Chemical warfare and survival strategies in bacterial range expansions

TL;DR

This study investigates how bacterial species coexist during range expansions, revealing key factors like inoculum composition, growth rates, and toxin range, supported by experiments and a calibrated computational model.

Contribution

The paper introduces a combined experimental and computational approach to identify determinants of bacterial coexistence during range expansions, challenging the idea that cyclic dominance is necessary.

Findings

Coexistence depends on inoculum composition, growth rates, and toxin range.

A calibrated model accurately predicts coexistence conditions.

Cyclic dominance is not essential for coexistence in bacterial expansions.

Abstract

Dispersal of species is a fundamental ecological process in the evolution and maintenance of biodiversity. Limited control over ecological parameters has hindered progress in understanding of what enables species to colonise new area, as well as the importance of inter-species interactions. Such control is necessary to construct reliable mathematical models of ecosystems. In our work, we studied dispersal in the context of bacterial range expansions and identified the major determinants of species coexistence for a bacterial model system of three Escherichia coli strains (toxin producing, sensitive, and resistant). Genetic engineering allowed us to tune strain growth rates and to design different ecological scenarios (cyclic and hierarchical). We found that coexistence of all strains depended on three strongly interdependent factors: composition of inoculum, relative strain growth rates, and effective toxin range. Robust agreement between our experiments and a thoroughly calibrated computational model enabled us to extrapolate these intricate interdependencies in terms of phenomenological biodiversity laws. Our mathematical analysis also suggested that cyclic dominance between strains is not a prerequisite for coexistence in competitive range expansions. Instead, robust three-strain coexistence required a balance between growth rates and either a reduced initial ratio of the toxin-producing strain, or a sufficiently short toxin range.