Mycoplasma hyopneumoniae is the causative agent of the porcine enzootic pneumonia. This disease is responsible of important economical losses for the swine industry. The P46 membranous and the P97 adhesin proteins have been described as amongst the most immunogenic proteins. The aim of the present study was to analyze the genetic and antigenic variations of those important proteins from different North-American strain/isolates. In particular, variations at the 3'-end of p97 were looked after. Indeed, this region contains two repetitive DNA sequences (RR1 and RR2) as described by Wilton et al (1998) and modifications (deletion or insertion) are reflected in the c-terminal of P97 adhesin and may be responsible for the antigenic variations.

Lung homogenates from pigs diagnosed with typical clinical signs of enzootic pneumonia were used to obtain 11 isolates of M. hyopneumoniae. After laboratory diagnostic confirmation by PCR, bacterial stocks from each isolate and the ATCC-25934 reference strain were used for total DNA extraction. The p46 and p97 genes from each strain/isolate have been amplified by PCR and then directly sequenced using standard protocols. Furthermore, monoclonal antibodies (MAbs) directed against E. coli-expressed recombinant fusion P46 and P97 proteins of the reference strain have been produced to establish the antigenic variations between the different strain/isolates by Western blot analysis.

Deduced amino-acid sequence analysis revealed variations in the P97 C-terminal region consisting of important sequence deletions or insertions either in the RR1 and the RR2. Five isolates displayed 2 RR1 insertions compared to the reference strain. The remaining six isolates displayed 2 to 7 RR1 deletions in comparison to the reference strain. However, 8 isolates displayed a partial or a complete deletion of the RR2 sequence but neither isolate showed any insertion in this region. Western immunoblot analysis using the P97-specific MAbs revealed different electrophoretic patterns of the P97 protein that correlated with the above genetic variations. The significance of these variations in the P97 of the different isolates could be investigated further by adhesion assays using ciliary cells of the swine respiratory tract. However, comparison of p46 gene sequence of all isolates revealed a high degree of amino acids conservation and Western blot analysis using the P46-specific MAbs did not revealed any variation in electrophoretic patterns. In conclusion, amongst these 11 North American isolates, the P46 protein seems very conserved and the P97 protein is more variable.