Jan Kreft from the Theoretical Biology Group of the University of Bonn, Germany, visited the Wessex Institute and gave an overview of microbial survival strategies involving competition, cooperation, and communication, explaining the importance of spatial dynamics for these processes in his talk on “Survival strategies of microorganisms in nature”.
Kinetic theory of optimal pathway length assumes that the total concentration of enzymes as well as metabolites in a cell is minimized because of the costs of enzyme synthesis and metabolite toxicity. Extending a given pathway by an extra step while constraining total concentration of enzymes and metabolites would lead to a reduction in the concentration of the already existing enzymes and intermediates. Assuming linear kinetics, where reaction rates are proportional to enzyme and metabolite concentrations, the rate of substrate consumption of a pathway decreases with the square of the pathway length, while the ATP yield of the pathway increases with its length, typically resulting in an optimal length in terms of the rate of ATP production or growth rate. The predictions of kinetic theory are in full agreement with observed lengths of pathways in nitrification, methane oxidation, aerobic and anaerobic sugar degradation, etc. However, a longer pathway would have a higher ATP or growth yield. This is the reason for the observed trade-off between growth rate and yield.
This trade-off has consequences for fitness, as can be seen by competing two growth strategies based on this trade-off. The strategy of fast growth at low yield, or low efficiency, (HighRate) is due to short pathways, whereas the strategy of slow growth at high yield (HighYield) is due to long pathways. In well-mixed systems, global competition for resources allows the HighRate strategy to win because it has the higher growth rate at any resource concentration. However, in spatially structured systems such as biofilms, the HighYield strategy has a higher fitness when the diffusional influx of substrate into the biofilm limits growth, because under such conditions, the higher efficiency of resource exploitation pays off, provided the neighbours cooperate in using the resource economically. This cooperation among neighbours can evolve because microbes attached to surfaces typically grow and multiply forming clusters of relatives; the offspring from a single ancestor stays together.
Microbes not only cooperate, they often do so only if they are not alone, and they use cell-to-cell communication to find out whether they are alone or in a group. They do this by producing a diffusible extracellular signal that switches on gene expression if a threshold concentration of the signal has been reached. However, the concentration of the signal not only depends on the density of signal producing cells, but also on diffusional constraints in the physical environment and on the degree of clustering of the cells. This clustering is essential for communication in complex natural environments, since it prevents interference from the multitude of other signalling species (practical problems) and exploitation by cheaters, cells that save costs because they do not produce signals or because they do not respond to the signal but nevertheless benefit from the signal producing and responding other cells (evolutionary problems).