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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 &tinobacillus pZeuropneumoniae RTX-toxin I) is proposed for the RTX-toxin produced by the reference strains for serotypes 1, 5a, 5b, 9,lO 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 apxZ operon will have the designations apxZC, apxZA, apxZB, and apxZD 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 m a s between 103 and 105 kDa. The genes of the apxZZ operon will have the designations apxZZC for the activator gene and apxZZA for the structural toxin gene. In the apxZZ operon, no genes for secretion proteins have been found. Secretion of ApxII seems to occur via the products of the secretion genes apxZB and apxZD of the apxZ operon. The designation ApxIII is proposed for the nonhaemolytic RTX-toxin of the reference strains for serotypes 2, 3, 4, 6 and 8, which was previously named cytolysin 111 (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 apxZZZ operon have the designations apxZZZC, apxZZZA, apxZZZB and apxZZZD for the activator gene, the structural gene and the two secretion genes respectively.

Monoclonal antibodies (MAbs) were raised against extracellular hemolytic and cytotoxic proteins of Actinobacilus pleuropneumoniae serotypes 2 and 9. MAbs were tested for inhibition of hemolytic and cytotoxic activities of the reference strains of A. pleuropneumoniae serotypes 1 to 12. Five MAbs inhibited hemolytic or cytotoxic activities of one or more serotypes and were used to probe Western blots (immunoblots) of cytotoxic and hemolytic culture filtrates of serotypes 1 to 12. The MAbs recognized three different proteins: (i) a protein of approximately 103 kDa that was associated with a weak hemolytic activity and a moderate cytotoxic activity, (ii) a protein of approximately 105 kDa that was associated with a strong hemolytic activity and a strong cytotoxic activity, and (iii) a protein of approximately 120 kDa that was associated with a strong cytotoxic activity but not with hemolytic activity. Serotypes 6, 7, and 12 produced only the 103-kDa protein, and serotype 10 produced only the 105-kDa protein. The other serotypes produced two proteins: serotypes 1, 5, 9, and 11 produced the 103and 105-kDa proteins, and serotypes 2, 3, 4, and 8 produced the 103-and 120-kDa proteins.

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SUMMARYTen transmission trials with Actinobacillus pleuropneumoniae were carried out. The observed transmission was highly variable, which was surprising since the design of the trials was very similar. We investigated whether the variable transmission could be explained by variation in infectivity of A. pleuropneumoniae infected pigs. We looked for measurable characteristics, which could be indicative for infectious pigs or for the level of infectivity. The characteristic that appeared to be most indicative for a pig being infectious was an A. pleuropneumoniae positive tonsil at necropsy. The characteristic that was correlated to the level of infectivity was the number of A. pleuropneumoniae colonies isolated from the nasal swab, i.e. the probability for an infectious pig to infect a susceptible pig was tenfold higher on days where at least ten colonies were isolated. In this study it is shown that it is possible to measure the bacterial transmission of A. pleuropneumoniae under controlled circumstances if variation in infectivity is taken into account.