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.

Biofilms of sulphate-reducing Desulfovibrio sp. EX265 were grown in square section glass capillary flow cells under a range of fluid flow velocities from 0.01 to 0.4 m/s (wall shear stress, tau(w), from 0.027 to 1.0 N/m(2)). In situ image analysis and confocal scanning laser microscopy revealed biofilm characteristics similar to those reported for aerobic biofilms. Biofilms in both flow cells were patchy and consisted of cell clusters separated by voids. Length-to-width ratio measurements (l(c):w(c)) of biofilm clusters demonstrated the formation of more "streamlined" biofilm clusters (l(c):w(c)=3.03) at high-flow velocity (Reynolds number, Re, 1200), whereas at low-flow velocity (Re 120), the l(c):w(c) of the clusters was approximately 1 (l(c):w(c) of 1 indicates no elongation in the flow direction). Cell clusters grown under high flow were more rigid and had a higher yield point (the point at which the biofilm began to flow like a fluid) than those established at low flow and some biofilm cell aggregates were able to relocate within a cluster, by travelling in the direction of flow, before attaching more firmly downstream.

Human pathogens often colonize their host by the formation of biofilms. These surface-attached aggregates of bacteria are characterized by a self-produced extracellular matrix, which makes them highly resistant towards antibiotic treatment. Their abilities to adhere to abiotic surfaces (e.g., catheters and other medical devices) also makes bacterial biofilm formation a challenge in modern medicine. Antimicrobial peptides have lately been introduced as a potential class of drug molecules for combating severe hospital-acquired infections. One of these peptides, human cathelicidin LL-37, has recently been demonstrated to bridge innate and adaptive host defence, in addition to facilitating a robust antibiofilm effect at sub-inhibitory concentrations. In this review we will discuss the evidence, potential and challenges for LL-37 as a candidate molecule for therapeutic use.

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