[Difference and variation of the sef14 operon gene clusters in S. enteritidis and closed serogroup-D Salmonella]

In order to reveal why SEF14 fimbriae are restrictively expressed on strains of serogroup D salmonella, mainly S. enteritidis and S. dublin, the difference and variation of the sef14 operon gene clusters in S. enteritidis and related serogroup-D Salmonella were analyzed.

The genes encoding subunits of sefA, sefD and sefR in S. pullorum, S. enteritidis and S. dublin were amplified by PCR method and then sequenced to analyze the the difference and variation, respectively.

The results of PCR amplification showed that prevalence of sefA, sefD and sefR genes in S. enteritidis and S. dublin was 100%. In 18 isolates of S. pullorum, the prevalence of sefA gene was 100%,while the prevalence of sefD and sefR genes was 38.9% (7/18), and 11 strains isolated after 1980s did not contain any gene sefDor sefR. The sequencing data of PCR products revealed that sequences of sefA, sefD and sefR genes in S. enteritidis and S. dublin were identical with those those from NCBI GenBank data which accession number were L11008, U07129 and AF233854, respectively. Interestingly, among the 7 strains of S. pullorum before 1980s, the sefD sequence has a missing base pair at position 196 and caused open reading frame (ORF) shift, resulting in a stop codon (TAG) at position 71 amino acid residual (Leu of TTA at position 214 - 216 shift into stop codon of TAG at position 215 - 217). Unlike S. pullorum, all S. enteritidis and S. dublin tested could express SEF14 fimbriae in vitro.

Based on the data of the difference and variation of sef14 operon gene clusters between S. enteritidis and S. pullorum, we may explain why SEF14 fimbriae in S. pullorum could not be expressed.