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)a | MIC (μg/ml) by Etest/susceptibilityb |
MIC (μg/ml) by microdilutionb,c |
MBC (μg/ml)c,d |
Mean FICIe,f |
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|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| CIP | GEN | SPE | TET | PEN | CFX | CRO | AZT | FOS | CRO | AZT | FOS | CRO | AZT | FOS | AZT + CRO | FOS + AZT | FOS + CRO | |
| AE-7570 (2011): ST1407, PBP1 (L421P), PBP2 (XXXIV), PorB (G120K, A121N), mtrR promoter (deletion A) | 12/R | 6/NA | 8/S | 1/I | 0.38/I | 0.125/S | 0.023/S | 0.38/I | 6/NA | 0.008/S | 1/R | 8/NA | 0.008 | 2 | 16 | 1.6 | 1.9 | 1.9 |
| AE-4615 (2011): ST8707, TEM-1, PBP1 (L421P), PBP2 (XIX), PorB (A121S), MtrR (A39T), Tet(M) | 1.5/R | 4/NA | 2/S | 24/R | 12/R | ≤0.016/S | 0.004/S | 0.064/S | 3/NA | 0.004/S | 0.25/S | 4/NA | 0.004 | 0.5 | 4 | 1.7 | 3.2 | 1.7 |
| AE-6772 (2012): ST8827, TEM-1, PBP1 (L421P), PBP2 (XIX), PorB (A120K, A121D), MtrR (A39T), Tet(M) | 2/R | 6/NA | 6/S | 16/R | 3/R | ≤0.016/S | ≤0.002/S | 0.023/S | 4/NA | 0.002/S | 0.064/S | 8/NA | 0.002 | 0.064 | 8 | 1.3 | 1.8 | 2.3 |
| AE-2655 (2012): ST8826, PBP1 (L421P), PBP2 (XXXIV), PorB (G120K, A121N), mtrR promoter (deletion A) | 24/R | 6/NA | 3/S | 0.25/S | 0.38/I | 0.094/S | 0.023/S | 0.125/S | 8/NA | 0.008/S | 0.5/I | 16/NA | 0.008 | 1 | 16 | 2.2 | 2.3 | 1.5 |
| AE-9562 (2012): ST437, PBP1 (L421P), PBP2 (V), PorB (G120K, A121D), mtrR promoter (deletion A) | 24/R | 8/NA | 3/S | 0.5/I | 0.25/I | 0.023/S | 0.023/S | 0.19/S | 4/NA | 0.008/S | 0.5/I | 8/NA | 0.008 | 1 | 8 | 2.4 | 2.3 | 1.4 |
| ATCC 49226 | 0.006/S | 12/NA | 12/S | 1.5/I | 0.75/I | 0.023/S | 0.016/S | 0.25/S | 32/NA | 0.008/S | 1/R | 64/NA | 0.008 | 1 | 64 | 1.6 | 2.0 | 1.2 |
| AZT-HRL (2011): ST285, 23S rRNA (3 alleles with mutation A2059G) | ≥32/R | 12/NA | 12/S | 4/R | 2/R | 0.047/S | 0.064/S | ≥256/Rf | 12/NA | 0.016/S | 512/R | 8/NA | 0.008 | 1,024 | 8 | 1.5 | 2.0 | 1.5 |
| F89 (2010): ST1407, PBP2 (XXXIV and A501P) | ≥32/R | 8/NA | 12/S | 3/R | 0.38/I | 2/R | 1.5/R | 0.25/S | 12/NA | 0.5/R | 1/R | 32/NA | 1 | 2 | 32 | 2.4 | 2.6 | 1.3 |
PBP1, penicillin-binding protein 1.
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.
All tests in broth microdilution were performed with FB broth supplemented with 1% IsoVitaleX and G6P (25 μg/ml).
MBC, minimum bactericidal concentration.
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).
For the time-kill experiments, the MIC was 256 μg/ml.