Actinobacillus pleuropneumoniae RTX-toxins: uniform designation of haemolysins, cytolysins, pleurotoxin and their genes

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Abstract
SUMMARY: The three different pore-forming RTX-toxins of Actinobacillus pleuropneumoniae are reviewed, and new and uniform designations for these toxins and their genes are proposed. The designation ApxI (for Actinobacillus pleuropneumoniae RTX-toxin I) is proposed for the RTX-toxin produced by the reference strains for serotypes 1, 5a, 5b, 9,10 and 11, which was previously named haemolysin I (HlyI) or cytolysin I (ClyI). This protein is strongly haemolytic and shows strong cytotoxic activity towards pig alveolar macrophages and neutrophils; it has an apparent molecular mass in the range 105 to 110 kDa. The genes of the apxI operon will have the designations apxIC, apxIA, apxIB, and apxID for the activator, the structural gene and the two secretion genes respectively. The designation ApxII is proposed for the RTX-toxin which is produced by all serotype reference strains except serotype 10 and which was previously named App, HlyII. ClyII or Cyt. This protein is weakly haemolytic and moderately cytotoxic and has an apparent molecular mass between 103 and 105 kDa. The genes of the apxII operon will have the designations apxIIC for the activator gene and apxIIA for the structural toxin gene. In the apxII operon, no genes for secretion proteins have been found. Secretion of ApxII seems to occur via the products of the secretion genes apxIB and apxID of the apxI operon. The designation ApxIII is proposed for the non-haemolytic RTX-toxin of the reference strains for serotypes 2, 3, 4, 6 and 8, which was previously named cytolysin III (ClyIII), pleurotoxin (Ptx), or macrophage toxin (Mat). This protein is strongly cytotoxic and has an apparent molecular mass of 120 kDa. The genes of the apxIII operon have the designations apxIIIC, apxIIIA, apxIIIB and apxIIID for the activator gene, the structural gene and the two secretion genes respectively.