penetrate into the water. Microbial biofilms are not homogeneous matrices, showing
regions with different concentrations of cells and polymers, together with water
containing “voids”. There is some controversy over whether the distribution of biomass is
based only on randomly localised “clusters” of cells plus biopolymers, separated by water
channels (de Beer and Stoodley, 1995), or based on superimposed porous layers of
biomass with different densities and compositions (Christensen and Characklis, 1990).
Both concepts seem to coexist in practical situations. Each cluster of biomass (cells and
biopolymers) may include different layers, containing either distinct microbial
populations, or similar populations in different metabolic states, or even just
Figure 10.3 Two conceptual models of
biofilm structure.
different concentrations of cells and non-living biomass. It may happen that the cluster
structure approaches the layer model as the biofilm grows older and the “void” spaces
(water channels) are filled with more biopolymers and cells, as well as with organic and
inorganic debris. This statement is supported by data showing the increase of biofilm
density with time (Trulear, 1983). The two structural models are schematically presented
in Figure 10.3.
Dynamics of Biofilm FormationThe build up of microbial films is usually preceded by the adsorption of a thin layer of
organic macromolecules to the adhesion surface, sometimes comprising also metallic
hydroxides or oxides and very fine mineral powders (Chamberlain, 1992). The effect of
organic films in promoting the biofouling of surfaces submerged in sea water had also
been reported long time ago by Zobell and Allen (1935).
Micro-organisms suspended in the liquid are transported by diffusion, convection or
self motility to the initially clean surface. Once at the solid surface, irreversible adhesion
is favoured by the formation of “polymer bridges” between the adsorbed layer of
macromolecules and the biopolymers excreted by the microbes (Characklis and Marshall,
1990). The “induction or lag period” of the biofilm development includes the adsorption
of the organic conditioning film plus the first phase of microbial attachment before
exponential growth of the biofilm begins.
Mass transfer mechanisms play an important role in the transport of dissolved
nutrients and substrates (oxygen, sugars, organic acids, etc,) to the micro-organisms on
Biofilm reactors 297