Fig. 1: A high-throughput competition-based assay identifies widespread selection in favour of mecA.
From: Ubiquitous selection for mecA in community-associated MRSA across diverse chemical environments
a Methicillin-resistant S. aureus (MW2 CA-MRSA, “mecA+”, DsRed, shown in red) and mecA-deleted (MW2 ΔmecA, “mecA−”, GFP, shown in green) strains were mixed in a 1:1 ratio, and grown in 1536-well plates in the presence of different test compounds and subinhibitory concentration of cefoxitin. In the Single-Dose assay, each well contained a single test compound at a single concentration (150 μM). In the Dose-Response assay, 5 or 9 wells contained a concentration gradient of each compound. b Dose-response curves for different β-lactam antibiotics (amoxicillin, turquoise triangle), non-β-lactam antibiotics (ciprofloxacin, turquoise square) and natural products (curcumin, turquoise diamond) that select for mecA+ (n = 2 dye-swapped replicates). Vertical lines are the calculated IC50s of each strain. c Density plot of the normalized fluorescent signals of 57,480 diverse compounds from the Single-Dose assay (density plot, grey dots). The different controls (n = 168–184 random representative wells of each control) are highlighted as only mecA− strain (green), only mecA+ strain (red), mecA− and mecA+ cells mixed in 1:1 ratio without treatment (blue), with 6 μM cefoxitin (Fox) which inhibits the mecA− strain but not mecA+ (magenta) and with 150 μM cefoxitin which inhibits both strains (brown). d Density plot of the normalized fluorescent signals for bioactive and diversity libraries in the Single-Dose assay (see Supplementary Fig. 9 for fluorescent signals for bioactive & diversity library). Test compounds are separated into four groups by using thresholds determined by the median and standard deviation of the controls: non-selecting (blue), strongly and moderately selecting for mecA+ strain compounds (orange and yellow, respectively) and compounds inhibiting both strains (brown). The illustrated upper threshold for compounds moderately selecting for mecA+ is the mean of all the per-plate thresholds. Pie charts represent the frequency of each group within each of the libraries. e The 50% inhibitory concentrations (IC50s) of mecA− versus mecA+ strains for all 1990 compounds measured in Dose-Response assay (mean of the two dye-swap replicates). Dashed red line represents equal IC50s of mecA− and mecA+ strains. For the majority of compounds, IC50 of the mecA+ strain is higher than the IC50 of the mecA− strain. Source data are provided as a Source Data file.
