Published online by Cambridge University Press: 03 June 2010
INTRODUCTION TO BIOFILM STRUCTURE
The general view on biofilm structure has dramatically changed during the last decade. It had been previously assumed that most biofilms are more or less homogeneous layers of micro-organisms in a slime matrix. The use of confocal scanning laser microscopy (CSLM) and computerized image analysis tools has revealed a more complex picture of biofilm morphology (Lawrence et al., 1991; Caldwell et al., 1993) and structural heterogeneities (Costerton et al., 1994; Gjaltema et al., 1994). Cell clusters may be separated by interstitial voids and channels, which create a characteristic porous structure. In some cases, biofilms grow as clusters taking a ‘mushroom’ shape, whilst in others, more compact and homogeneous layers can be observed.
Types of biofilm heterogeneity
In many biofilms, the reported non-uniformities are unidirectional and perpendicular to the substratum. Three-dimensional variation of microbial species, biofilm porosity, substrate concentration and diffusivity have been repeatedly reported in biofilm. It is becoming clear that there are many forms of heterogeneity in biofilms, and a definition of biofilm heterogeneity is needed. According to Bishop & Rittmann (1995), heterogeneity may be defined as ‘spatial differences in any parameter we think is important’. An adapted list from Bishop & Rittmann (1995) summarizes a few examples of possible biofilm heterogeneity:
(1) Geometrical heterogeneity: biofilm thickness, biofilm surface roughness, biofilm porosity and substratum surface coverage.
(2) Chemical heterogeneity: diversity of chemical solutes (nutrients, metabolic products, inhibitors), pH variations, diversity of reactions (aerobic/anaerobic, etc.).
(3) Biological heterogeneity: microbial diversity of species and their spatial distribution, differences in activity [growing cells, extracellular polymeric substances (EPS) producers, dead cells, etc.].