A Study Of Attaching-behaviors And -Phenotype Variation In Planktonic Cells Of Pseudomonas Aeruginosa

Biofilms are typically composed of a dense layer of bacteria growing in a self-produced matrix that holds the cells together to form surface-associated communities,thereby enabling them to survive or thrive in various environments ranging from mineral surfaces to human tissue.Individual cells growing in biofilms are physiologically and phenotypically distinct even if the biofilms is formed by a genetically identical species.This is partially because of the nonhomogeneous distribution of nutrients,electron ac-ceptors(oxygen),metabolic waste and signaling molecules in biofilms,where the local concentrations of these small molecules are determined by their diffusion rates,cell densities,and external cycling conditions.By contrast,exponentially growing cells in a well-mixed planktonic culture are traditionally assumed to be physiologically and phe-notypically uniform.Increasingly,researchers have reported cell-to-cell heterogeneity in a variety of physiological parameters including growth rate,chemotaxis,metabolism,nutritional acquisition,and tolerance to noxious stimuli including antibiotics,and this phenotypic heterogeneity in an isogenic population has been observed in many different bacterial species.The attachment of planktonic cells to surfaces is the first and most crucial step in biofilm formation.In our text,we focused on the study of attaching-behaviors and-phenotype variation in planktonic cells of Pseudomonas aeruginosa.Using a combina-tion of high spatiotemporal microscopy and a bacterial tracking algorithm,we showed that the planktonic cells of P.aeruginosa differently attach to surfaces even when they remained in the exponential phase.Typically,in the later exponential phase,approx-imately 80%of the cells can quickly attach to surfaces within 15 minutes,whereas approximately 20%of the cells slowly attach to surfaces.Notably,we consistently ob-served that fast-and slow-attaching phenotypes coexist in planktonic cells,regardless of their growth phase.Furthermore,we found that 1)the distinct attaching phenotypes of planktonic cells resulted from the differential production of polysaccharides Psl,and that 2)the RsmYZ/RsmA signaling pathway mainly regulated the differential produc-tion of Psl.The differential production of Psl in P.aeruginosa plays a significant role in biofilm development and formation in the presence of flows.For instance,the differential pro-duction of Psl 1)enables subpopulations of planktonic cells to quickly attach to surfaces in the initial stage;2)enables subpopulations of surface-associated cells to differentiate into ones that can detach from surfaces in the early stage;and 3)subsequently enables young biofilms to form matrix-sophisticated structures in the mature stage.We speculate that different approaches used by planktonic cells to attach on sur-faces constitute a bet-hedging strategy that has naturally evolved in P.aeruginosa.The strategy enables only a subset of the population to attach on the surface that might be or might not be a suitable habitat for biofilm formation;however,the remainder of the population can live in the mobile phase regardless of whether the attaching cells suc-ceed in forming biofilms.This strategy is expected to provide evolutionary benefits to P.aeruginosa in overcoming unpredictable environmental perturbations.