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Abstract: AasP, an autotransporter serine protease of Actinobacillus pleuropneumoniae, has been shown to be expressed in necrotic porcine lung tissue. Based on the hypothesis that AasP might play an important role in A. pleuropneumoniae adhesion and virulence by processing other surface-associated proteins, the predicted catalytic site of AasP was deleted and the isogenic mutant, AP76ΔaasP, was compared to the wild-type strain in a biofilm assay as well as an aerosol infection model. AP76ΔaasP showed increased adherence compared to the wild-type strain under standard culturing conditions as well as under NAD restriction. No significant differences between AP76 wild-type and AP76ΔaasP were observed upon experimental infection of pigs, indicating that AasP does not play a crucial role in A. pleuropneumoniae virulence. [Copyright &y& Elsevier]

Abstract: Actinobacillus pleuropneumoniae, a gram-negative rod of the Pasteurellaceae family, causes pleuropneumonia in pigs. Establishing A. pleuropneumoniae free herds is difficult due to the occurrence of persistently infected animals. The ApxIV toxin is expressed by A. pleuropneumoniae in vivo and an ELISA based on the toxin is used to detect infection and to differentiate between infected and vaccinated animals. In this study, we have identified a 1070bp insertion element of the IS30 family, designated ISApl1, in the A. pleuropneumoniae serotype 7 strain AP76. ISApl1 contains a 924bp ORF encoding a transposase, which is flanked by 27bp inverted repeats showing six mismatches. We investigated the occurrence of ISApl1 in other A. pleuropneumoniae strains, and its possible interference with virulence associated factors. Four insertion sites were identified in AP76: within the apxIVA toxin ORF, within a putative autotransporter adhesin ORF, upstream of a capsular polysaccharide biosynthesis gene cluster, and downstream of a beta-lactamase gene. ISApl1 is also present in some serotype 7 field isolates, but not in reference or field strains of other serotypes. In A. pleuropneumoniae AP76, the transposase gene is transcribed in vitro. The insertion in the apxIVA toxin gene remains stable after animal passage. Since this insertion should disrupt toxin expression, we tested 7 pigs infected with AP76 at day 21 post-infection. All were negative in the ApxIV ELISA but four out of seven were positive in an ApxII toxin ELISA. These results show that insertion elements can affect the detection of A. pleuropneumoniae infected animals. [Copyright &y& Elsevier]