Analysis of Haemophilus parasuis by multilocus enzyme electrophoresis

The diversity among 40 Australian isolates and eight reference strains of Haemophilus parasuis was examined using multilocus enzyme electrophoresis. Thirty-four electrophoretic types (ETs) were recognized with a mean diversity per locus of 0.405. One Australian isolate was located in an ET separated by a considerable distance ( > 0.8) from the rest of the isolates, suggesting that it may represent a different species or subspecies. The remaining 33 ETs formed two distinct divisions (A and B), separated from each other by a distance of 0.506. All 12 Australian isolates of serovar 5 plus the two reference strains for this serovar were included in Division A. The only other isolates present in this Division were Australian isolates of serovars 4 and 13 and two nontypeable isolates. Division B contained a diverse range of serovars — Australian isolates of serovars 1, 2, 7 / 10, 9 and 13 as well as the reference strains for serovars 1, 2, 3, 4, 8 and 9. These results supported other studies which demonstrated considerable diversity, and indicate that the population of H. parasuis may contain more than a single species or subspecies. There was considerable diversity even amongst isolates of the same serovar, indicating that serotyping is not a particularly suitable technique for strain typing in epidemiological studies.