中国罗非鱼主养区无乳链球菌的分子流行特征及其传播方式

为分析2007—2015年中国罗非鱼主养区无乳链球菌(Streptococcus agalactiae)的分子特征和流行情况,分离并收集了248株罗非鱼源无乳链球菌。通过分子血清型、MLST、毒力基因和前噬菌体等分型方法对248株无乳链球菌进行了分子遗传特征分析。结果表明,229株无乳链球菌(92.3%)的分子血清型是Ⅰa型,其余19株均是Ⅰb型(7.7%)。MLST分析结果表明,所有Ⅰa型无乳链球菌都是ST7型,所有Ⅰb型无乳链球菌都是ST261型。毒力基因检测结果发现,229株Ⅰa-ST7型无乳链球菌的毒力基因型相同,即V1型;19株Ⅰb-ST261型无乳链球菌的毒力基因型相同,即V2型。前噬菌体检测结果表明,Ⅰa-ST7型无乳链球菌可分为两种前噬菌体基因型,分别是P1型(36株)和P2型(193株);Ⅰb-ST261型无乳链球菌的10个前噬菌体基因都是阴性,即P3型。根据以上4种分子分型方法可将248株无乳链球菌分为3种基因型,即Ⅰa-ST7-V1-P1型、Ⅰa-ST7-V1-P2型和Ⅰb-ST261-V2-P3型。2010—2011年主要流行菌株由Ⅰa-ST7-V1-P1型转变为Ⅰa-ST7-V1-P2型,其中Ⅰa-ST7-V1-P1型是2011年之前的主要流行菌株,Ⅰa-ST7-V1-P2型在2011年及之后成为主要流行菌株。研究表明,近年来我国罗非鱼源无乳链球菌发生了明显的遗传变异,同时,根据我国罗非鱼主养区无乳链球菌的流行特点,推测我国罗非鱼源无乳链球菌是通过苗种或水体等介质进行传播的,属于输入性传播方式。

Molecular characteristics and transmission of Streptococcus agalactiae in a major tilapia culturing area of China

Streptococcosis in cultured tilapia (Oreochromis spp.) caused by Streptococcus agalactiae has increased during the past decade in China. In this study, a total of 248S. agalactiae strains were isolated from diseased tila-pia, collected in the major tilapia-culturing area of China, during 2007?2015. The aim of the study was to analyze the characteristics of the 248 tilapiaS. agalactiae strains. In molecular characterization assays, four genotypic categories comprised of molecular serotype, multilocus sequence typing (MLST), virulence genes profiling pat-terns and prophage genotype, were used to analyze the genotypic diversity of theseS. agalactiae strains. The re-sults showed that 229 of the 248S. agalactiae strains were of molecular serotype Ia, and the remaining 19 were of type Ib. MLST revealed that all the serotype Ia strains were ST7, and all the serotype Ib strains were ST261. The results of virulence genes detection indicated that the 229 Ia-ST7S. agalactiae strains have the same genotype of virulence genes (V1), which carried 9 virulence genes (9/11: 81.8%), except thatscpB andlmb virulence genes were negative for them. The 19 Ib-ST261S. agalactiae strains have the same genotype of virulence genes (V2), which possessed 6 virulence genes (6/11: 54.5%) detected by PCR. Prophage detection suggested that the 229 Ia-ST7S. agalactiae strains could be divided into two genotypes: type P1 (36 strains) and P2 (193 strains). All the Ib-ST261 strains were negative for 10 prophage genes, and they belonged to type P3. According to the four mo-lecular typing methods, all 248S. agalactiae strains could be divided into three genotypes: namely, Ia-ST7-V1-P1, Ia-ST7-V1-P2, and Ib-ST261-V2-P3. Notably, the predominant strains ofS. agalactiae in China had shifted from type Ia-ST7-V1-P1 to type Ia-ST7-V1-P2 during 2010?2011. In addition, the type Ia-ST7-V1-P1 strain was pre-dominant before 2011, and the type Ia-ST7-V1-P2 strain predominated thereafter. In conclusion, theS. agalactiae strains collected from tilapia in China showed low genetic diversity. Through analyses of the characteristics of the S. agalactiae strains isolated from deceased tilapia in the major tilapia-culturing area of China, we can deduce that the transmission ofS. agalactiae relies on the import of either new tilapia stock or culture water.