Fig. 2. Fitness and selection consequences of differential effects of antibiotic concentration on growth rate.

a Theoretical max growth rates in pure culture of ‘isogenic’ strains differing only in resistance or sensitivity to antibiotic. Shaded area represents the range of antibiotic concentrations in subsequent panels (b–d) exploring competition outcomes and minimal selective concentration (MSC) based on these relative growth rates. b The MSC is defined as the antibiotic concentration at which growth rate of the resistant strain exceeds that of the sensitive strain (relative fitness >1). c, d In a community, the MSC can be increased by two basic mechanisms. One is increased costs of resistance, which may arise by increased competition for nutrients (c) and the other is reduced antibiotic effect upon sensitive strains, which may arise by community protection (d). e, f The emergence of de novo resistance mutations can be altered by community protection. e In the absence of a protective community antibiotic exposure above the MIC acts upon standing genetic variation, often selecting for a single high resistance, high cost mutation. e Communities that provide exposure protection may reduce the realised antibiotic exposure to sub-MIC levels, allowing for the sequential accumulation of low cost, low resistance mutations that together provide high levels of resistance for example via epistasis. Letters inside panels (S, R, A, AB, etc.) represent the accumulation of different mutations during selective sweeps.