Isolation And Investigation Of Streptococcus Suis And The Pathogenicity Of Coinfection In Mice

Streptococcus suis is an important zoonotic pathogen,which can cause septicemia,meningitis,arthritis,pneumonia and other diseases in pigs.In recent years,coinfection of Streptococcus suis and other pathogenic bacteria has been frequently reported,which greatly increases the difficulty of prevention and control of pig farms.The relationship between other pathogenic bacteria and Streptococcus suis under coinfection,especially its impact on the pathogenicity of Streptococcus suis,is rarely reported.Therefore,this study firstly conducted an epidemiological investigation on the prevalence of Streptococcus suis,and isolated and identified the main pathogenic bacteria;Subsequently,the pathogenicity of mixed infection in mice was confirmed by mouse infection experiments,and its pathogenic mechanism was initially explored;finally,the immunogenicity of the isolated Streptococcus suis attenuated strain to mice was tested,which provided a theoretical basis for the development of attenuated vaccines.In this study,bacterial isolation and identification and pathogenicity detection were carried out on the tissue samples of diseased pigs from some pig farms in Northeast China by means of colony morphology observation,staining microscopy,biochemical identification,16s r RNA sequencing,and mouse infection experiments.A total of 58 strains of 9 species of bacteria were isolated and identified from 30 tissue disease materials of the pig farm,including 17 strains of Staphylococcus,12 strains of Streptococcus(2 attenuated strains of Streptococcus suis type 16,1 attenuated strain of Streptococcus suis type 29),12 strains of Escherichia coli and 10 strains of Enterococcus;the mouse infection test results showed that Streptococcus suis,Staphylococcus,and Enterococcus had no obvious pathogenicity to mice,while Klebsiella pneumoniae,Serratia marcescens,Enterobacter cloacae,Escherichia coli,Moraxella wisconsin,Erwinia rhubarb are highly pathogenic to mice and can lead to death of mice;the glutamate dehydrogenase-encoding gene gdh of Streptococcus suis in serum was detected by nested PCR method,and the epidemiological investigation of Streptococcus suis was carried out on serum samples from some pig farms in Shandong,Changchun of Jilin,Inner Mongolia and other places,and 409 serum samples were tested.The results showed that Streptococcus suis was prevalent in some pig farms in Shandong,Changchun of Jilin,and Inner Mongolia,and the detection rates were 46.1%,47.9%,and 56.7%,respectively.Another batch of 132 sera from Shandong was tested for Streptococcus suis type 2 antibody by ELISA,the positive rate was 50%.Secondly,since Escherichia coli,Staphylococcus and Streptococcus suis were isolated from multiple organs in sick pigs at the same time,we carried out a pathogenic study on mice with mixed infection of the above bacteria.The test results showed that compared with the single infection of Escherichia coli,Staphylococcus simulans,and Streptococcus suis,coinfection significantly increased the lethality,clinical symptom score and body weight loss of mice,Significantly promoted the distribution and colonization of various bacteria in the heart,liver,spleen,lung and kidney tissues of mice,and aggravated tissue lesions;flow cytometry results showed that the numbers of macrophages and neutrophils in the above organs of mice after mixed infection decreased significantly,and the proportion of anti-inflammatory M2cells decreased.The relative increase in the proportion of pro-inflammatory PMN-I cells may be the reason for the enhanced pathogenicity.Finally,the immune protection of two isolated Streptococcus suis attenuated strains SS₁₆and SS₂₉on mice was studied.The results showed that the SS₁₆attenuated strain had a 40%cross-immunity protection rate against Streptococcus suis serotype 2,and delayed the death of the mice in the Streptococcus suis serotype 7 challenge group,while the SS₂₉attenuated strain was only 20%resistant to Streptococcus suis serotype2.The rate of cross-immunity protection was higher,and there was no cross-immunity protection effect on Streptococcus suis type 7.In summary,this study preliminarily clarified the prevalence of Streptococcus suis in some pig farms in Shandong,Changchun of Jilin Province and Inner Mongolia,and the pathogenicity of Escherichia coli,Staphylococcus simulans and Streptococcus suis in coinfection and the immune protection effect of the isolated Streptococcus suis attenuated strain were studied and evaluated.This study provides a reference for revealing the pathogenic mechanism of coinfection in clinical practice and effectively preventing Streptococcus suis related diseases.