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.表面附着是细菌启动各类表面相关行为的必要初始步骤，此类行为包括定植、生物被膜形成以及宿主定向毒力。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、生物被膜起始等行为。缺失GTP酶FlhF（GTPase FlhF）的铜绿假单胞菌会组装出随机定位的鞭毛且仍保有运动能力；但我们观察到，该ΔflhF突变株的生物被膜形成缺陷程度与无运动能力的无鞭毛细菌相当。该表型与ΔflhF细菌在表面附着后的早期行为改变密切相关。正向与反向遗传筛选结果显示，FlhF可与FimV相互作用以调控表面附着状态下的鞭毛旋转，并提示环腺苷酸（cAMP）信号通路参与该调控通路。尽管cAMP可调控铜绿假单胞菌的众多转录程序，但已知的该第二信使靶标并不参与调控表面附着细菌的鞭毛旋转。相反，鞭毛马达的切换行为改变似乎源于cAMP对鞭毛旋转开关复合物蛋白及其相关定子复合物的直接或间接作用。