THE RTX HEMOLYSINS APXI AND APXII ARE MAJOR VIRULENCE FACTORS OF THE SWINE PATHOGEN ACTINOBACILLUS-PLEUROPNEUMONIAE - EVIDENCE FROM MUTATIONAL ANALYSIS

The involvement of the RTX haemolysins (ApxI and ApxII) of the swine p athogen Actinobacillus pleuropneumoniae in virulence was investigated using haemolysin-deficient mutants constructed by a mini-Tn10 mutagene sis procedure. Two types of haemolysin mutant with single insertions o f the transposon were obtained from a serotype 1 strain producing both ApxI and ApxII. One presented a complete loss of haemolytic activity because of the absence of ApxI and ApxII production. The other display ed weaker haemolysis than the wild type and produced only ApxII. The c hromosomal regions flanking mini-Tn10 were cloned and sequenced. In th e non-haemolytic mutant, the transposon had inserted in apxIB, a gene involved in the exportation of ApxI and ApxII toxins. The weakly haemo lytic mutant resulted from the disruption of the structural gene for A pxI. Both mutations in the apxI operon were associated with a signific ant loss of virulence for mice and pigs, demonstrating that haemolysin s are involved in A. pleuropneumoniae pathogenicity. The non-haemolyti c mutant was apathogenic and the weakly haemolytic mutant retained som e virulence for pigs, suggesting that both ApxI and ApxII are needed f or full virulence.