Introduction At present, gentamicin is a promising antibiotic for the treatment of multidrug resistant N. gonorrhoeae isolates. Therefore, the knowledge of N. gonorrhoeae susceptibility to gentamicin is required. The CLSI guidelines do not list breakpoints for gentamicin. However, MIC interpretive criteria have been proposed. Moreover, a recent report comparing the disk diffusion with Etest has established tentative gentamicin zone breakpoints for the CLSI method. The proposed breakpoints are ≥16 mm for susceptible, 13–15 mm for intermediate and ≤12 mm for resistant. The aim of this study was to compare the disk diffusion method with the agar dilution test, and to analyse the suitability and reliably of disk diffusion to monitor the susceptibility to gentamicin.
Methods We studied 237 n. gonorrhoeae isolates obtained in 2013 and 2015 from the GASSP-AR. The MIC determination and disk diffusion tests to gentamicin were performed according to CLSI, and tentative breakpoints previously reported were used. The 2008 WHO and ATCC 49226 reference strains were used as control. The inhibition diameters by disk diffusion were tested by correlation with the MIC value.
Results Gentamicin MICs ranged from 2 to 16 µg/ml, and the MIC50/90 were both 8 µg/ml. The Pearson correlation between disk diffusion and agar dilution was −0.67 (p<0.001). No very major or major discrepancies were detected with disk diffusion as compared to agar dilution. However, a high percentage of minor discrepancies (39.9%) was observed. By adjusting the susceptible breakpoint for disk diffusion to S≥17 mm, the minor discrepancies rate was reduced from 39.9% to 18.4%.
Conclusion: N. gonorrhoeae isolates with resistance to gentamicin were not observed. The disk diffusion had good correlation when compared with the agar dilution method. Although a high percentage of minor discrepancies was observed, the error rate was reduced adjusting the breakpoint. Until it becomes standardised, the disk diffusion can be a screening method in clinical laboratories to detect the gentamicin susceptibility of N. gonorrhoeae.