What is the 'minimum inhibitory concentration' (MIC) of pexiganan acting on Escherichia coli? - A cautionary case study

TL;DR

This study investigates the variability and limitations of MIC measurements for pexiganan against E. coli, revealing complex bacterial responses and questioning the MIC's reliability as a sole indicator of antimicrobial activity.

Contribution

It provides a detailed analysis of MIC variability, introduces a pharmacodynamic model, and highlights the importance of population-level studies for accurate antimicrobial assessment.

Findings

MIC measurements show intrinsic variability.

Sub-MIC pexiganan kills some bacteria, reducing peptide activity.

Population-level studies are crucial for interpreting MIC results.

Abstract

We measured the minimum inhibitory concentration (MIC) of the antimicrobial peptide pexiganan acting on Escherichia coli, and report an intrinsic variability in such measurements. These results led to a detailed study of the effect of pexiganan on the growth curve of E. coli, using a plate reader and manual plating (i.e. time-kill curves). The measured growth curves, together with single-cell observations and peptide depletion assays, suggested that addition of a sub-MIC concentration of pexiganan to a population of this bacterium killed a fraction of the cells, reducing peptide activity during the process, while leaving the remaining cells unaffected. This pharmacodynamic hypothesis suggests a considerable inoculum effect, which we quantified. Our results cast doubt on the use of the MIC as 'a measure of the concentration needed for peptide action' and show how 'coarse-grained' studies at the population level give vital information for the correct planning and interpretation of MIC measurements.