Abstract
Serpulina hyodysenteriae and Serpulina pilosicoli are the respective antiological agents of swine dysentery and intestinal spirochaetosis. Both species have been shown to be genetically diverse but the mechanisms accounting for the variation have not been investigated. Originally, most bacterial species were thought to be clonal and although recombination occurred it was presumed to be too infrequent to disrupt linkage disequilibrium. The recent application of the index of association (Ia) to MEE data sets has shown that bacteria may occupy a spectrum of population structures. Whilst the majority of species are clonal, some naturally transformable species have been shown to be panmicitc (ie high levels of recombination have disrupted linkage disequilibrium) whilst others have an intermediate population structure. The Ia was applied to 231 S. hyodysenteriae isolates that were divided into 50 electrophoretic types (ETs), and the population structure was found to be epidemic (ie significant levels of background recombination occurring with a large number of isolates restricted to certain ETs that may have survival or virulence advantages). In contrast 145 S. pilosicoli isolates obtained from humans in the Highlands of PNG divided in 27 ETs had a completely panmictic population structure. The results may reflects the different epidemiological aspects of each disease. The potential mechanisms for recombination in each species require further investigation.