Microorganisms live in ecologically dynamic environments that inevitably fluctuate between mild and harsh conditions. Critically, the latter gives rise to higher demographic noise and random extinctions that govern evolutionary dynamics, which in turn might shape the environmental conditions through large-scale feedback loops. Although the independent contributions of environmental variability (EV) and demographic fluctuations (DF) have been extensively studied, the emerging `eco-evo dynamics' resulting from the EV and DF joint interplay remains largely unknown due to its challenging complexity and interdisciplinary nature. This poses an open problem that is crucial for many relevant, real world living systems.
In this study we focus on the eco-evo dynamics in the paramount case of AntiMicrobial Resistance (AMR), which currently causes 7*10^5 deaths every year, and is estimated to become the cause of 10 million yearly deaths by 2050. AMR is typically characterized by cooperative behaviour: a mutant strain generates a Public Good (PG; e.g., extracellular enzyme) at some metabolic cost (reduced birth rate) and the PG inhibits the antimicrobial drug for all the population (antimicrobial-independent rates for all strains). Furthermore, PG is typically shared only when the number of cooperators Nc overcomes a given threshold N_c>N_th. Below N_th the Good is held private, within cooperators' intracellular medium, and non-PG-producers become vulnerable to antimicrobial drugs.
We will thus discuss the AMR eco-evo dynamics of a well-mixed, finite microbial population of fluctuating size composed of a cooperative, PG-producer strain and a `free-rider', defector strain. The former has a constant but lower birth rate, while the latter has a higher birth rate that drastically reduces in the presence of antimicrobial drugs, unless the fraction of cooperators is high and PG is shared. The resulting eco-evo dynamics, arising from the direct coupling between the evolutionary state (level of cooperation) and the environmental condition (active/inhibited antimicrobial drug for defectors), and that enhances species coexistence, will be presented. And, finally, we will discuss the impact of a more realistic time-dependent, fluctuating environment.