| Streptococcus suis is not only an important pathogen in pigs,but also a significant zoonotic pathogen worldwide.Based on the antigenicity of its capsular polysaccharide(CPS),S.suis can be divided into 29 traditional serotypes,27 new capsular gene clusters(NCL)and untyped strains.Nine serotypes are known to infect people,including serotypes 2,4,5,9,14,16,21,24,and 31,which pose a serious threat to public health.The first case of human infection with S.suis serotype 31 was reported in Thailand in 2015,but there are few studies on the pathogenicity and drug resistance characteristics of S.suis serotype 31.During 2017-2019,tonsils and nasal swabs from healthy pigs were collected from slaughterhouses in Jiangsu province,and then S.suis from these samples were isolated and identified.We found that the highest isolation rate was S.suis serotype 31.Therefore,in this study we continued to isolate and identify S.suis from healthy pigs from slaughterhouses in this region,and performed genome-wide sequencing,drug resistance assay,and zebrafish infection experiment with S.suis serotype 31 strains isolated from healthy and diseased pigs.The research results not only help to understand the incidence of S.suis in the region and formulate strategies for the prevention and control of swine streptococcosis,but also contribute to further understanding of the pathogenicity and drug resistance characteristics of S.suis serotype 31.1.Epidemiological survey of S.suis in a slaughterhouse in Jiangsu ProvinceIn 2019,76 tonsil and nasal swab samples from healthy pigs were collected from slaughterhouses in this region,of which 43 were S.suis positive(56.58%).Together with the samples isolated in 2017 and 2018 from slaughterhouses in this region,454 samples were collected in total and 311 samples were S.suis positive(68.50%).From 2017 to 2019,a total of 405 S.suis strains were isolated and further serotyped by PCR.There were 342(84.44%)traditional serotypes,20(4.94%)NCL strains and 43 strains(10.62%)of unknown serotypes.It was found that the isolation rate of serotype 31 was the highest,accounting for 16.96%(58/342).The remaining strains were serotype 9(14.04%),serotype 16(11.70%),serotype 2(9.65%),serotype 3(8.19%),serotype 4(8.19%),serotype 5(6.14%),and serotype 7(5.85%),serotype 8(4.97%).serotype 28(2.92%),serotype 15(2.05%),serotype 11(1.75%),serotype 21(1.75%),serotype 12(1.46%),serotype 29(1.17%),serotype 23(0.88%),serotype 25(0.88%).serotype 30(0.88%),serotype 13(0.29%)and serotype 17(0.29%).2.Pathogenicity and resistance characteristics of S.suis serotype 3153 strains of S.suis 31 strains isolated from healthy and diseased pigs were subjected to whole genome sequencing,and a core gene single nucleotide polymorphism(SNP)evolutionary analysis was performed with the 23 genome sequences of serotype 31 from the NCBI database.The genomes of all strains can be divided into two large clusters:cluster A and cluster B.Multilocus sequence typing(MLST)analysis revealed that only 7 of the 53 strains had known ST and the rest were unknown,and all strains had no genetic relationship with the reported human isolate 43640(ST221)in Thailand.The susceptibility of 53 strains of S.suis serotype 31 to antibiotics was tested,including penicillin,amoxicillin,ceftiofur,rifampicin,vancomycin,linezolid,enrofloxacin,moxafloxacin,chloramphenicol,flufenicol,lincomycin,clindamycin,tiamulin,wunimulin,gentamicin,spectinomycin,telmicoxin,tyrasin,erythromycin,doxycycline,and azithromycin.The results showed severe multidrug resistance;strain WUSS375 isolated from a healthy pig was resistant to sixteen antibiotics;34.0%(18/53)of strains were resistant to ten antibiotics.Moreover,serotype 31 strains were highly resistant to macrolides(100%),tetraflavins(100%),and lincosamides(100%);they were sensitive to rifampicin(1.9%),vancomycin(1.9%),and ?-lactams(9.4%).88.68%isolates(47/53)contain macrolide resistance genes;90.57%isolates(48/53)contain tetracycline resistance genes;77.36%isolates(41/53)contain aminoglycoside antibiotic resistance genes.The detection rate of erm(B),ant(6)-Ia and tet(O)respectively reached 84.91%(45/53),71.70%(38/53),and 52.83%(28/53),which roughly matched its drug-resistant phenotype.But the detection rate of lincosamide resistant genes was only 24.53%(13/53).18 strains were selected for zebrafish infection experiment.The results showed that 15 strains were pathogenic to zebrafish,with zebrafish mortality of 70-100%when challenged at 106CFU/fish.Four strains were 100%lethal to zebrafish,three were isolated from diseased pigs,and one were isolated from healthy pigs.In summary,S.suis serotype 31 strins show the genetic diversity,and the presence of virulent and multi-drug-resistant S.suis serotype 31 strains in slaughterhouses has potential public health significance. |
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