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 N. 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 (25 566) 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 G. mellonella infection model: larvae were infected with N. gonorrhoeae 25 566 and treated with nitrofurantoin at 5 mg/kg or 50 mg/kg. Larval survival and melanization were monitored up to 48 hours. We found that the nitrofurantoin MICs were between 0.5 and 24 µg/ml. WGS identified a canonical point mutation at nfsB at position 155 (GàA) 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 Galleria mellonella infection model. Constant exposure to nitrofurantoin over 10 days resulted in increases in MIC in three of four strains.