Microbial populations often live in small patchy habitats, which repeatedly or continuously exchange biomass. Recent advances in millifluidic technology allows to mimic such large collections of microbial populations, by enclosing a few cells in an aqueous droplet and allow them to grow in an experimentally controlled environment. Resources in droplets are depleted on a relatively fast timescale, thus one way to prolong growth is to repeatedly pool all cells in droplets and seed a new set of droplets by dilution. We analyze the ensuing eco-evolutionary dynamics where selection can act on multiple levels: Within droplets, fast utilization of resources allows a strain to gain in frequency. However, a slower, but more efficient, strain could increase its final population size. The main parameter that influences the favorable strategy in this dynamics is the dilution rate, which is how many cells are inoculated on average in a new droplet. If the average inoculum is small, multiple strains can mutually coexist among droplets along a trade-off in fast growth and high efficiency.

In addition to this basic framework, that already exhibits coexistence, we subsequently include environmental and ecological challenges: bacteria need to produce extracellular beta-lactamase to degrade antibiotics stopping their growth, or can extract more resources by expressing and excreting iron-binding siderophores, that increase availability of iron for growth. Both of these excreted molecules can be seen as a public good, helping the whole microbial community. In particular, we ask how producing strains fare in the presence of non-producing strains, and how individual cost is offset by the gain on the population level within a droplet. As a result, we can characterize the parameter space into regions of stable coexistence, limit cycles, but also extinction of some or all strains, when growth occurs in the separated habitat of droplets.

These ecological interactions of microbes lets us speculate on costly altruistic and cooperative traits in the course of evolution. It seems the brief and temporary spatial separation during growth is often already enough to allow for these costly traits to be effectively neutral -- they can coexist stably within the global population.