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. 2023 Feb 15;12(2):389. doi: 10.3390/antibiotics12020389

Figure 1.

Figure 1

Fractional inhibitory concentration index (FICI) of PMB and LL-37 against (A) E. coli and (B) P. aeruginosa CIs, where FICI ≤ 0.5 is synergistic, 0.5 < FICI ≤ 1.0 is additive, 1.0 < FICI ≤ 4.0 is indifferent, and FICI > 4.0 is antagonistic. The synergistic responses of LL-37 and PMB against clinical, drug-resistant isolates of E. coli and P. aeruginosa were evaluated using checkerboard assays. The microtiter plates were incubated at 37 °C under static conditions for 18–22 h before 100 μL of 0.2 mM resazurin was added to all wells. After an additional incubation of 2 h, the levels of reduced resazurin were quantified at 560 nm. Control wells included MH media and bacteria in the absence of antimicrobial agents as sterility and growth controls, respectively. The mean FICI and standard deviations for 10 different clinical, drug-resistant isolates were calculated from at least three independent experiments.