Biofilm structure

In natural streambed biofilms, the biomass of colloidal carbohydrates was found to be 5 times greater than the biomass of bacteria inhabiting the biofilm (Hall and Meyer, 1998). The biofilm volume/bacterial volume ratio is much higher, because the fraction of the biofilm volume actually occupied 

Decaying eucaryotic alga, POM Active bacteria 0 o Dormant bacteria 

FIGURE i Diagrammatic representation of the structure and function of bacterial biofilms. Dissolved organic matter (DOM) is sorbed onto the biofilm (1), additional DOM is released from algae and organic particles (2). The organic matter is cleaved by extracellular enzymes (3). Interactions can occur between clones of syntrophic or competing bacteria (4). See Section II for details. 

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Freitas dos Santos, L.M., Livingston, A.G., Membrane attached biofilms novel technique for measurement of biofilm thickness, density and diffusivity, Proc. lAWQ Conf. Workshop on Biofilm Structure, Growth and Dynamics - Need for New Concept , Noordwijkerjout, The Netherlands, August 1995. 

Fig. 5.1.4 Ti me lapse T2 maps of biofilm structures as a function of growth. The T2 maps are collected at different stages in the growth of a biofilm at the time shown after initial inoculation. Light colors indicate water...

Figure 5.1.7 shows the propagator of the motion measured for a clean and a biofilm impacted capillary [14,15] and the residence time distributions calculated for each from these velocity distributions. The clean capillary gives an experimental propagator equal to the theoretical velocity distribution convolved with a Gaussian diffusion curve [14], as shown in Figure 5.1.2. For the flow around the biofilm structure note the appearance of a high velocity tail indicating higher probability of large displacements relative to the clean capillary. The slow flow peak near zero displacement results from the protons trapped within the EPS gel matrix where the...

Fig. 5.1.7 (a) Propagators in both a clean square capillary (dotted) and for flow around a biofilm structure (dashed) for an observation time, A = 15 ms. (b) Residence time distribution functions calculated from the propagator data shown in (a). The induction of a high... 

On the other hand, cell immobilization on carriers definitively improves bioreactor efficiency. Cell aggregation in a biofilm structure increases process stability and tolerance to shock loadings. A proper selection of operating conditions allows... 

The association of human pathogens with biofilms formed by resident epithytes is considered to enhance survival on leaf surfaces. It has been estimated that 10-40% of the total bacteria on the surface of parsley and broad-leaf endive are associated with biofilms (Lindow and Brandi, 2003). However, studies performed with E. coli 0157 H7 or Salmonella would suggest that bacterial cells tend to aggregate between the grooves of epidermal cells rather than associate with biofilm structures (Warriner et al, 2003a). 

Biofilms enhance bacteria-DOM interactions by several means. Their spatial and chemical heterogeneity provides additional sorption sites for DOM compared with clean surfaces. Their loose architecture with interstitial voids and channels increases diffusivity and to some extent allows convective flow within biofilm structures. Because bacteria metabolize organic matter sorbed to the biofilm, a diffusion flux from the free water to the biofilm is maintained. Large proportions of organic matter sorbed to the biofilm are not instantly turned over but remain in the biofilm as a reservoir, which buffers direct effects of DOM depletion in the water column. 

Stoodley, P., J. D. Boyle, D. deBeer, H. M. Lappin-Scott. 1999a. Evolving perspectives of biofilm structure. Biofouling 14 75-90. 

Mass transfer is related to velocity (i.e. turbulence) so that as the velocity increases the availability of biocide at the water/biofilm interface is facilitated. Furthermore as the biocide concentration at the interface is increased there is increased concentration driving force for mass transfer through the biofilm, thereby ensuring deeper penetration of the biofilm structure in a given time. 

Two Common Mistakes about Biofilms The very word biofilm is in fact wrong for one thing, it is not a film. In other words, biofilms have a rather patchy and nonuniform structure, which renders them opposite to what is implied by a film, which has a uniform fabric. Figure 4.10 shows two examples of biofilm structure. 

Yang, X., Beyenal, H., Harkin, G and Lewandowski, Z. (2000). Quantifying biofilm structure using image analysis. J. Microbiol. Methods, 39, 109-119. 

Elimination of bacteria in this mode of growth is challenging due to the resistance of biofilm structures to both antimicrobials and host defences. 

There is an implicit assumption, however, that EABs growing on electrode surfaces can be described as a well-controlled condition in which CV can be applied to study reaction mechanisms as in pure electrochemical systems. Beyond reproducibility of the biofilm electrode surface, simply characterizing biofilm structure itself has historically been difficult [114-116]. Furthermore, the result of biofilm heterogeneity is local variation of not only diffusion coefficients, but also flow velocities [117-120]. The unknown mass transfer conditions suggest that not all cells in the EAB contribute equally to current production. Several chapters in this book are dedicated to the use of CV to characterize electron transfer processes in EABs. 

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