Objective:

To analyse the effect of reserpine in the MIC of tetracycline in N. gonorrhoeae.

Methods:

Seventeen clinical isolates of N. gonorrhoeae and three strains with a characterised mutation in the mtrCDE operon were analysed. The MIC of tetracycline both in the absence or the presence of reserpine was performed by E-test and read both at 24 and 48 h. Moreover, the MIC of erythromycin was performed following the same methodology in the three control isolates. Presence of tetA and tetM were established by PCR. To discharge a direct effect of reserpine the MIC of this compound was calculate.

Results:

Twelve intermediate and five tetracycline-resistant isolates were studied. All tetracycline-resistant strain carried the tetM gene (Dutch variant) whereas none presented the tetA gene. Nor tetA neither tetM genes were present among the intermediate isolates. In all strains the MIC of tetracycline was affected by reserpine. Thus, among intermediate isolates the MIC in presence of reserpine decreased from 4 to >62-fold (from 0.5 to 1 mg/L to <0.016 to 0.25 mg/L), without differences when the plates were read at 24 or 48 h, while those strains carrying tetM gene presented a MIC ranged from 12 to 16 mg/L and 1–1.5 mg/L at 24 h which was determined in the absence and the presence of reserpine, respectively, and of 32 mg/L, and 6–8 mg/ml when read in absence and presence of reserpine at 48 h. The isolates were able to grow on concentrations of reserpine higher than 100 mg/L.

The effect of reserpine on mtrCDE system was discharged due to its null effect inhibiting the MIC of erythromycin.

Conclusion:

Our results suggest an intrinsic mechanism of resistance to tetracycline in Neisseria gonorrhoeae clinical isolates probably associated with the basal expression of some reserpine inhibitable efflux pump different from mtrCDE.