RTX TOXIN GENOTYPES AND PHENOTYPES IN ACTINOBACILLUS-PLEUROPNEUMONIAEFIELD STRAINS

Actinobacillus pleuropneumoniae serotype reference strains and 204 A. pleuropneumoniae field strains representing all 12 serotypes and both biovars 1 and 2, obtained from laboratories from various countries wor ldwide, were analyzed for the presence of the toxin genes apxlC, apxIA , apxIB, apxID, apxIIC, apxIIA, apxIIIC, apxIIIA, apxIIIB, and apxIIID by DNA-DNA hybridization with specific gene probes. Expression of the toxins ApxI, ApxII, and ApxIII was assessed by immunoblot analysis wi th monoclonal antibodies. The results show that the patterns of apr ge nes and those of the expressed Apr toxins in biovar 1 field strains ar e the same as those of the genes and toxins of corresponding serotype reference strain. We found only three strains which had certain apr ge nes missing compared with the genes in their serotype reference strain s. Analysis of the expression of the three toxins showed that nearly a ll strains expressed their apr genes and produced the same Apr toxins as their serotype reference strain. We found only one strain that did not produce ApxI, although it contained the apxICABD genes, and one st rain which did not express ApxII but which contained apxIICA. Several field strains which initially showed that their serotype did not corre spond to the apr gene profile of the reference strain and which had an unexpected virulence for the given serotype revealed that their initi al serotyping was erroneous. We show that the apr gene profiles are in herent to a given serotype. The method cannot differentiate between al l 12 serotypes. However, it allowed us to distinguish five groups of t oxin gene patterns which showed Pathological, toxicological, and epide miological significance. None of the biovar 2 strains contained apxIII genes. The apxI and apxII genes in the biovar 2 strains, however, wer e the same as those found in the serotype reference strains of biovar 1.