TABLE 1

Susceptibility test results for five N. gonorrhoeae isolates, the ATCC 49226 control, and two strains (AZT-HLR and F89) highly resistant to azithromycin and ceftriaxone, respectively

Isolate (yr of detection): ST by NG-MAST, main molecular characteristics (7, 8, 25)aMIC (μg/ml) by Etest/susceptibilitybMIC (μg/ml) by microdilutionb,cMBC (μg/ml)c,dMean FICIe,f
CIPGENSPETETPENCFXCROAZTFOSCROAZTFOSCROAZTFOSAZT + CROFOS + AZTFOS + CRO
AE-7570 (2011): ST1407, PBP1 (L421P), PBP2 (XXXIV), PorB (G120K, A121N), mtrR promoter (deletion A)12/R6/NA8/S1/I0.38/I0.125/S0.023/S0.38/I6/NA0.008/S1/R8/NA0.0082161.61.91.9
AE-4615 (2011): ST8707, TEM-1, PBP1 (L421P), PBP2 (XIX), PorB (A121S), MtrR (A39T), Tet(M)1.5/R4/NA2/S24/R12/R≤0.016/S0.004/S0.064/S3/NA0.004/S0.25/S4/NA0.0040.541.73.21.7
AE-6772 (2012): ST8827, TEM-1, PBP1 (L421P), PBP2 (XIX), PorB (A120K, A121D), MtrR (A39T), Tet(M)2/R6/NA6/S16/R3/R≤0.016/S≤0.002/S0.023/S4/NA0.002/S0.064/S8/NA0.0020.06481.31.82.3
AE-2655 (2012): ST8826, PBP1 (L421P), PBP2 (XXXIV), PorB (G120K, A121N), mtrR promoter (deletion A)24/R6/NA3/S0.25/S0.38/I0.094/S0.023/S0.125/S8/NA0.008/S0.5/I16/NA0.0081162.22.31.5
AE-9562 (2012): ST437, PBP1 (L421P), PBP2 (V), PorB (G120K, A121D), mtrR promoter (deletion A)24/R8/NA3/S0.5/I0.25/I0.023/S0.023/S0.19/S4/NA0.008/S0.5/I8/NA0.008182.42.31.4
ATCC 492260.006/S12/NA12/S1.5/I0.75/I0.023/S0.016/S0.25/S32/NA0.008/S1/R64/NA0.0081641.62.01.2
AZT-HRL (2011): ST285, 23S rRNA (3 alleles with mutation A2059G)≥32/R12/NA12/S4/R2/R0.047/S0.064/S≥256/Rf12/NA0.016/S512/R8/NA0.0081,02481.52.01.5
F89 (2010): ST1407, PBP2 (XXXIV and A501P)≥32/R8/NA12/S3/R0.38/I2/R1.5/R0.25/S12/NA0.5/R1/R32/NA12322.42.61.3
  • a PBP1, penicillin-binding protein 1.

  • b CIP, ciprofloxacin; GEN, gentamicin; SPE, spectinomycin; TET, tetracycline; PEN, penicillin; CFX, cefixime; CRO, ceftriaxone; AZT, azithromycin; FOS, fosfomycin; R, resistant; NA, not available; S, susceptible; I, intermediate. The MIC interpretation according to the 2014 EUCAST criteria (26) was determined as follows: for PEN, S at ≤0.06 μg/ml and R at ≥2 μg/ml; for CFX and CRO, S at ≤0.125 μg/ml and R at ≥0.25 μg/ml; for AZT, S at ≤0.25 μg/ml and R at ≥1 μg/ml; for CIP, S at ≤0.025 μg/ml and R at ≥0.125 μg/ml; for GEN, not available; for SPE, S at ≤64 μg/ml and R at ≥128 μg/ml; and for TET, S at ≤0.25 μg/ml and R at ≥2 μg/ml.

  • c All tests in broth microdilution were performed with FB broth supplemented with 1% IsoVitaleX and G6P (25 μg/ml).

  • d MBC, minimum bactericidal concentration.

  • e FICI, fractional inhibitory concentration index. The mean FICI was obtained by calculating the average from at least three independent checkerboard tests. It was interpreted as synergistic, additive, indifferent, or antagonist effect (<0.5, 0.5 to 1, >1 to 4, and >4, respectively) (7).

  • f For the time-kill experiments, the MIC was 256 μg/ml.