Microbial DNA and Protein Synthesis

DNA synthesis (an indicator of microbial growth, mainly bacteria) has been determined by the incorporation of 3H- or 14C-thymidine into microbial DNA. Briefly, a soil extract is incubated with radiolabeled thymidine for a short time, and then the amount of radiolabel in the cells is measured (Baath 1992; Baath et al. 2001). This method must meet with the following requirements if the results are to be interpreted correctly: (a) DNA synthesis must be linearly related with cell growth; (b) all bacteria must take up thymidine through the cell membrane, and this should not be metabolized, and; (c) the radioactive label should not be exchanged with other molecules.

The synthesis of microbial proteins is also highly correlated with microbial activity and can be determined with 3H- or 14C-leucine methodologies. The leucine incorporation method (Baath 1994) is similar to the thymidine incorporation method and has similar drawbacks. However, the measurement of protein synthesis is more accurate than the measurement of DNA synthesis due to the relatively high protein content of cells. In addition, few microbial species have been found that are unable to take up leucine (Baath 1998).

Recently, these thymidine and leucine incorporation techniques have been adapted to study the tolerance of soil microbial communities to heavy metals. Microbial growth rates determined with the thymidine and leucine incorporation techniques were found to be sensitive to heavy metal pollution in short-term experiments (Rajapaksha et al. 2004). However, this was not the case in some long-term experiments performed under laboratory conditions (Diaz-Ravina and Baath 1996b; Rajapaksha et al. 2004) or under field conditions (Baath et al. 2005).

The application of these techniques in soils that were experimentally polluted with metals indicates that the tolerance of the microbial community may change with time. It has been suggested that there is an immediate increase in tolerance due to the initial toxicity, followed by a more gradual increase due to the different competitive abilities of the surviving bacteria and/or their different physiological and/or genetic adaptations (Diaz-Ravina and Baath 2001).

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Body Detox Made Easy

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