  Understanding why strains with different metabolic pathways that compete for
a single limiting resource coexist is a challenging issue within a theoretical
perspective. Previous investigations rely on mechanisms such as group or
spatial structuring to achieve a stable coexistence between competing metabolic
strategies. Nevertheless, coexistence has been experimentally reported even in
situations where it cannot be attributed to spatial effects [Heredity {\bf
100}, 471 (2008)]. According to that study a toxin expelled by one of the
strains can be responsible for the stable maintenance of the two strain types.
We propose a resource-based model in which an efficient strain with a slow
metabolic rate competes with a second strain type which presents a fast but
inefficient metabolism. Moreover, the model assumes that the inefficient strain
produces a toxin as a byproduct. This toxin affects the growth rate of both
strains with different strength. Through an extensive exploration of the
parameter space we determine the situations at which the coexistence of the two
strains is possible. Interestingly, we observe that the resource influx rate
plays a key role in the maintenance of the two strain types. In a scenario of
resource scarcity the inefficient is favored, though as the resource influx
rate is augmented the coexistence becomes possible and its domain is enlarged.

Populations and Evolution