Figure 4 Broad-spectrum bactericidal effects of three-drug combinations. Fifteen three-drug targeted drug combinations (TDCs) were tested at their IC90 concentrations against 10 multidrug-resistant strains, including K. pneumoniae (KPNIH776 and KPNIH892), A. baumannii (ABNIH144, ABNIH233 and ABNIH333), P. aeruginosa (PANIH338 and PANIH668), C. freundii (CFB10), E. cloacae (ECB2) and E. coli (ECOB11). (A) Heatmap of bactericidal effects of 15 TDCs against 10 MDR bacteria; 0% viability (red), 20% viability (white), 50% viability (blue) and 100% viability (black). Comb1 (rifabutin—0.052 μM, polymyxin B—1 μM and zidovudine—1 μM); Comb2 (rifabutin—0.056 μM, polymyxin B—1 μM and trimethoprim—4 μM); Comb3 (rifabutin—0.056 μM, polymyxin B—1 μM and aztreonam—4 μM); Comb4 (rifabutin—0.06 μM, polymyxin B—1 μM and ceftazidime—15 μM); Comb4* (rifabutin—0.06 μM, polymyxin B—1 μM and ceftazidime—4 μM); Comb5 (rifabutin—0.09 μM, polymyxin B—1 μM and imipenem—16 μM); Comb5* (rifabutin—0.09 μM, polymyxin B—1 μM and imipenem—8 μM); Comb6 (rifabutin—0.052 μM, colistin—2.1 μM and zidovudine—1 μM); Comb7 (rifabutin—0.056 μM, colistin—2.1 μM and trimethoprim—4 μM); Comb8 (rifabutin—0.056 μM, colistin—2.1 μM and aztreonam—4 μM); Comb9 (rifabutin—0.06 μM, colistin—2.1 μM and ceftazidime—15 μM); Comb9* (rifabutin—0.06 μM, colistin—2.1 μM and ceftazidime—8 μM); Comb9# (rifabutin—0.06 μM, colistin—2.1 μM and ceftazidime—4 μM); Comb10 (rifabutin—0.09 μM, colistin—2.1 μM and imipenem—16 μM); Comb10* (rifabutin—0.09 μM, colistin—2.1 μM and imipenem—8 μM); Comb11 (colistin—1.2 μM, auranofin—1 μM and imipenem—16 μM); Comb11* (colistin—1.2 μM, auranofin—1 μM and imipenem—8 μM); Comb12 (colistin—1.9 μM, auranofin—1 μM and rifabutin—0.2 μM); Comb13 (colistin—1.7 μM, auranofin—1 μM and ceftazidime—15 μM); Comb13* (colistin—1.7 μM, auranofin—1 μM and ceftazidime—8 μM); Comb13# (colistin—1.7 μM, auranofin—1 μM and ceftazidime—4 μM); Comb14 (colistin—2.1 μM, auranofin—1 μM and zidovudine—1 μM); Comb15 (polymyxin B—1.8 μM, auranofin—1 μM and ceftazidime—15 μM); Comb15* (polymyxin B—1.8 μM, auranofin—1 μM and ceftazidime—8 μM); Comb15# (polymyxin B—1.8 μM, auranofin—1 μM and ceftazidime—4 μM); n=4; * and # represent the same drugs are in combination but with different concentrations. (B) Top three TDCs and dimethylsulphoxide (DMSO) control were plotted as % normalized viability of different MDR isolates: (i) colistin—1.96 μg/mL, auranofin—0.68 μg/mL and ceftazidime—8.20 μg/mL (green) (Comb13 in A; (ii) colistin—2.19 μg/mL, auranofin—0.68 μg/mL and rifabutin—0.17 μg/mL (blue) (Comb12 in A; (iii) rifabutin—0.08 μg/mL, colistin—2.43 μg/mL and imipenem—4.80 μg/mL (purple) (Comb10 in A; n=4. Bar graph represent mean, and error bars represent the SEM. (C) Clinical breakpointsCitation24, Citation29 and drug concentrations in three-drug TDCs. Bars represent drug concentrations of colistin (black), auranofin (green), ceftazidime (blue), rifabutin (purple) and imipenem (orange), and corresponding colored dashed lines represent individual drug susceptibility breakpoints. Breakpoints were selected for auranofinCitation30 and rifabutinCitation31 based on the selected literature. Imipenem and ceftazadime breakpoints were based on CLSI guidelines for Enterobacteriaceae. Imipenem breakpoint is 2 μg/mL and ceftazadime breakpoint is 8 μg/mL for Acinetobacter spp. and Pseudomonas spp.Citation24