A Systematic Taxonomic Study And Analysis Of Integrated Junctional Elements Of The Genus Schwannella

The genus Shewanella belonging to the family Shewanellaceae as a sole genus,has been reported sixty-eight recognized species.The majority of Shewanella species inhabit a wide range of marine niches and creatures.The genus Shewanella plays a critical role in bioremediation and microbial fuel cells.Multiple Shewanella species are regarded as causative agents of food spoilage and opportunistic pathogens of aquatic animals as well as human diseases.More attention has been paid to enhance the knowledge of the genus Shewanella.There is rare information on the phylogenetic taxonomy,evolutionary relationship and population characteristics of the Shewanella species.In this study,a standard multilocus sequence analysis(MLSA)based on six housekeeping genes(gyrA,gyrB,infB,recN,rpoA and topA)was established as a rapid,accurate and economical classification tool by assessing the nucleotide sequences and phylogenies of almost sixty type Shewanella strains.This method yielded a sufficient resolving power in view of enough informative sites,adequate sequences divergence and distinct interspecies branches.A notable gap between the ranges of interspecies and intraspecies similarities made taxonomy unambiguous with 96.8%cutoff value for Shewanella novel species delineation.The MLSA approach also provided a robust phylogentic system to explicit widespread picture of evolutionary relationship in Shewanella genus.The reliability of phylogeny was supported by high bootstrap and similar topology.The splitstree network proposed twelve distinct evolutionary clades with identical G+C contents,higher genetic similarity and similar geographical niches.Eighty-six tested strains were subjected to explore the population diversity in China.The most prevalent Shewanella species was Shewanella algae,followed by Shewanella xiamenensis,Shewanella chilikensis,Shewanella indica,Shewanella seohaensis and Shewanella carassii.Some species frequently isolated from clinical and food samples highlighted the necessity to perform diagnose and surveillance in Shewanella strains.Finally,the genetic,genomic and biochemical analysis indicated that Shewanella haliotis and Shewanella upenei should be reclassified as synonyms of S.algae,and Shewanella pacifica should be reclassified as a synonym of Shewanella japonica to refine the taxonomic structure of Shewanella genus.Integrative conjugative elements(ICEs)seving as self-transmissible elements play a significant role in the genetic evolution and environmental adaptation of bacteria.The SXT/R391 element is the largest family belonging to ICEs and has been recorded among Vibrio and Proteus species in China.However,rare information is available on the distribution and genetic characteristics of SXT/R391 ICEs among Shewanella species.In this study,eighty-six Shewanella strains collected from diverse samples in China were subjected to determine the presence of SXT/R391 ICEs by targeting the intSXT gene.Three positive strains classified as Shewanella algae were obtained from both diarrhea patients and washing water in a local mill.The pulsed field gel electrophoresis profile and sequence analysis indicated that they shared closely clonal relationships and high sequence similarities,therefore,a representative ICE was selected and designed as ICESa/CHN110003 to perform the subsequent analysis.The BLASTn results showed that the most related ICEs of ICESalCHN110003 was ICE VchBan5,with low coverage of 76%and high identity of 99%.The phylogenetic tree of concatenated core genes demonstrated that ICESalCHN 110003 formed a distinct evolutionary branch separated from all the other ICEs.The graphical composition of genetic structure exhibited that ICESalCHN110003 encoded uncommon genes in the inserted hotspots and none of recognized ICEs were completely identical to that.Combined with the lower coverage in sequence analysis,independent clade in phylogenetic tree and unique fragments in hotspots,the ICESalCHN 110003 was regarded as a novel SXT/R391 element which enriched the members of ICEs.Furthermore,multiple antibiotic resistance of positive isolates and cross-species transfer capability of SXT/R391 ICEs were also discovered.Therefore,it was necessary to reinforce continuous surveillance for SXT/R391 ICEs in the genus Shewanella and take actions to prevent the widespread dissemination of ICEs.