Vet. Res. 34 (2003) 353-359
DOI: 10.1051/vetres:2003010

Phylogenetic relationship of equine Actinobacillus species and distribution of RTX toxin genes among clusters

Peter Kuhnert^a, Hélène Berthoud^a, Henrik Christensen^b, Magne Bisgaard^b and Joachim Frey<sup>a

a</sup>  Institute of Veterinary Bacteriology, Laenggass-Str. 122, University of Bern, 3012 Bern, Switzerland
^b  Department of Veterinary Microbiology, The Royal Veterinary and Agricultural University, 1870 Frederiksberg C, Denmark

(Received 16 August 2002; accepted 2 December 2002)

Abstract
Equine Actinobacillus species were analysed phylogenetically by 16S rRNA gene (rrs) sequencing focusing on the species Actinobacillus equuli, which has recently been subdivided into the non-haemolytic A. equuli subsp. equuli and the haemolytic A. equuli subsp. haemolyticus. In parallel we determined the profile for RTX toxin genes of the sample of strains by PCR testing for the presence of the A. equuli haemolysin gene aqx, and the toxin genes apxI, apxII, apxIII and apxIV, which are known in porcine pathogens such as Actinobacillus pleuropneumoniae and Actinobacillus suis. The rrs-based phylogenetic analysis revealed two distinct subclusters containing both A. equuli subsp. equuli and A. equuli subsp. haemolyticus distributed through both subclusters with no correlation to taxonomic classification. Within one of the rrs-based subclusters containing the A. equuli subsp. equuli type strain, clustered as well the porcine Actinobacillus suis strains. This latter is known to be also phenotypically closely related to A. equuli. The toxin gene analysis revealed that all A. equuli subsp. haemolyticus strains from both rrs subclusters specifically contained the aqx gene while the A. suis strains harboured the genes apxI and apxII. The aqx gene was found to be specific for A. equuli subsp. haemolyticus, since A. equuli subsp. equuli contained no aqx nor any of the other RTX genes tested. The specificity of aqx for the haemolytic equine A. equuli and ApxI and ApxII for the porcine A. suis indicates a role of these RTX toxins in host species predilection of the two closely related species of bacterial pathogens and allows PCR based diagnostic differentiation of the two.

Key words: Actinobacillus / horse / RTX toxin / phylogeny / diagnostic

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© INRA, EDP Sciences 2003