Table 4.
Synergy assays and culturable enumeration against multidrug resistant E. coli and A. baumanii.
| Bacterial Strain | MBC 154 | MBC A22 | FIC 154 | FIC A22 | FICI | Log CFU mL−1 | SD CFU mL−1 |
|---|---|---|---|---|---|---|---|
| E. coli 25922 | >32 | >64 | 4 | 8 | 0.25 | 3.2 | 0.2 |
| A. baumannii BC5a | 16 | >64 | 4 | 2 | 0.281 | BLD | — |
| A. baumannii BC5 | 16 | >64 | 4 | 4 | 0.312 | BLD | — |
| A. baumannii BC5 | 16 | >64 | 2 | 8 | 0.25 | 2.8 | 0.2 |
| MBC H33258 | MBC A22 | FIC H33258 | FIC A22 | FICI | LOG CFU ml −1 | SD CFU ml −1 | |
| E. coli H4Ha | >32 | >64 | 8 | 32 | 0.75 | 4.2 | 0.2 |
| E. coli 25922 | >32 | >64 | 32 | 8 | 1.125 | 4.5 | 0.1 |
| A. baumannii BC5 | >32 | >64 | 8 | 32 | 0.75 | 3.9 | 0.1 |
| MBC Cipro | MBC A22 | FIC Cipro | FIC A22 | FICI | LOG CFU ml −1 | SD CFU ml −1 | |
| A. baumannii BC5 | >32 | 64 | 8 | 32 | 0.75 | 3.6 | 0.1 |
| MBC 156 | MBC A22 | FIC 156 | FIC A22 | FICI | LOG CFU ml −1 | SD CFU ml −1 | |
| A. baumannii BC5 | >32 | >64 | 16 | 32 | 1 | 4.7 | 0.1 |
| E. coli 25922 | >32 | >64 | 32 | 64 | 2 | 9.3 | 0.3 |
| E. coli H4H | >32 | >64 | 32 | 64 | 2 | 9.6 | 0.1 |
Synergy was evaluated between A22 and DPA154, H33258, ciprofloxacin or DPA156 by the checkerboard method following the broth microdilution as outlined by the Clinical Institute for Laboratory Standards. DPA 154, but not 156 demonstrates synergy against MDR A. baumanii. Hoechst 33258, ciprofloxacin, and DPA 156 demonstrate no synergy. Bacteria were incubated overnight at 37 °C for 16 h for each assay in duplicate. Growth inhibition was determined by optical density at A595 nm and enumeration of colony-forming units (CFU) per mL was determined by spot plating with 10 µL from each well of serial 1:5 dilutions performed in triplicate. BLD: below the limit of detection (<2 LOG CFU/mL) where no colonies were observed; SD (standard deviation). Cipro: ciprofloxacin, MBC: minimal bactericidal concentration, FIC: fractional inhibitory concentration and FICI: fractional inhibitory concentration index as calculated accordingly: FICI = FIC1/MBC1 + FIC2/MBC2. aMulitdrug-resistant strains. A. baumannii BC5 is a ciprofloxacin-resistant strain (MIC > 100 µM) while both E. coli strains are sensitive to ciprofloxacin (MIC 0.19–39 µM). Spot plate follow-up for validation and checkerboards and additional replicate synergy data are available in Supplemental Material (Figs S21–S23).