Characterization of atypical Actinobacillus pleuropneumoniae serovar 1 isolates

Area of Science:

• Veterinary Microbiology
• Bacterial Pathogenesis
• Molecular Diagnostics

Background:

• Actinobacillus pleuropneumoniae (APP) causes significant economic losses in the swine industry.
• Accurate serotyping of APP is crucial for epidemiological surveillance and vaccine development.
• Previous typing methods have limitations in identifying atypical strains.

Purpose of the Study:

• To investigate the characteristics of two APP isolates from Japan that were nontypeable by conventional methods.
• To elucidate the genetic basis for the atypical phenotype observed in these APP isolates.
• To assess the impact of this atypicality on serologic typing.

Main Methods:

• Agar gel diffusion (AGD) test for serotyping.
• Polymerase chain reaction (PCR) for capsular serovar identification.
• Nucleotide sequence analysis of gene clusters involved in polysaccharide biosynthesis.
• Transmission electron microscopy (TEM) for visualizing capsular material.

Main Results:

• The two APP isolates were nontypeable by AGD but PCR-positive for serovar 1.
• Sequence analysis revealed genetic defects in capsular polysaccharide (CPS) biosynthesis loci, specifically a 7-nucleotide deletion in the cps1D gene.
• TEM confirmed the absence of detectable capsular material in these isolates.
• Despite genetic similarity to serovar 1, the isolates failed to produce CPS.

Conclusions:

• A defect in CPS production, caused by a specific gene mutation, leads to nontypeable APP strains.
• This CPS-deficient phenotype significantly complicates conventional serologic typing.
• Understanding these atypical strains is vital for improving diagnostic accuracy and disease management in porcine pleuropneumonia.