Caboni Mariaelena, Pédron Thierry, Rossi Omar, Goulding David, Pickard Derek, Citiulo Francesco, MacLennan Calman A, Dougan Gordon, Thomson Nicholas R, Saul Allan, Sansonetti Philippe J, Gerke Christiane
Novartis Vaccines Institute for Global Health, Siena, Via Fiorentina, Italy.
Institut Pasteur, Unité de Pathogénie Microbienne Moléculaire, INSERM U1202, Paris, France.
PLoS Pathog. 2015 Mar 20;11(3):e1004749. doi: 10.1371/journal.ppat.1004749. eCollection 2015 Mar.
Shigella is the leading cause for dysentery worldwide. Together with several virulence factors employed for invasion, the presence and length of the O antigen (OAg) of the lipopolysaccharide (LPS) plays a key role in pathogenesis. S. flexneri 2a has a bimodal OAg chain length distribution regulated in a growth-dependent manner, whereas S. sonnei LPS comprises a monomodal OAg. Here we reveal that S. sonnei, but not S. flexneri 2a, possesses a high molecular weight, immunogenic group 4 capsule, characterized by structural similarity to LPS OAg. We found that a galU mutant of S. sonnei, that is unable to produce a complete LPS with OAg attached, can still assemble OAg material on the cell surface, but a galU mutant of S. flexneri 2a cannot. High molecular weight material not linked to the LPS was purified from S. sonnei and confirmed by NMR to contain the specific sugars of the S. sonnei OAg. Deletion of genes homologous to the group 4 capsule synthesis cluster, previously described in Escherichia coli, abolished the generation of the high molecular weight OAg material. This OAg capsule strongly affects the virulence of S. sonnei. Uncapsulated knockout bacteria were highly invasive in vitro and strongly inflammatory in the rabbit intestine. But, the lack of capsule reduced the ability of S. sonnei to resist complement-mediated killing and to spread from the gut to peripheral organs. In contrast, overexpression of the capsule decreased invasiveness in vitro and inflammation in vivo compared to the wild type. In conclusion, the data indicate that in S. sonnei expression of the capsule modulates bacterial pathogenesis resulting in balanced capabilities to invade and persist in the host environment.
志贺氏菌是全球范围内痢疾的主要病因。脂多糖（LPS）的O抗原（OAg）的存在及其长度，与几种用于入侵的毒力因子一起，在发病机制中起着关键作用。福氏志贺氏菌2a具有以生长依赖方式调节的双峰OAg链长分布，而宋内志贺氏菌的LPS则包含单峰OAg。在这里，我们发现宋内志贺氏菌而非福氏志贺氏菌2a拥有一种高分子量、具有免疫原性的4型荚膜，其特征是与LPS OAg结构相似。我们发现，宋内志贺氏菌的galU突变体无法产生带有OAg的完整LPS，但仍能在细胞表面组装OAg物质，而福氏志贺氏菌2a的galU突变体则不能。从未内志贺氏菌中纯化出未与LPS相连的高分子量物质，经核磁共振证实含有宋内志贺氏菌OAg的特定糖类。删除与先前在大肠杆菌中描述的4型荚膜合成簇同源的基因，消除了高分子量OAg物质的产生。这种OAg荚膜强烈影响宋内志贺氏菌的毒力。无荚膜的基因敲除细菌在体外具有高度侵袭性，在兔肠道中具有强烈炎症反应。但是，缺乏荚膜降低了宋内志贺氏菌抵抗补体介导杀伤以及从肠道扩散到外周器官的能力。相比之下，与野生型相比，荚膜的过表达降低了体外侵袭性和体内炎症反应。总之，数据表明在宋内志贺氏菌中，荚膜的表达调节细菌发病机制，从而导致在宿主环境中入侵和持续存在的能力达到平衡。
