Identification and characterization of key regulatory elements involved in virulence modulations of Pseudomonas aeruginosa

Abstract

The opportunistic pathogen Pseudomonas aeruginosa is a significant cause of infection in immunocompromised individuals, cystic fibrosis patients, and burn victims. For its survival advantage, the bacterium adapts to a motile or sessile lifestyle when infecting the host. The active type III secretion system in motile bacterial cells enables a cytotoxic effect on the host. In contrast, an operative type VI secretion in sessile bacterial cells embedded in biofilms helps survive against bacterial competitors. P. aeruginosa switches between the lifestyles through regulatory pathways, including Gac-Rsm and secondary messengers like c-di-GMP. In this thesis, different molecular techniques were applied to understand the regulatory pathways of H1-T6SS, and the newly identified modulators were characterized. The results revealed the RNA binding protein RtcB as a switch controller of a motile and sessile bacterial lifestyle. Wildtype PAO1 in in-vitro conditions, armed with an active T3SS and inactive T6SS. In contrast, PAO1(∆retS) bears an active T6SS and repressed T3SS. Deletion of rtcB led to simultaneous expression of T3SS and T6SS in both PAO1 and PAO1(∆retS). Furthermore, the deletion of rtcB increased the biofilm formation in PAO1(∆rtcB) and restored the motility of PAO1(∆rtcB∆retS). The killing assay showed that H1-T6SS was activated in PAO1(∆rtcB) and could compete against Escherichia coli. Transcriptome analysis was performed for PAO1, PAO1(∆rtcB), PAO1(∆retS), and PAO1(∆rtcB∆retS). It revealed 370 genes of PAO1(∆rtcB) and 1030 genes in PAO1(∆rtcB∆retS) were differentially expressed. That includes genes encoding various virulence factors and the four types of secretion systems. Quantification of c-di-GMP showed elevated levels in PAO1(∆rtcB), which contributed to the altered phenotype and characteristics of PAO1(∆rtcB). The long-chain-fatty-acid—CoA ligase FadD1 was also identified as a new player in H1-T6SS regulation in this thesis. It deactivated the expression of H1-T6SS in the absence of sensor kinase RetS. In PAO1(ΔfadD1∆retS), the expression of sRNAs RsmY and RsmZ which was essential for activation of H1-T6SS, was repressed, and the intracellular concentrations of c-di-GMP diminished. This alteration led to an elevation in swarming motility, higher cAMP, and restoration of T3SS, wherein PAO1(ΔretS) was inactive, indicating that fadD1 plays a role in influencing both H1-T6SS and other virulence factors. In summary, work from this thesis has revealed a pivotal role of RtcB and FadD1 in the virulence of P. aeruginosa. Furthermore, it gives new insights into the complex regulatory network that modulates the switch between T6SS and T3SS in P. aeruginosa.