Type VI secretion system of Pseudomonas aeruginosa is associated with biofilm formation but not environmental adaptation.

Pseudomonas aeruginosa encodes three type VI secretion systems (T6SSs), namely H1-, H2-, and H3-T6SS. P. aeruginosa hemolysin-coregulated protein (Hcp) is the effector protein and the hallmark of T6SS. Although T6SS is ubiquitous and affects ecology and human health, its general mechanism and physiological role are still not fully understood. Therefore, in this study, we investigated the impact of the P. aeruginosa T6SS on biofilm formation and environmental adaptation. To this end, we collected P. aeruginosa clinical isolates, divided them into strong biofilm formation (SBF) and nonbiofilm formation (NBF) groups based on their biofilm-forming ability, and compared their associated clinical characteristics. The duration of hospitalization was longer in patients infected with SBF than those infected with NBF strains. The expression levels of T6SS-related genes (hcp1 and hcp3) and a quorum-sensing gene (lasR) were higher in the SBF group as compared to those in the NBF group. In addition, the expression level of lasR was negatively associated with that of hcp1, but was positively associated with those of hcp2 and hcp3. Moreover, we evaluated the expression of T6SS- and biofilm-associated genes in planktonic and biofilm cells of the P. aeruginosa strain PAO1, and constructed strain PAO1△clpV1 to study the adaptation characteristics of H1-T6SS. The expression levels of hcp1, hcp2, hcp3, lasR, and other biofilm-associated genes were significantly higher in PAO1 biofilm cells as compared to those of planktonic cells. However, except for swarming ability as a vital feature for biofilm formation, there were no significant differences in the biofilm-forming ability and expression of biofilm-associated genes, adherence ability, growth characteristics, resistance to acid and osmotic pressure, surface structure, and morphology between the PAO1△clpV1 and PAO1 wild-type strains. Collectively, our results suggest that T6SS might play a role in biofilm formation and that H1-T6SS does not contribute to environmental adaptation in P. aeruginosa.