Research On The Relativity Between The Capsule Synthesis Gene Cluster Variation And Blood Resistance In Streptococcus Suis Isolated From Human

Streptococcus suis is a severe zoonotic pathogen,responsible for acute diseases in pigs and human,including septicemia,arthritis,endocarditis and meningitis,even sudden death.It not only brings huge economic losses to the swine industry,but also poses a hazardous threat to the public health.S.suis infection in human not only via skin injuries,also can via the gastrointestinal tract.Therefore,more and more researchers believed that S.suis should be considered an emerging foodborne pathogen.In the process of S.suis infection,blood is an important immune barrier.Among numerous virulence factors that have been reported,capsule is the first exposed layer to the blood during the infection.It is also an important virulence factor of S.suis which can assist S.suis in resisting innate immunity in blood.Capsule of S.suis is synthesized by a cluster of genes in the genome.To investigate the relation between resistance of S.suis to blood and capsule synthesis gene cluster,we used the European standard strain P1/7 and the Sichuan outbreak strain SC19 as reference strains,selected a S.suis strain LSM128 from 103 S.suis human isolates,which has strong ability to resist blood,and attempted to find the reason why it has strong ability of resistance to blood through variation loci of capsule synthesis gene cluster.The main results are as follows:(1)Through bioinformatics analysis,we found that there was no variation in the capsule synthesis gene cluster of strain LSM128 in comparison with strain P1/7.However,a missense mutation occurred at the 1219 base site on a transcriptional regulation gene,gntR,which in the upstream of the capsule synthesis gene cluster.This missense mutation leaded guanine to adenine,resulting alanine mutated to threonine in amino acid level.(2)Through homologous recombination,we constructed the gntR gene deletion and replacement mutants of strain P1/7 and strain LSM128 successfully.According to phenotypic assays,we found that gntR was not the main gene regulating the expression of the capsule.(3)Through the virulence evaluation experiment of strain P1/7,LSM128 and their gntR gene deletion and replacement mutants,we found that gntR was associated with the virulence of S.suis,but different strains showed different sides.In strain P1/7,the deletion of gntR can lead to the increase of virulence,but it is not observed in strain LSM128.It is worth noting that the missense mutation occurred in the gntR of LSM128 can significantly improve the resistance to blood and virulence of strain P1/7.In conclusion,we consider that gntR is involved in resistance to blood and the expression of virulence,but it seems not universality,and this phenomenon don't affected by the capsule.And the 1219 base site of gntR is an important site for its influence on the resistance to blood and virulence.