Biofilm structure has been studied using many techniques, including transmission electron microscopy, scanning electron microscopy (SEM), and confocal scanning laser microscopy (CSLM). SEM has been used extensively to study the surface architecture of biofilms. The resulting images reveal an uneven outer surface topography, with the high resolution achieved by this method allowing individual cells to be clearly distinguished among a condensed matrix. Although this technique provides valuable information regarding the nature of biofilms, it is not entirely useful because it is well-known that the dehydration stages of sample preparation for SEM can destroy the EPS matrix. CSLM allows nondestructive in situ analysis of hydrated biofilms in combination with a wide range of fluorescent compounds. This technique can be used to form 3-D computer reconstructions of biofilms. These show a variable distribution of biomass with bacteria aggregating at different horizontal and vertical sites, with the highest cell densities at the biofilm base or at the top of the biofilm, forming "mushroom," "cone," or "stacks" shapes. Where biofilms have developed under turbulent conditions, they form additional structures termed "streamers." CSLM has also shown that biofilms are highly hydrated and that the total biofilm volume is made up of cell clusters, horizontal and vertical interstitial voids, and conduits beneath the clusters. These clusters and channels produce biofilms of varying depth and structure. Species composition has been shown to be an important determining factor in biofilm structure.
Recently, it was suggested that the structural complexity of biofilms is determined by the organisms through signaling molecules. It has been established that a family of diffusible chemical signals (N-acyl homoserine lactones) can regulate the production of virulence determinants and secondary metabolites, in suspended cultures, in a cell density-dependent manner. Also known as quorum sensing, it is thought that this may be important for the formation of biofilms which also contain densely packed cells. Evidence based on a pure culture Pseudomonas aeruginosa biofilm growing in laminar flow by Davies and coworkers (1998) supports this theory. They reported that N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL) was required for the biofilm to develop a complex structure by comparing wild-type biofilms with a Lasl defective mutant (Lasl directs the synthesis of OdDHL). This work illustrates the interest in this field, which has a huge potential for biofilm control. However, since quorum sensing is a concentration-dependent phenomenon, it will be strongly influenced by mass transfer processes. It may be expected that quorum sensing will have a greater significance in diffusion-dominated regions such as those found in large cell clusters or channels when bulk liquid flow is very low.
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