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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