猪源ST9型耐甲氧西林金黄色葡萄球菌中前噬菌体的流行状况与转导分析

本研究旨在分析猪源ST9型耐甲氧西林金黄色葡萄球菌(Staphylococcus aureus,MRSA)中前噬菌体的流行情况、结构特点和转导能力，探究前噬菌体在猪源MRSA流行克隆形成中的作用。基于全基因组信息，分析了近年来从我国多省分离的131株ST9型MRSA中前噬菌体的流行率、分型、亲缘关系和结构特征；选取含不同分型前噬菌体的菌株进行诱导，对诱导获得的噬菌体颗粒进行转导，测定转导子的耐药表型及体外适应性。研究结果显示：猪源ST9型MRSA的前噬菌体携带率为78.6%(103/131株)，其中，63株携带完整前噬菌体序列，所有前噬菌体序列均不含耐药基因，仅2.9%(3/103株)的前噬菌体序列含毒力基因；前噬菌体谱型丰富，其整合酶分型主要为Sa2int和Sa4int；各型别前噬菌体结构同源性较高，完整前噬菌体可被诱导为长尾噬菌体；噬菌体颗粒可包装供体菌的aadD、tet(L)耐药基因并转导至受体菌中；转导子可获得卡那霉素、四环素耐药表型，体外生长能力与受体菌株无明显差异(P>0.05)。研究结果表明：猪源ST9型MRSA的前噬菌体携带率较高，谱型丰富，不携带耐药基因，部分噬菌体可包装供体菌的耐药基因转导至受体菌，产生的适应性代价小。

Prevalence and Transduction of Prophages in Methicillin-resistant Staphylococcus aureus ST9 of Swine Origin

The study aimed to analysis the prevalence, structural characteristics and transduction ability of prophages in porcine methicillin-resistant Staphylococcus aureus (MRSA) ST9, and to explore the role of prophages in the formation of porcine MRSA epidemic clones. Based on whole genome sequences, we analyzed the prevalence, typing, phylogeny, and structural characteristics of prophages in 131 MRSA ST9 strains isolated from several provinces of China in recent years. Strains with prophages of various types were selected to be induced into phage particles which were then performed transduction experiment. We determined the antimicrobial resistance and in vitro fitness cost of the transductants. The results showed that the carriage rate of prophages in porcine MRSA ST9 was 78.6% (103/131), of which 63 isolates carried prophages with complete structure. None of prophage harbored resistance gene, while only 2.9% sequences (3/103) carried virulence genes. The profiles of prophages were abundant, of which the integrase typing were mainly Sa2int and Sa4int. Different types of prophages had high structural homology, and the complete prophages could be induced into Siphoviridae. Resistance genes aadD and tet(L) of donor strains could be packaged into phage particles and transduced to recipient strain. The transductants acquired resistance phenotypes to kanamycin and tetracycline, while the growth ability of transductants had no significant difference with the recipient strains in vitro (P>0.05). These results indicated that the prophages of porcine ST9 MRSA have high carriage rate, various types, and do not carry resistance gene. Resistance genes of donor strains could be packaged and transduced into recipient strains by part of phages, resulting in little fitness costs.