Characterization of atypical <i>Actinobacillus pleuropneumoniae</i> serovar 1 isolates

Two <i>Actinobacillus pleuropneumoniae</i> (APP) isolates from clinical cases of porcine pleuropneumonia in Japan, positive for ApxIA, ApxIIA, and ApxIVA, were nontypeable using the agar gel diffusion (AGD) test but positive in the capsular serovar 1-specific PCR assay. Nucleotide sequence analysis revealed that gene clusters involved in the biosynthesis of the capsular polysaccharide (CPS) and lipopolysaccharide O-polysaccharide of the isolates were identical to those of serovar 1 reference strain 4047. The main difference found in the CPS loci was a loss of 7 nucleotides at the 3'-end of the <i>cps1D</i> gene in the atypical isolates, which is responsible for the defect in CPS production. Consistent with the serologic and molecular findings, transmission electron microscopic analysis confirmed the absence of detectable capsular material in the 2 atypical isolates. Collectively, our results suggest that this type of APP, defective in CPS production, may severely hamper serologic typing of the pathogen.