Importance of twitching and surface-associated motility in the virulence of Acinetobacter baumannii

Acinetobacter baumannii is a pathogen of increasing clinical importance worldwide, especially given its ability to readily acquire resistance determinants. Motile strains of this bacterium can move by either or both of two types of motility: (i) twitching, driven by type IV pili, and (ii) surface-associated motility, an appendage-independent form of movement. A. baumannii strain MAR002 possesses both twitching and surface-associated motility. In this study, we isolated spontaneous rifampin-resistant mutants of strain MAR002 in which point mutations in the rpoB gene were identified that resulted in an altered motility pattern. Transcriptomic analysis of mutants lacking twitching, surface-associated motility, or both led to the identification of deregulated genes within each motility phenotype, based on their level of expression and their biological function. Investigations of the corresponding knockout mutants revealed several genes involved in the motility of A. baumannii strain MAR002, including two involved in twitching (encoding a minor pilin subunit and an RND [resistance nodulation division] component), one in surface-associated motility (encoding an amino acid permease), and eight in both (encoding RND and ABC components, the energy transducer TonB, the porin OprD, the T6SS component TagF, an IclR transcriptional regulator, a PQQ-dependent sugar dehydrogenase, and a putative pectate lyase). Virulence assays showed the reduced pathogenicity of mutants with impairments in both types of motility or in surface-associated motility alone. By contrast, the virulence of twitching-affected mutants was not affected. These results shed light on the key role of surface-associated motility and the limited role of twitching in the pathogenicity of A. baumannii.

鲍曼不动杆菌作为一种在全球范围内日益重要的病原体，其易于获得耐药决定因子的能力尤为显著。该细菌的游动菌株可通过以下两种游动方式之一或两种同时进行游动：一是通过Ⅳ型菌毛驱动的牵拉运动，二是表面相关游动，这是一种独立于附属器官的运动形式。鲍曼不动杆菌菌株MAR002兼具牵拉运动和表面相关游动。在本研究中，我们分离出了一种自发性利福平耐药突变体，其中在rpoB基因中发现了导致游动模式改变的点突变。对缺乏牵拉运动、表面相关游动或两者兼有的突变体进行转录组分析，基于其表达水平和生物学功能，识别出每个游动表型中的失调基因。对相应的敲除突变体的研究揭示了若干与鲍曼不动杆菌菌株MAR002游动相关的基因，包括两个与牵拉运动相关（编码次要菌毛亚单位和RND[抗性结节分化]组分），一个与表面相关游动相关（编码氨基酸渗透酶），以及八个与两者都相关（编码RND和ABC组分、能量转换器TonB、孔蛋白OprD、T6SS组分TagF、IclR转录调控因子、PQQ依赖性糖脱氢酶和假定的果胶裂解酶）。致病性试验表明，同时损害两种类型游动或仅损害表面相关游动的突变体表现出降低的致病性。相反，受牵拉运动影响的突变体的致病性未受影响。这些结果揭示了表面相关游动在鲍曼不动杆菌致病性中的关键作用以及牵拉运动在致病性中的有限作用。