TABLE 1.

Summary of antimicrobial synergy testing of PDR A. baumanii clones by Etest 2, checkerboard, and time-kill methods

Clone no.Susceptibility results with indicated test and antibiotic combinationa
Etest 2bCheckerboardcTime-killd
TGC+CSTIPM+TGCIPM+CSTAMK+TGCTGC+CSTIPM+TGCIPM+CSTAMK+TGCIPM+TGC+AMKIPM+TGCIPM+CSTIPM+TGC+AMK
1I/I/II/SS/SI/II/IA/IA/AI/IA/INTSND6 he
2I/II/SS/SI/II/IA/AS/AI/II/I6 he6, 24 h6, 24 h
3I/IS/SS/SI/II/IS/AA/AI/II/I6, 24 hSNDNT
4I/II/SA/SI/II/IA/IA/IA/AS/I6 heSND6 he
5I/IA/SS/SI/II/II/IA/AI/IA/INT6, 24 h6, 24 h
6I/IS/SS/SI/II/IA/IA/II/II/INTSND6 he
7I/IS/SS/SI/IA/AA/AA/IA/AS/ISNDSND6 he
8I/IS/SS/SI/IA/AA/AS/AI/II/I6, 24 h6 he6, 24 h
  • a TGC, tigecycline; CST, colistimethate; IPM, imipenem; AMK, amikacin; S, synergy; I, indifference; A, additiveness; SND, synergy not detected at 6 or 24 h; NT, not tested.

  • b Etest 2 synergy testing was performed for all eight clonal types with the combinations TGC+CST, IPM+TGC, IPM+CST, and AMK+TGC. For Etest method 2, TGC and CST were added at 0.25 and 0.5 times the MIC into the agar medium. Results represent the interpretation of the change in the MIC of the drug in the E strip when added to the agar medium containing TGC or CST at 0.25 or 0.5 times the MIC compared to the MIC of the drug in the E strip in the agar medium containing no antibiotic.

  • c Checkerboard synergy testing was performed for all eight clonal types with the indicated combinations. Results represent the interpretations of the minimum and maximum FICI values.

  • d Time-kill analysis was performed only for the clones that showed synergy or additivity with Etest 2 and in more than two wells with CB testing. Results represent the duration (in hours) for which synergy was detected.

  • e Synergy was not detected at 24 h.