A new subtype of serovar 6, K6b:O3, of Actinobacillus pleuropneumoniae based on genotypic analysis.

We have analyzed the capsule (CPS) and the lipopolysaccharide O-Antigen (O-Ag) biosynthesis loci of fifteen field isolates of Actinobacillus pleuropneumoniae, including eleven North American and four Japanese ones, reactive to antisera against serovars 3, 6, 8 and/or 15. Ten North American isolates amplified a serovar 6-indicative fragment derived from the capsular loci, whereas one North American isolate and all four Japanese isolates amplified the serovar 6-indicative fragment as well as the serovar 3-indicative fragment. The five isolates producing a 3/6 banding pattern contain a type I CPS locus, named K6b, similar to serovar 6, but with differences in the cpxABCD and cpsABC gene sequences and the length of intergenic regions (modF-cpxA, and cpsC-cpsD). The main difference found between the K6 and K6b cps genes is a loss of function of a 113 AA UDP-glycosyltransferase found in type 6b due to the amino acid substitutions in the C-terminal domain of Cps6bA. Additionally, the isolates harbor a LPS O-Ag locus highly identical to those of field and reference strains of serovars 3, 8, 15, 17 and 19 but different from that of serovar 6. Taken together, our results indicate the existence of a subtype of A. pleuropneumoniae, serovar 6, that we called "K6b:O3'', and we propose isolate EH1248 as the reference strain.