Tigecycline Nonsusceptibility Occurs Exclusively in Fluoroquinolone-Resistant Escherichia coli Clinical Isolates, Including the Major Multidrug-Resistant Lineages O25b:H4-ST131-H 30 R and O1-ST648

Tigecycline (TGC) is a last-line drug for multidrug-resistantEnterobacteriaceae . We investigated the mechanism(s) underlying TGC nonsusceptibility (TGC resistant/intermediate) inEscherichia coli clinical isolates. The MIC of TGC was determined for 277 fluoroquinolone-susceptible isolates (ciprofloxacin [CIP] MIC, 2 mg/liter). The MIC50 and MIC90 for TGC in fluoroquinolone-resistant isolates were 2-fold higher than those in fluoroquinolone-susceptible isolates (MIC50 , 0.5 mg/liter versus 0.25 mg/liter; MIC90 , 1 mg/liter versus 0.5 mg/liter, respectively). Two fluoroquinolone-resistant isolates (O25b:H4-ST131-H 30R and O125:H37-ST48) were TGC resistant (MICs of 4 and 16 mg/liter, respectively), and four other isolates of O25b:H4-ST131-H 30R and an isolate of O1-ST648 showed an intermediate interpretation (MIC, 2 mg/liter). No TGC-resistant/intermediate strains were found among the fluoroquinolone-susceptible isolates. The TGC-resistant/intermediate isolates expressed higher levels ofacrA andacrB and had lower intracellular TGC concentrations than susceptible isolates, and they possessed mutations inacrR and/ormarR . The MICs ofacrAB -deficient mutants were markedly lower (0.25 mg/liter) than those of the parental strain. After continuous stepwise exposure to CIPin vitro , six of eight TGC-susceptible isolates had reduced TGC susceptibility. Two of them acquired TGC resistance (TGC MIC, 4 mg/liter) and exhibited expression ofacrA andacrB and mutations inacrR and/ormarR . In conclusion, a population of fluoroquinolone-resistantE. coli isolates, including major extraintestinal pathogenic lineages O25b:H4-ST131-H 30R and O1-ST648, showed reduced susceptibility to TGC due to overexpression of the efflux pump AcrAB-TolC, leading to decreased intracellular concentrations of the antibiotics that may be associated with the development of fluoroquinolone resistance.