Evolutionary Analysis On Salmonella Flic And Fljb Flagellin Gene Clusters

As an important food-borne pathogen,Salmonella spreads through the food chain among human and animals,leading the largest number of foodborne illness.As an extension of comparative genomics,pan-genomics is of great significance to characterize the diversity of Salmonella species,serovars and their evolutionary and genomic structure or virulence recognition.Flagellin and its phase variation play an important role in the evolution and adaptation of Salmonella.Serotypes on Salmonella isolates are indicated biphasic or monophasic according to the ‘phase variation' of flagellar gene fliC and fljB,expressing both or one flagellar phases.In this study,214 Salmonella genomes were downloaded from the NCBI database,which covered 205 genomes of 87 different serovars of S.enterica subsp.enterica.According to pan-genome analysis,14,583 protein families were found among 214 Salmenalla genomes.Moreover,Salmonella displayed an open-genome structure.A phylogenetic tree constructed based on Single Nucleotide Polymorphisms(SNPs)of core genes was used to reflect the essential evolutionary relationships among various serovars.The COG(Cluster of Orthologous Groups of proteins)function categories showed that the function of ‘Carbohydrate transport and metabolism' stood the first place.The evolution characterization of flagellin gene clusters was carried out.The fliC and fljB genes were identified in 207 and 138 genomes,respectively.Congruence analysis was performed among the core genome and each gene of fliC and fljB gene clusters.Only fliA and fliS showed congruence to Salmonella core genome.Congruence was also observed among fliB,fliC/fljB,and fliD genes,and their phylogeny revealed a division into two major groups,which strongly corresponded to monophasic and biphasic serovars.Besides,homologous recombination events referring fliB,fliC/fljB and fliD were found to have mainly occurred within each group.Based on the two evolutionary patterns of flagellin gene,we further analyzed the expression differences of four mono-and bi-Salmonella strains by total RNA.Compared with mono-FliC expression,the bi-FliC expression demonstrated a higher level,which was consistent with the diversities on biSalmonella strains.In summary,our study provided a robust genetic framework for future studies on the evolution of flagellar fliC and fljB characterization in Salmonella enterica.