Isolation and characterization of a capsule-deficient mutant of Actinobacillus pleuropneumoniae serotype 1

The capsular polysaccharides (CPS) play a major role in pathogenicity of Ac tinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumon ia. The purpose of the present study was to isolate a mutant in CPS biosynt hesis by using a mini-Tn10 transposon mutagenesis system and evaluate its a dherence to host cells. One mutant apparently did not possess CPS as it did not react with a monoclonal antibody against A. pleuropneumoniae serotype 1 capsular antigen. Absence of capsule was confirmed by flow cytometry and also by transmission electron microscopy after polycationic ferritin labell ing. The site of insertion of the mini-Tn10 was determined and found to be in the cpxC gene. Its gene product, CpxC, is a protein involved in polysacc haride transport across the cytoplasmic membrane during CPS biosynthesis. U se of piglet tracheal frozen sections indicated that the CPS mutant adhered significantly (P=0.0001) more than the parent strain. The non-capsular mut ant was less virulent in pigs compared to the parent strain and showed no m ortality in experimentally infected pigs. The CPS mutant was however resist ant to pig serum. This CPS mutant is the first A. pleuropneumoniae mutant i n a CPS transport gene. It is also the first time that adherence of a CPS m utant of A. pleuropneumoniae is evaluated. Our observations indicate that c apsular polysaccharides of A. pleuropneumoniae serotype 1 are not involved in adherence to piglet tracheal frozen sections but rather mask, at least i n part, the adhesive functions. (C) 2000 Academic Press.