Factors Influencing Biofilm Structure

Hydrodynamics

Shear - biofilms growing in low shear environments tend to form isolated microcolonies often of irregular shape with little or no indication of direction of flow.

Biofilms growing in high shear environments tend to form long filaments or streamers leading to the down stream direction.  These streamers are visco-elastic, that is they stretch out when ambient flow increases and retract when flow decreases. They also wave back and forth in the stream flow much like a flag in the wind.

In flow cell experiments, biofilms have been seen to flow as ripples over the glass surface in the down stream direction looking much like a series of ocean waves (Stoodley film).  These ripple-like biofilms are commonly observed to detach from the substrate surface and to be carried with the bulk flow downstream.   In other instances ball shaped masses of biofilm material  have been seen to “roll” down stream, remaining in contact with the glass surface.

Nutrition

It has been shown that biofilms will grow on surfaces in nutrients solutions so dilute that they will not support the growth of planktonic cells.  This observation supports the contention that biofilm acts as an adsorbent layer concentrating organic materials from the bulk fluid.  High nutrient concentrations tend to produce biofilms that are thicker and denser than those grown in low nutrient concentrations.

Paul Stoodley’s lab demonstrated that a mature mixed species biofilm could change its morphology from ripples and streamers to densely packed mound-like structures when the nutrient concentration was increased ten fold (Stoodley et al. 1999c).  These more densely packed biofilms had a greater tendency to slough off the substrate surface than did the ripples and streamers formed at lower concentration.  At the other extreme, however, Stoodley also showed that the rate of detachment from the substrate in an Aeromonas hydrophila biofilm was also increased by nutrient limitation. So perhaps either too much or too little nutrient predisposes to detachment.

It seems clear however that it is more than simply the nutrient concentration that influences the architecture of a mature biofilm.  Klausen et al.. 2003 showed that different carbon sources greatly affected the structure of biofilms in Pseudomonas aeruginosa PAO1.  Glucose grown biofilms produce the tower and mushroom shapes separated by water channels typical of this organism.  When grown under the same conditions with citrate substituted for glucose, however, the biofilms formed by PAO 1 are flat, homogeneous and lack evidence of water channels.

Costerton paints an image of planktonic bacteria oxidizing substrates and extruding protons via electron transport mechanisms across their cell membranes into the external environment (Mitchell and Moyle 1965).  He postulates that this would work well in a nutrient rich low flow environment where protons might accumulate to sufficient concentrations so as to reenter the membrane with subsequent ATP production by oxidative phosphorylation.  In a high flow, turbulent environment these protons would rapidly be lost to the cell of group of cells producing them.  In such an environment, cells encased in a matrix rich biofilm would be as a significant advantage.  Nutrients, even those in low concentration, delivered to the biofilm in the bulk fluid through the complex system of water channels would be adsorbed into the biofilm matrix where they become available to the cells.  Costerton further argues that this efficiency of energy gathering and utilization might have been a major selective advantage in the evolution and dominance of biofilms particularly in turbulent ecosystems such as mountain streams where nutrient concentrations are low and contact between nutrients and cells is transient.