Team:Goettingen/Model

Modelling of glyphosate-dependent growth inhibition

The bacterial growth model

Under lab conditions, the right evironmental parameters and nutrients provided, bacterial growth can be observed in form of growth curves, plotting the bacterial mass as a function of time. A convenient way to observe those growth curves is provided by measuring the optical density also referred to as the turbidity.
During the growth of bacteria three distinct phases can be differentiated (Figure 1), all of which can be associated with physiological changes in the cell cultures: (1)

  1. During lag phase the bacteria are suggested to adapt to physiological changes in the medium. Adaption can be achieved by e.g. changes in the Proteom of the Bacteria in order to meet the new culture requirements. Bacterial growth is strongly reduced
  2. After adaption bacterial growth enters the log phase alternatively named exponential phase. During this phase the bacteria show exponential growth, the most rapid growth possible under given conditions. The actual double-timing of a certain strain is determined by the growth rate, the increase of cell mass per time.
  3. In the stationary phase no net growth can be observed. Nutrients and carbon sources are used up and increase in cell mass is no longer possible. Dead cells can lyse and provide another source of nutrients, explaining a small increase in cell mass that sometimes still occurs.

To be scientifically correct a fourth phase can be observed in the bacterial growth curve namely the death phase. As we measured our bacterial mass in terms of optical density this phase couldn't be observed in our experiments and has no relevance for our modeling approaches.

Correlation between growth rate and glyphosate concentration

As described in detail in our Background section glyphosate (N-(phospho­monomethyl)glycine) specifically inhibits enol­pyruvyl-shikimate-3-phosphate (EPSP) synthase, an enzyme present in plants, fungi, bacteria and archea (2-5). It catalyses the conversion of shikimate-3-phosphate (S3P) and phosphoenolpyruvate (PEP) to EPSP an intermediate in the shikimate pathway, which eventually enables the synthesis of aromatic amino acids. Interaction of glyphosate with EPSP therefore leads to depletion of cellular levels of aromatic amino acids resulting in an inhibition of growth. (6).

Obtaining the growth rate μ

To

References

  1. Environmental Microbiology Second Edition, Maier Pepper Gerba, 2009
  2. Amrhein et al. (1983) FEBS Lett. 157: 191-196.
  3. Comai et al. (1983) Science 221: 370-371.
  4. Schulz et al. (1984) Arch. Microbiol. 137: 121-123
  5. Steinrücken & Amrhein (1980) Biochem. Biophys. Res. Commun. 94: 1207-1212.
  6. Biochemical basis for glyphosate-tolerance in a bacterium and a plant tissue culture Nikolaus Amrhein, Detlef Joharming, Joachim Schab and Arno Schulz e