A virulence-associated filamentous bacteriophage of Neisseria meningitidis increases host-cell colonisation

Neisseria meningitidis is a commensal of human nasopharynx. In some circumstances, this bacteria can invade the bloodstream and, after crossing the blood brain barrier, the meninges. A filamentous phage, designated MDAΦ for Meningococcal Disease Associated, has been associated with invasive disease. In this work we show that the prophage is not associated with a higher virulence during the bloodstream phase of the disease. However, looking at the interaction of N. meningitidis with epithelial cells, a step essential for colonization of the nasopharynx, we demonstrate that the presence of the prophage, via the production of viruses, increases colonization of encapsulated meningococci onto monolayers of epithelial cells. The analysis of the biomass covering the epithelial cells revealed that meningococci are bound to the apical surface of host cells by few layers of heavily piliated bacteria, whereas, in the upper layers, bacteria are non-piliated but surrounded by phage particles which (i) form bundles of filaments, and/or (ii) are in some places associated with bacteria. The latter are likely to correspond to growing bacteriophages during their extrusion through the outer membrane. These data suggest that, as the biomass increases, the loss of piliation in the upper layers of the biomass does not allow type IV pilus bacterial aggregation, but is compensated by a large production of phage particles that promote bacterial aggregation via the formation of bundles of phage filaments linked to the bacterial cell walls. We propose that MDAΦ by increasing bacterial colonization in the mucosa at the site-of-entry, increase the occurrence of diseases.脑膜炎奈瑟菌（Neisseria meningitidis）是人类鼻咽部的共生菌群成员。在特定条件下，该细菌可侵入血流，并在穿过血脑屏障（blood brain barrier）后累及脑膜。一种被命名为MDAΦ（脑膜炎球菌疾病相关噬菌体，Meningococcal Disease Associated）的丝状噬菌体（filamentous phage），曾被关联至侵袭性疾病的发生。本研究证实，该前噬菌体（prophage）并不会在疾病的血流侵袭阶段提升细菌的毒力。然而，通过研究脑膜炎奈瑟菌与上皮细胞（epithelial cells）的相互作用——这是鼻咽定植（colonization）的关键步骤——我们发现，该前噬菌体通过产生噬菌体颗粒，可增强有荚膜脑膜炎球菌在上皮细胞单层上的定植能力。对覆盖上皮细胞的生物膜进行分析后发现，脑膜炎球菌通过数层高菌毛化的细菌结合于宿主细胞的顶膜表面（apical surface）；而在生物膜的上层区域，细菌不再具有菌毛，但被噬菌体颗粒所包裹：（i）形成丝状束，且/或（ii）在部分区域与细菌结合。后者极有可能对应着噬菌体从细菌外膜（outer membrane）挤出过程中正在增殖的噬菌体颗粒。上述数据表明，随着生物膜体量增加，上层区域细菌丧失菌毛后无法通过IV型菌毛（type IV pilus）实现细菌聚集，但这一缺陷可通过大量产生噬菌体颗粒得到弥补——这些噬菌体通过形成与细菌细胞壁相连的丝状束，促进细菌聚集。我们据此提出，MDAΦ可通过增强病原菌在入侵部位黏膜处的定植能力，提升疾病的发生风险。