Modulation of flagellar rotation in surface-attached bacteria: A pathway for rapid surface-sensing after flagellar attachment

Attachment is a necessary first step in bacterial commitment to surface-associated behaviors that include colonization, biofilm formation, and host-directed virulence. The Gram-negative opportunistic pathogen Pseudomonas aeruginosa can initially attach to surfaces via its single polar flagellum. Although many bacteria quickly detach, some become irreversibly attached and express surface-associated structures, such as Type IV pili, and behaviors, including twitching motility and biofilm initiation. P. aeruginosa that lack the GTPase FlhF assemble a randomly placed flagellum that is motile; however, we observed that these mutant bacteria show defects in biofilm formation comparable to those seen for non-motile, aflagellate bacteria. This phenotype was associated with altered behavior of ΔflhF bacteria immediately following surface-attachment. Forward and reverse genetic screens led to the discovery that FlhF interacts with FimV to control flagellar rotation at a surface, and implicated cAMP signaling in this pathway. Although cAMP controls many transcriptional programs in P. aeruginosa, known targets of this second messenger were not required to modulate flagellar rotation in surface-attached bacteria. Instead, alterations in switching behavior of the motor appeared to result from direct or indirect effects of cAMP on switch complex proteins and/or the stators associated with them.

附着是细菌启动一系列表面相关行为的必要前置步骤，这些行为包括定殖、生物膜形成以及宿主靶向毒力。革兰氏阴性机会致病菌铜绿假单胞菌（Pseudomonas aeruginosa）最初可通过其单极鞭毛附着于表面。尽管多数细菌会快速脱离，但部分细菌会转变为不可逆附着状态，并表达IV型菌毛（Type IV pili）等表面相关结构，同时启动抽动运动、生物膜起始等相关行为。缺失GTP酶FlhF的铜绿假单胞菌可组装出随机分布的可运动鞭毛；但本研究观察到，这类突变株的生物膜形成缺陷与非运动性、无鞭毛菌株的缺陷程度相当。该表型与ΔflhF菌株在表面附着后即刻的行为改变相关。正向与反向遗传筛选发现，FlhF可与FimV相互作用以调控表面环境下的鞭毛旋转，并证实cAMP信号通路参与该调控过程。尽管cAMP可调控铜绿假单胞菌的诸多转录程序，但该第二信使的已知靶标并不参与调控表面附着细菌的鞭毛旋转。取而代之的是，鞭毛马达的切换行为改变似乎是cAMP对切换复合物蛋白及其相关联的定子结构产生直接或间接作用所导致的。