subsp multocida) were assigned to a single cluster (] ), together with the single biovar 14 isolate, which was the only other lac tose-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 ac ross multiple farms, and across multiple states, individual farms can posse ss multiple ribotypes of P multocida. Some of the unusual biochemical varia nts of P multocida present in Australian pigs have a very limited genetic d iversity. The nature of pig production in Australia, primarily involving co ntinuous flow systems with few closed herds, has possibly contributed to th e widespread distribution of a limited number ribotypes.