Rosini Roberto, Campisi Edmondo, De Chiara Matteo, Tettelin Hervé, Rinaudo Daniela, Toniolo Chiara, Metruccio Matteo, Guidotti Silvia, Sørensen Uffe B Skov, Kilian Mogens, Ramirez Mario, Janulczyk Robert, Donati Claudio, Grandi Guido, Margarit Immaculada
Novartis Vaccines and Diagnostics Srl, Siena, Italy.
Institute for Genome Sciences, University of Maryland School of Medicine, Baltimore, Maryland, United States of America.
PLoS One. 2015 May 6;10(5):e0125985. doi: 10.1371/journal.pone.0125985. eCollection 2015.
The human and bovine bacterial pathogen Streptococcus agalactiae (Group B Streptococcus, GBS) expresses a thick polysaccharide capsule that constitutes a major virulence factor and vaccine target. GBS can be classified into ten distinct serotypes differing in the chemical composition of their capsular polysaccharide. However, non-typeable strains that do not react with anti-capsular sera are frequently isolated from colonized and infected humans and cattle. To gain a comprehensive insight into the molecular basis for the loss of capsule expression in GBS, a collection of well-characterized non-typeable strains was investigated by genome sequencing. Genome based phylogenetic analysis extended to a wide population of sequenced strains confirmed the recently observed high clonality among GBS lineages mainly containing human strains, and revealed a much higher degree of diversity in the bovine population. Remarkably, non-typeable strains were equally distributed in all lineages. A number of distinct mutations in the cps operon were identified that were apparently responsible for inactivation of capsule synthesis. The most frequent genetic alterations were point mutations leading to stop codons in the cps genes, and the main target was found to be cpsE encoding the portal glycosyl transferase of capsule biosynthesis. Complementation of strains carrying missense mutations in cpsE with a wild-type gene restored capsule expression allowing the identification of amino acid residues essential for enzyme activity.
人源和牛源细菌病原体无乳链球菌（B族链球菌，GBS）表达一种厚厚的多糖荚膜，它是主要的毒力因子和疫苗靶点。GBS可分为十种不同的血清型，其荚膜多糖的化学成分各不相同。然而，经常从定植和感染的人和牛中分离出不与抗荚膜血清发生反应的不可分型菌株。为了全面了解GBS中荚膜表达缺失的分子基础，通过基因组测序对一组特征明确的不可分型菌株进行了研究。基于基因组的系统发育分析扩展到大量已测序菌株，证实了最近在主要包含人源菌株的GBS谱系中观察到的高度克隆性，并揭示了牛源群体中更高程度的多样性。值得注意的是，不可分型菌株在所有谱系中均匀分布。在cps操纵子中鉴定出一些明显导致荚膜合成失活的不同突变。最常见的基因改变是导致cps基因中出现终止密码子的点突变，并且发现主要靶点是编码荚膜生物合成的门户糖基转移酶的cpsE。用野生型基因对cpsE中携带错义突变的菌株进行互补，恢复了荚膜表达，从而确定了酶活性所必需的氨基酸残基。
