Bacterial Colonies

Figure 1. The histograms of the velocities of E. coli bacteria in a colony. The red curve is in the absence of glucose while the green curve is 20 minutes after exposure to glucose. Note the bimodality of the green curve, indicating a differentiation within the colony.
     The development of bacterial colonies is a major example of pattern formation. Depending on the medium and the geometry, different structures are observed, going from concentric rings and foci in a simple radial geometry to dendritic movements. We observed experimentally for several species of bacteria that the velocity of bacterial fronts is a decreasing function of the concentration of nutrients initially present of the Petri dish where colonies are grown. This behavior is quite puzzling as standard models, e.g. the Fisher-Kolmogorov model, predict a behavior either constant or even increasing. In [1] we developed a quantitative model to understand the phenomenon, based on the experimental observation reported in Figure 1, that is the bimodality in the bacterial mobility observed after exposure to the nutrients. The observed slow-down suggests a description of the colony in different sub-populations (two in the simplest possible setting), characterized by different velocities. Transition rates from the fast to the slow sub-populations increase with the concentration of nutrients available. Such a model has a clear physiological sense (bacteria move less if their metabolic needs are satisfied) and is able to quantitatively account for the observed behavior of the bacterial front velocity, as we show analytically and numerically in [1].

[1]. Repulsion and metabolic switches in the collective behavior of bacterial colonies. A. Sekowska, J.B. Masson, A. Celani, A. Danchin, M. Vergassola, Biophys. J., to appear