Nitrofurantoin activity against Neisseria gonorrhoeae : an in vitro and in vivo study in Galleria mellonella

We assessed the in vitro and in vivo efficacy of nitrofurantoin in a panel of Neisseria gonorrhoeae strains. Four N. gonorrhoeae strains-three WHO reference strains (WHO U, WHO P, WHO V) and a clinical isolate from a patient with symptomatic urethritis (25566) were tested for nitrofurantoin susceptibility using E-tests on GC agar. To investigate resistance emergence, each strain was serially passaged on GC agar with a nitrofurantoin gradient E-tests for ten consecutive days. Nitrofurantoin-resistant isolates (n = 3) underwent whole genome sequencing (WGS) to identify resistance-associated mutations. In vivo efficacy was evaluated using the Galleria mellonella infection model: larvae were infected with N. gonorrhoeae 25566 and treated with nitrofurantoin at 5 mg/kg or 50 mg/kg. Larval survival and melanization were monitored up to 48 h. We found that the nitrofurantoin MICs were between 0.5 and 24 & micro;g/ml. WGS identified a canonical point mutation at nfsB at position 155 (G & agrave;A) for WHO U-S3 isolate, whereas WHO P-S2, WHO V-S4, showed no new nfsB point mutations despite MIC increases. A dose of 50 mg/kg was effective against N. gonorrhoeae in the G. mellonella infection model. Constant exposure to nitrofurantoin over 10 days resulted in increases in MIC in three of four strains.We found evidence that nitrofurantoin had in vivo and in vitro activity against Neisseria gonorrhoeae.