Chemical warfare and survival strategies in bacterial range 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 colonize new areas, as well as the importance of interspecies 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.

Stochasticity enhances the gaining of bet-hedging strategies in contact-process-like dynamics

“…Given that these conditions are often met by biological populations-as for instance, in bacterial colonies competing at the front of a range expansion in noisy environments [34][35][36]-our results support the importance of bet-hedging in nature. This being said, of course, more realistic models-including some realistic ingredients such as, for example, the possibility of "dormant" states and not just birth and death processes-would be required to approach viral or bacterial communities and their bet-hedging more closely.…”

The impact of resource dependence of the mechanisms of life on the spatial population dynamics of an in silico microbial community

“…These emergent spatial patterns broadly agree with those observed in experimental studies of three bacterial strains in cyclic competition. 68 We notice a significant difference in behaviour between resource-dependent reproduction and competition when compared with resource-dependent mobility. In the first two cases, a notable zone of activity emerges where interactions occur more frequently due to the relatively higher concentration of environmental substrate compared to the outer areas of the grid.…”

Range expansions transition from pulled to pushed waves as growth becomes more cooperative in an experimental microbial population

“…For example, range expansions of microbial populations have revealed the dependence of the invasion velocity on the supply of resources (37) and demographic stochasticity (33). Experiments with microbes have also demonstrated the strong effect of range expansion on competition (38)(39)(40)(41)(42)(43)(44) and neutral evolution via the founder effect or gene surfing (45,46). In this study, we focus on expansions with and without the Allee effect and quantify their differences.…”