Mol. Syst. Biol. 11, 822 (2015)

The minimum inhibitory concentration (MIC) of an antibiotic is defined as the lowest concentration that inhibits bacterial growth over a defined period of time in culture, generally starting from a standard initial cell density of 5 × 105 cells/ml (MIC*). This measurement is important in determining the resistance of bacterial cells to specific antibiotics. Artemova et al. argued that MIC is insufficient to quantify the fitness of a resistant bacterial strain in cases where resistance is mediated by degradation of the antibiotic, as with β-lactam antibiotics. This can be attributed to the so-called inoculum effect, whereby the cooperative inactivation of the antibiotic by the bacterial population causes the MIC to depend on the initial cell density, complicating the dynamics of the population. The authors sought a metric for quantifying antibiotic resistance that removed the effect of initial cell density. They measured the MIC for the β-lactam cefotaxime against a resistant Escherichia coli strain at dozens of initial cell densities. They found that the MIC varied by three orders of magnitude depending on the initial cell density: as cell density decreased, MIC asymptotically approached a limit corresponding to the level of resistance of a single cell (scMIC). The cefotaxime scMIC obtained with this liquid dilution method was within a factor of 2 of that obtained with single cells grown on agar, and both were at least an order of magnitude smaller than the MIC*. This consistency in the scMIC measurement carried over to experiments where two strains were in competition and in laboratory evolution experiments. These experiments indicated that the scMIC specifies the concentration at which selection for increased resistance (evolution) occurs. The authors built a mathematical model to describe this predictive power of the scMIC, which validated their in vitro and in vivo (in animal host) experiments. Because scMIC directly measures the fitness of an individual cell, and because selection acts on individual cells and favors cells that perform better as individuals, the scMIC is superior to MIC as a metric for quantifying cell fitness.