Beyond the FIC index: the extended information from fractional inhibitory concentrations (FICs)
Nikos Fatsis-Kavalopoulos, Dione L. Sánchez‐Hevia, Dan I. Andersson · Journal of Antimicrobial Chemotherapy · 2024
Various assays, such as the commonly used checkerboard assay, quantify antibiotic interactions by calculating the fractional inhibitory concentration index (FICi). where MICA and MICB are the minimum inhibitory concentrations of antibiotics A and B, and CA and CB are the concentrations inhibiting bacterial growth in combination. According to Loewe’s additivity model, in an additive interaction the FICi equation simplifies to 1. Deviations from FICi = 1 indicate interactions: FICi > 1 shows negative interaction, while FICi 4 translates to FICB→A=FICA→B=2. This yields the following limits of the unilateral interactions: Applying this analysis to an already published dataset of ampicillin and gentamicin interactions in E. coli isolates5 shows a more complete picture then the original analysis (Figure 1). While the original FICi metric (Figure 1b) suggested additive interactions, individual FIC values reveal a range of effects (Figure 1c), Specifically, gentamicin consistently inhibits the efficacy of ampicillin to varying degrees (FICGEN→AMP, in Figure 1c), and ampicillin has a variable impact on the efficacy of gentamicin (FICAMP→GEN, in Figure 1c), which was previously masked by the FICi metric. We suggest that this easy analysis will improve our understanding of how antibiotics interact. Gentamycin (GEN) and ampicillin (AMP) checkerboard assays and new data based on the FIC index. (a) A schematic representation of a checkerboard assay indicating the concentrations present of both antibiotics in the wells of the microtitre plate. (b) Data previously published on the FICi index of the interaction between AMP and GEN in a collection of 254 clinical E. coli isolates.5 (c) the same experiments as (b) but reanalysed and illustrated as separate FIC values. The dotted lines indicate the limits of synergy and antagonism in the FIC representation (blue for synergy red for antagonism). The continuous lines represent the FICi limits of synergy and antagonism similarly (FICi = 0.5 for synergy and FICi = 4 for antagonism). To allow for intuitive reading of the FIC figure, values are represented on log2 axes starting at 0.5. This way, values to the left of the axes origin represent positive effects (synergistic) and to the right negative effects (antagonistic). Grey lines are standard deviations based on at least n = 3 for every data point. Funding was provided by the Swedish Research Council (Vetenskapsrådet) and the Knut and Alice Wallenberg Foundation by research grants to D.I.A. Funders had no input on data analysis, design or decision to publish. None to declare.