Extracellular vesicles as viral countermeasures: dampening of oscillations and reduction of extinction risk

Microbes produce extracellular vesicles (EVs, tiny membrane enclosures) that can transport some "cargo" (signaling molecules, proteins/enzymes, toxins, and nucleic acids) away from themselves or to other cells. EVs have also been shown to adsorb virus (phage) particles and inhibit infection, so another potential function is to serve as decoys for virus infection. However, the fitness benefit has not been explored quantitatively. Here, three existing mathematical models are extended to include EVs and parameterized based on literature. Simulations include a number of environments (lab culture and ambient), conditions (equilibrium and oscillating, i.e. predator-prey cycles), and bacteria (including enteric Escherichia coli and marine Prochlorococcus). Hosts invest, on average, ∼10% of resources into EV production. The models predict that producing EVs typically results in relatively minor increases in average host concentration (average ∼4.3% of log concentration). However, under oscillating conditions, EVs can substantially dampen and, in most cases, completely eliminate fluctuations, thereby increasing the minimum concentration and reducing extinction risk. These results provide insights into the fitness benefit of EVs as viral countermeasures, and they constitute a starting point for including EVs in ecosystem models.