title: What Is the 'Minimum Inhibitory Concentration' (MIC) of Pexiganan Acting on Escherichia coli?-A Cautionary Case Study.
abstract: We measured the minimum inhibitory concentration (MIC) of the antimicrobial peptide pexiganan acting on Escherichia coli , and found 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.
authors.affiliation: SUPA and School of Physics and Astronomy, The University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, EH9 3FD, Scotland, UK.
authors.forename: Alys K
authors.initials: AK
authors.lastname: Jepson
authors.affiliation: SUPA and School of Physics and Astronomy, The University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, EH9 3FD, Scotland, UK.
authors.forename: Jana
authors.initials: J
authors.lastname: Schwarz-Linek
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK.
authors.forename: Lloyd
authors.initials: L
authors.lastname: Ryan
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK.
authors.forename: Maxim G
authors.initials: MG
authors.lastname: Ryadnov
authors.affiliation: SUPA and School of Physics and Astronomy, The University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, EH9 3FD, Scotland, UK. w.poon@ed.ac.uk.
authors.forename: Wilson C K
authors.initials: WC
authors.lastname: Poon
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D023181:Antimicrobial Cationic Peptides / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000187:drug effects* / Q000254:growth & development; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D015203:Reproducibility of Results; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D023181:Antimicrobial Cationic Peptides; C093828:pexiganan
keywords: Antimicrobial peptide; Escherichia coli; Inoculum effect; Killing curves; Minimum inhibitory concentration; Pexiganan