The CAMP effect of Actinobacillus pleuropneumoniae is caused by Apx toxins.
A. pleuropneumoniae shows synergistic haemolysis when cocultured with Staphylococcus aureus on blood agar plates. This CAMP effect has been attributed to a discrete CAMP factor, but also to the A. pleuropneumoniae-RTX-toxins I, II and III. The CAMP effect of recombinant Escherichia coli strains that secreted each of these toxins, and of A. pleuropneumoniae mutant strains that were devoid of one or more of these toxins, was investigated. The E. coli strains were CAMP positive, whereas the A. pleuropneumoniae strain devoid of functional toxin genes was CAMP negative. It was concluded that the CAMP effect of A. pleuropneumoniae is caused by the toxins and that there is no discrete CAMP factor.