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The presence and the transformations of organic fractions within biofilms are not usually included in biofilm models, mainly owing to the lack of knowledge of the important processes, the stoichiometry and the kinetics. A conceptual biofilm model taking into account microbial physiology with regard to production of extracellular polymers and their transformation within the biofilm is suggested. The suggested components in the biofilm are the cell biomass and the various extracellular polymeric substances (EPS), such as polysaccharides, protein, humic substances and nucleic acids. In addition, environmental factors that influence the microbial exopolymer production and composition are taken into account. The suggested conceptual model can be helpful in structuring the design and evaluation of biofilm experiments focusing on biofilm structure and properties.

More deoxyribonucleic acid (DNA) was measured than could be accounted for by cell counts in activated sludge flocs, a mixed culture of sludge bacteria, and a pure culture of Pseudomonas putida. This means that estimations of cell number or biomass by measurement and correlation with DNA content will result in an overestimation of the cell number or biomass by up to a factor 10. There could be four reasons for the fact that the DNA was present: There were no specific enzymes present that degrade DNA (DNAses), DNAses were inhibited by humic or humic-like substances, the DNA was protected by coiling with metal ions or the turnover of DNA was very high. For activated sludge it is unlikely that there were no DNAses present but inhibition of DNAses and protection of DNA by metal ions are the most probable reasons why the DNA stays in the systems. The DNA was found in the activated sludge floc exopolymeric matrix whereas DNA from the bacterial cultures was found mainly in the bulk phase (in the slimes).

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