hlyIA-like sequence present in a genetic locus with low homology to appA in Actinobacillus pleuropneumoniae serotype 10.

Using the appA gene encoding Actinobacillus pleuropneumoniae-RTX-toxin II (ApxII) as a probe, Southern hybridization demonstrated the presence of appA-sequence in all 12 serotypes except serotype 10. Serotype 10 strains had a genetic locus with low homology to appA (tentatively designated Lha). DNA sequence analysis revealed that Lha contained a part of hlyIA gene encoding a 105-kDa hemolysin ApxI. Southern hybridization demonstrated the presence of Lha (hlyIA) sequence not only in serotype 10 but also in serotypes 1, 5a, 5b, 9, and 11, which had strong hemolytic activity with sheep red blood cells and were highly virulent for mice. These results suggest that ApxI may be associated with the virulence phenotype of the organism.