Molecular characterization of cytotoxins produced by Haemophilus, Actinobacillus, Pasteurella.

The leukotoxin of Pasteurella haemolytica has been implicated as one of the major virulence factors contributing to the pathogenesis of bovine pneumonic pasteurellosis. To gain a better understanding of the various biochemical, antigenic and toxigenic properties of the toxin, the genetic determinant (lkt) which encodes the leukotoxin has been cloned from P haemolytica serotype 1. Results from the molecular characterization of this determinant showed that four genes are required to synthesize and secrete the active toxin from P. haemolytica. This information provides the basis for genetic manipulation of the determinant to produce different "forms" of the leukotoxin for various studies. In addition, lkt was found to be homologous to the alpha-hemolysin determinant (hly) of Escherichia coli, Proteus vulgaris, P. mirabilis and Morganella morganii. Further evidence is being accumulated demonstrating the presence of the lkt/hly determinant in several species of Actinobacillus and Haemophilus. This suggests the wide dissemination of the lkt/hly determinant in many pathogenic bacteria and established the family of Rtx cytotoxins.