Identification of two new serotypes within serogroup B of Dichelobacter nodosus

The present study records the strain-specific molecular typing system for Dichelobacter nodosus (D. nodosus) based on genetic analysis of fimA locus. Based on the study two new serotypes B5 and B6 are reported within the serogroup B. Out of 200 swab samples collected randomly from foot lesions of footrot affected sheep from all the districts of Kashmir, India, 122 (61.0%) detected positive for D. nodosus. Serogroup B was predominantly prevalent in 83.60% of positive samples. Restriction fragment length polymorphism (RFLP) of polymerase chain reaction (PCR) amplified fimA gene of D. nodosus serogroup B revealed only two fingerprint patterns (FP) designated as FP1 and FP2. The FP1 was most prevalent and depicted by 82.35% of the samples with serogroup B while, FP2 was depicted by rest (17.65%) of the samples. Though the FP1 fimA sequence had the homology of 95% to D. nodosus fimA of serotype B4 isolate VRS 54, but there were 14 nucleotide differences and four nucleotide insertions/deletions in the coding sequence between these two strains resulting in eight amino acid substitutions in the fimbrial subunit. Similarly the FP2 fimA showed the sequence homology of 97% with D. nodosus fimA of serotype B2 isolate 183, with 10 nucleotide differences and three nucleotide insertions/deletions between these two sequences. This resulted in six amino acid substitutions, plus an amino acid length variation in the subunit protein. Thus it was presumed that these FP1 and FP2 strains represented new serotypes (B5 and B6, correspondingly) within the B serogroup as the degree of amino acid sequence difference with their nearest homologous strains was much greater than that within a serotype (0-5 amino acid differences), but comparable to that between serotypes (8-15 amino acid differences). This presumption was confirmed by cross tube agglutination test.