Abstract
The opportunistic pathogen Pseudomonas aeruginosa is a significant cause of infection in immunocompromised individuals, cystic fibrosis patients, and burn victims. To benefit its survival, the bacterium adapt to either a motile or sessile lifestyle when infecting the host. The motile bacterium has an often activated type III secretion system (T3SS), which is virulent to the host, whereas the sessile bacterium harbors an active T6SS and lives in biofilms. Regulatory pathways involving Gac-Rsm or secondary messengers such as c-di-GMP determine which lifestyle is favorable for P. aeruginosa. Here, we introduce the RNA binding protein RtcB as a modulator of the switch between motile and sessile bacterial lifestyles. Using the wild-type P. aeruginosa PAO1, and a retS mutant PAO1(∆retS) in which T3SS is repressed and T6SS active, we show that deleting rtcB led to simultaneous expression of T3SS and T6SS in both PAO1(∆rtcB) and PAO1(∆rtcB∆retS). The deletion of rtcB also increased biofilm formation in PAO1(∆rtcB) and restored the motility of PAO1(∆rtcB∆retS). RNA-sequencing data suggested RtcB as a global modulator affecting multiple virulence factors, including bacterial secretion systems. Competitive killing and infection assays showed that the three T6SS systems (H1, H2, and H3) in PAO1(∆rtcB) were activated into a functional syringe, and could compete with Escherichia coli and effectively infect lettuce. Western blotting and RT-PCR results showed that RtcB probably exerted its function through RsmA in PAO1(∆rtcB∆retS). Quantification of c-di-GMP showed an elevated intracellular levels in PAO1(∆rtcB), which likely drove the switch between T6SS and T3SS, and contributed to the altered phenotypes and characteristics observed. Our data demonstrate a pivotal role of RtcB in the virulence of P. aeruginosa by controlling multiple virulence determinants, such as biofilm formation, motility, pyocyanin production, T3SS, and T6SS secretion systems towards eukaryotic and prokaryotic cells. These findings suggest RtcB as a potential target for controlling P. aeruginosa colonization, establishment, and pathogenicity.
Highlights
We explored the role of RtcB beyond its stress response activity (RNA 20, 30 -cyclic phosphate, and 50 -OH ligase) in P. aeruginosa
We investigated the role of RtcB P. aeruginosa in pathogenicity and the underlying mechanisms using the wild type P. aeruginosa PAO1, which has an active T3SS and inactive T6SS under normal lab conditions, and a retS mutant
Protein blast analysis of RtcB in E. coli and P. aeruginosa showed that the proteins are 70% identical, expectedly fulfilling similar functions [18]
Summary
P. aeruginosa is a motile Gram-negative bacterium and a nosocomial pathogen causing life-threatening infections in humans [1]. P. aeruginosa produces a variety of virulence factors, including cell-associated structural features such as flagellum and pili, and extracellular effectors such as exotoxins and pyocyanin [3]. These exotoxins are secreted to the environment or directly to the host and/or other bacteria [4,5,6]. During the early stages of infection, the bacterial pathogen in a planktonic and motile lifestyle induces tissue damage and inflammation in the host [7]
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