Biofilms were grown from wild-type (WT) Pseudomonas aeruginosa PAO1 and the cell signaling lasI mutant PAO1-JP1 under laminar and turbulent flows to investigate the relative contributions of hydrodynamics and cell signaling for biofilm formation. Various biofilm morphological parameters were quantified using Image Structure Analyzer software. Multivariate analysis demonstrated that both cell signaling and hydrodynamics significantly (P < 0.000) influenced biofilm structure. In turbulent flow, both biofilms formed streamlined patches, which in some cases developed ripple-like wave structures which flowed downstream along the surface of the flow cell. In laminar flow, both biofilms formed monolayers interspersed with small circular microcolonies. Ripple-like structures also formed in four out of six WT biofilms, although their velocity was approximately 10 times less than that of those that formed in the turbulent flow cells. The movement of biofilm cell clusters over solid surfaces may have important clinical implications for the dissemination of biofilm subject to fluid shear, such as that found in catheters. The ability of the cell signaling mutant to form biofilms in high shear flow demonstrates that signaling mechanisms are not required for the formation of strongly adhered biofilms. Similarity between biofilm morphologies in WT and mutant biofilms suggests that the dilution of signal molecules by mass transfer effects in faster flowing systems mollifies the dramatic influence of signal molecules on biofilm structure reported in previous studies.Cells in bacterial biofilms are often less susceptible to host immune responses and antibiotics than cells grown in suspension (18). Biofilms may also provide a protective environment for pathogens, which, when released from the biofilm, may result in contamination of drinking water and medical fluids in delivery devices such as dialysis machines, venous catheters, dental water lines, and airway ventilators. Life-threatening infection caused by Pseudomonas aeruginosa in cystic fibrosis patients is a well-known example (8). Since biofilm formation in itself can be considered a virulence factor, it is important to understand the mechanisms which influence biofilm accumulation, structure, and behavior. Both hydrodynamics and cell signaling have been found to influence the structure of P. aeruginosa PAO1 biofilms. Stoodley et al. (27) reported that, under conditions of low-shear laminar flow, the biofilm consisted of a monolayer of cells with mound-shaped circular microcolonies but under high-shear, turbulent flow conditions, the biofilm formed filamentous streamers. Davies et al. (3) found that N-3-oxo-dodecanoyl homoserine lactone (OdDHL), a cell signal molecule involved in quorum sensing (QS) (reports regarding putative regulatory QS pathways and the role of QS in pathogenicity can be found elsewhere [5,16,20]), was required for the differentiation of biofilms into complex mushroom-and tower-like structures, which they described as characteristic of normal biofilms....

An understanding of the material properties of biofilms is important when describing how biofilms physically interact with their environment. In this study, aerobic biofilms of Pseudomonas aeruginosa PAO1 and anaerobic sulfate-reducing bacteria (SRB) biofilms of Desulfovibrio sp. EX265 were grown under different fluid shear stresses (tau g) in a chemostat recycle loop. Individual biofilm microcolonies were deformed by varying the fluid wall shear stress (tau w). The deformation was quantified in terms of strain (epsilon), and the relative strength of the biofilms was assessed using an apparent elastic coefficient (Eapp) and residual strain (epsilon r) after three cycles of deformation. Aluminium chloride (AlCl3) was then added to both sets of biofilm and the tests repeated. Biofilms grown under higher shear were more rigid and had a greater yield shear stress than those grown under lower shear. The addition of AlCl3 resulted in a significant increase in Eapp and also increased the yield point. We conclude that the strength of the biofilm is in part dependent on the shear under which the biofilm was grown and that the material properties of the biofilm may be manipulated through cation cross-linking of the extracellular polysaccharide (EPS) slime matrix.