PhD defence: Growing Bacterial Populations: Fluctuations, Oscillations, and Selection Bias

PLEASE NOTE: If a candidate gives a layman's talk, the livestream will start fifteen minutes earlier.

Bacterial cells reproduce by growing in size and dividing into two daughter cells with identical genes. In this thesis, we model how a growing population of such bacteria behaves. Even though all cells in these populations are genetically identical, there are small but important random fluctuations in their phenotypes, such as the growth rates and division timings. We combine biologically realistic division mechanisms and growth fluctuations into a single model that builds upon existing models. With this model, we link the statistical properties of growing populations to the properties of single-cells.

Our model leads to new insights on various measurable properties of the population that people have long been interested in: the long-term growth rate, oscillations in this growth rate, and the distributions of phenotype and cell size in the population. We find that the different types of phenotype fluctuations, namely in growth and division, have different effects on population properties. Most notably, the population growth rate is independent of the division mechanism, and the cell size distribution of a population is independent of the single-cell growth rates.

We also make a number of predictions about the oscillations in the growth rate. Unlike previous work, we investigate how they would appear in the population growth rate in a single experiment, rather than in some theoretical average of repeated experiments. The sizes of these oscillations are now random themselves as they depend on the fluctuations in the early population. We discover a transition point: if individual cell growth rates fluctuate too much, then the population oscillations will be inseparable from the background fluctuations. We also outline how fluctuations in growth and division affect the distribution of phenotype measurements in a population, such as cell fluorescence.