Population genomics of Actinobacillus pleuropneumoniae reveals lineage-associated K:O antigen and Apx toxin diversity

Actinobacillus pleuropneumoniae (APP) is the causative agent of porcine pleuropneumonia, causing high mortality and morbidity and substantial economic losses in the swine industry. Nineteen capsular (K) serotypes, defined by variation in capsular polysaccharide (CPS) biosynthesis loci, ten O serotypes, defined by variation in lipopolysaccharide (LPS) biosynthesis loci, characteristic Apx toxin profiles, and genomic diversity have been increasingly described; however, relationships among genomic lineage, surface antigen genotype, and Apx toxin repertoire remain incompletely resolved. Here, we analyzed 328 APP genomes, including 153 newly sequenced isolates and 175 public genomes from 21 countries. Core-genome SNP phylogeny and FastBAPS identified 23 genomic lineages. Nineteen capsular K types and 10 O-antigen types formed 26 K:O genotypes, of which 24 were restricted to a single lineage. Adjusted Wallace analysis showed greater concordance of K:O genotype with lineage than K type alone (0.881 versus 0.759), and with ApxI, ApxII and ApxIII genotype (0.949 versus 0.787). ApxIVA related sequence was detected in all genomes, while 56 complete loci revealed recurrent length variation within the N-terminus or the RTX repeat rich region and an additional divergent ApxIVA related homolog. In conclusion, our comprehensive population genomic study gives a more informative picture of APP genomics and virulence-associated diversity, providing a useful genomic basis for future diagnostic test and vaccine development.