Molecular Characterization of Streptococcus agalactiae Isolated from Bovine Mastitis in Eastern China

One hundred and two Streptococcus agalactiae (group B streptococcus [GBS]) isolates were collected from dairy cattle with subclinical mastitis in Eastern China during 2011. Clonal groups were established by multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE), respectively. Capsular polysaccharides (CPS), pilus and alpha-like-protein (Alp) family genes were also characterized by molecular techniques. MLST analysis revealed that these isolates were limited to three clonal groups and were clustered in six different lineages, i.e. ST (sequence type) 103, ST568, ST67, ST301, ST313 and ST570, of which ST568 and ST570 were new genotypes. PFGE analysis revealed this isolates were clustered in 27 PFGE types, of which, types 7, 8, 14, 15, 16, 18, 23 and 25 were the eight major types, comprising close to 70% (71/102) of all the isolates. The most prevalent sequence types were ST103 (58% isolates) and ST568 (31% isolates), comprising capsular genotype Ia isolates without any of the detected Alp genes, suggesting the appearance of novel genomic backgrounds of prevalent strains of bovine S. agalactiae. All the strains possessed the pilus island 2b (PI-2b) gene and the prevalent capsular genotypes were types Ia (89% isolates) and II (11% isolates), the conserved pilus type providing suitable data for the development of vaccines against mastitis caused by S. agalactiae.2011年间，于中国东部地区从患有亚临床乳腺炎的奶牛体内分离得到102株无乳链球菌（Streptococcus agalactiae，又称B群链球菌GBS）菌株。本研究分别通过多位点序列分型（multilocus sequence typing，MLST）与脉冲场凝胶电泳（pulsed-field gel electrophoresis，PFGE）对上述菌株进行克隆群分型；同时借助分子生物学技术对荚膜多糖（capsular polysaccharides，CPS）、菌毛及α样蛋白（alpha-like-protein，Alp）家族基因开展特征分析。多位点序列分型结果显示，本次分离的菌株仅隶属于3个克隆群，可划分为6个不同谱系，即序列型（sequence type，ST）103、ST568、ST67、ST301、ST313与ST570，其中ST568与ST570为新型基因型。脉冲场凝胶电泳分析则将受试菌株分为27种PFGE型别，其中7、8、14、15、16、18、23及25型为主要型别，合计覆盖近70%（71/102）的受试菌株。本研究中最流行的序列型分别为ST103（占总菌株数的58%）与ST568（占31%），二者均对应荚膜基因型Ia且未检出任何Alp家族基因，提示奶牛源无乳链球菌的流行菌株存在全新的基因组背景。所有受试菌株均携带菌毛岛2b（pilus island 2b，PI-2b）基因，优势荚膜基因型为Ia型（占89%）与II型（占11%）；保守的菌毛类型可为无乳链球菌所致乳腺炎的疫苗开发提供适宜的靶标数据。