Evidence for Serpulina hyodysenteriae being recombinant, with an epidemic population structure

The population structure of Serpulina hyodysenteriae was investigated using multilocus enzyme electrophoresis. A total of 231 isolates were divided into 50 electrophoretic types (ETs), with a mean genetic diversity of 0.29 for the number of ETs and 0.23 for the number of isolates. Subsets of isolates from two Australian states (71 isolates from Victoria and 68 isolates from Queensland) exhibited as much genetic variation as the entire collection. The calculated index of association (IA) for the number of ETs (0.29 +/- 0.17) was not significantly different from zero, and hence provided evidence for the occurrence of significant genetic recombination accounting for the observed variation between strains. In contrast, the IA for the number of isolates (3.93 +/- 0.03) was significantly different from zero, with seven of the 50 ETs (ETs 4, 6, 13, 14, 20, 33 and 35) containing 51% of all the isolates. Even when multiple isolates from the same farm were removed from the analysis, the IA value for the number of isolates remained significantly greater than zero (IA 9.87 +/- 0.04), indicating that it was not biased by their inclusion. The results suggest that S. hyodysenteriae has an epidemic population structure.