The polymerase chain reaction (PCR) has only recently become a powerful tool for the detection of microbial organisms from environmental samples. The application of this molecular technology has so far been hampered by the presence of various impurities co-extracted with soil DNA, such as humic acids and metal ions. These interfering substances can inhibit PCR most likely due either to chelation of humic acids with magnesium ions required by Taq polymerase, or to the binding of primers that reduce the sensitivity of detection [1]. A number of works devoted to methodological approaches outline the difficulty to recover pure DNA extracted from soil or sediment samples. Removing humic substances from the DNA samples represents actually a methodological challenge. Time consuming methods like CsCl gradients [2,3] or extensive and repetitive precipitation steps [4] have been developed. Methods such as diluting crude extract samples have been used but this latter reduces the detection limit [1]. In order to simplify the purification steps, the use of various minicolumns has been recently tested by some authors [1,[5][6][7][8].Our laboratory has focused attention for several years on the impact of tropical termites on physical and chemical soil properties and more specifically on the distribution of humic substances [9,10]. Our aim was to design molecular tools in order to study bacterial changes induced by termite activities. However, the samples of soil-feeding termite nests typically impede the purification of DNA because such nests are mostly made of termite dejections which are rich in organic matter and humic substances. Here we compare electrophoretic and chromatographic methods to purify DNA samples. The quality of DNA was then tested by performing amplification by Polymerase Chain Reaction (PCR). Considering physical and chemical properties of both DNA and humic substances, we also sought to explain how these molecules compete and interact during purification procedures. MaterialThe samples originated from the rainforest of the NsimiZoetele region (South Cameroun). Termite mounds samples belong to the soil-feeding species Cubitermes subarquatus (Termitinae). Soils without termite activity were defined as controls. Termite mounds of C. subarquatus contain around 60% of clay, 45 mg/g of Total Organic Carbon (TOC) including 15% of humic acids. Control soils A2 (2 -5 cm) and A6 (100 -140 cm) have 44.4 and 67.4% of clay, 18.8 and 5.76 mg/g of TOC with 9% and 7% of humic acids, respectively. DNA extraction and purification were made in duplicate on termite mound and A2 samples. No DNA was noticeable in the A6 deeper strata. To test the efficiency of the procedures A2 and A6 sterilised subsamples were inoculated with a bacterial strain of Escherichia coli (Pharmacia, strain NM 522). DNA extractionThe detailed procedure is described elsewhere [11]. Briefly, 0.4 g samples are submitted to a lysis solution containing lysozyme, proteinase K, and sodium dodecyl sulfate. Protein and cell debris were eliminated by a salting ...

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The peats of the Jambi region of Sumatra have been cultivated for many years by the villagers, after minor draining works. Their humification process was investigated using a chronosequence: T0 (control soil), 3 years, 6 years and 9 years of cultivation. The extraction of the humic substances at different pH reveals that the pH 12 extractant gives rise to false conclusions concerning the polymerized nature of the humic acids extracted. Elemental and molecular analyses confirmed their aliphatic nature, and hence their neoformed character.

The growth response of a wild achlorophyllous Euglena gracilis mutant was studied during exposure to cadmium and pentachlorophenol (PCP). Cadmium gradually reduced the growth rate and terminal cell density; PCP only lengthened the initial lag phase relative to control cultures. Flow cytometry showed that cadmium altered the cell cycle by delaying late S and G2/M phases; PCP did not disturb the cell cycle, but markedly affected DNA staining: the intercalating dyes ethidium bromide and propidium iodide showed little staining compared to controls. However, replication and transcription processes were not altered by PCP, as cell division occurred normally. Cells surviving after PCP treatment apparently developed an adaptative response during the lag phase.

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