The Pel Polysaccharide Can Serve a Structural and Protective Role in the Biofilm Matrix of Pseudomonas aeruginosa

Bacterial extracellular polysaccharides are a key constituent of the extracellular matrix material of biofilms. Pseudomonas aeruginosa is a model organism for biofilm studies and produces three extracellular polysaccharides that have been implicated in biofilm development, alginate, Psl and Pel. Significant work has been conducted on the roles of alginate and Psl in biofilm development, however we know little regarding Pel. In this study, we demonstrate that Pel can serve two functions in biofilms. Using a novel assay involving optical tweezers, we demonstrate that Pel is crucial for maintaining cell-to-cell interactions in a PA14 biofilm, serving as a primary structural scaffold for the community. Deletion of pelB resulted in a severe biofilm deficiency. Interestingly, this effect is strain-specific. Loss of Pel production in the laboratory strain PAO1 resulted in no difference in attachment or biofilm development; instead Psl proved to be the primary structural polysaccharide for biofilm maturity. Furthermore, we demonstrate that Pel plays a second role by enhancing resistance to aminoglycoside antibiotics. This protection occurs only in biofilm populations. We show that expression of the pel gene cluster and PelF protein levels are enhanced during biofilm growth compared to liquid cultures. Thus, we propose that Pel is capable of playing both a structural and a protective role in P. aeruginosa biofilms.

细菌胞外多糖（bacterial extracellular polysaccharides）是生物被膜（biofilm）胞外基质的关键组成成分。铜绿假单胞菌（Pseudomonas aeruginosa）是生物被膜研究的模式菌株，可合成三种与生物被膜形成相关的胞外多糖：藻酸盐（alginate）、Psl及Pel。目前针对藻酸盐与Psl在生物被膜形成中的功能已有大量研究，但学界对Pel的了解仍十分有限。本研究证实，Pel在生物被膜中可承担两种功能：本研究采用新型光镊（optical tweezers）实验体系，证实Pel在PA14菌株的生物被膜中对维持细胞间相互作用至关重要，是该群落的核心结构支架；敲除pelB基因会导致生物被膜形成能力严重缺陷。有趣的是，该效应具有菌株特异性：在实验室菌株PAO1中阻断Pel合成，并不会影响其黏附能力或生物被膜形成过程，此时Psl才是维持生物被膜成熟的核心结构多糖。此外，本研究证实Pel可通过提升对氨基糖苷类抗生素的抗性发挥第二种功能，且这种保护作用仅存在于生物被膜菌群中。研究发现，相较于液体培养条件，生物被膜生长过程中pel基因簇的表达水平与PelF蛋白含量均有所上调。综上，本研究提出：Pel在铜绿假单胞菌生物被膜中可同时发挥结构支撑与保护防御的双重作用。