Isolated cell behavior drives the evolution of antibiotic resistance

Abstract

Bacterial antibiotic resistance is typically quantified by the minimum inhibitory concentration (MIC), which is defined as the minimal concentration of antibiotic that inhibits bacterial growth starting from a standard cell density. However, when antibiotic resistance is mediated by degradation, the collective inactivation of antibiotic by the bacterial population can cause the measured MIC to depend strongly on the initial cell density. In cases where this inoculum effect is strong, the relationship between MIC and bacterial fitness in the antibiotic is not well defined. Here, we demonstrate that the resistance of a single, isolated cell—which we call the single-cell MIC (scMIC)—provides a superior metric for quantifying antibiotic resistance. Unlike the MIC, we find that the scMIC predicts the direction of selection and also specifies the antibiotic concentration at which selection begins to favor new mutants. Understanding the cooperative nature of bacterial growth in antibiotics is therefore essential in predicting the evolution of antibiotic resistance.

Small Molecule Restores Itaconate Sensitivity in Salmonella enterica: A Potential New Approach to Treating Bacterial Infections

“…For this reason, the standard protocol to measure the minimum inhibitory concentration (MIC) had to be modified by increasing both starting cell density and incubation time; this yielded an altered MIC (aMIC) as detailed in the Experimental Section of this work. Based on the “inoculum effect”, it is possible that the aMIC values reported here are overestimated, thus making our compounds look less potent than they are.…”

Antibiotic efficacy — context matters

“…Microbial ecologists have largely viewed population heterogeneity as a form of bet-hedging, conserved to facilitate the adaptation to new environments [93]. In support of this, several studies demonstrate how persister mechanisms can help pathogens survive inside host cells [94,95] and provide a reservoir for evolving antibiotic resistance [96-98]. Investigations have also shown how social behaviors such as quorum sensing [99], signaling [100] and cooperative mutualism [101] can confer protection against antibiotic lethality in mixed-species environments.…”

“Antibiotic inhibition of bacteria growth in droplets reveals heteroresistance pattern at the single cell level”

“…[We looked how susceptibility to the drug changes with increasing bacteria inoculum density] β-lactam antibiotics are often subject to the inoculum effect (IE) where the efficiency of the drug depends on the starting inoculum density of the bacteria 20 . IE is often overlooked in traditional MICassays due to the inaccuracy in setting inoculum density to recommended level using conventional ODmeasurement 13 .…”