Reinfection and ceftriaxone tolerance in a clinical case of recurrent gonorrhoea: a case report supported by in vitro and in vivo models

Background:
Antibiotic tolerance slows bacterial killing during drug exposure without changing minimum inhibitory concentration (MIC) and is often missed by MIC-based testing in Neisseria gonorrhoeae.

Objectives:
To quantify ceftriaxone tolerance using complementary laboratory assays and to integrate these readouts with genomic typing to interpret recurrent urethritis within one patient series.

Methods:
Four clinical isolates obtained between 2022 and 2025 underwent tolerance disc testing. From the most recent isolate (25355), tolerant and non-tolerant variants were derived for functional study, including growth curves (log10 CFU/mL), minimum duration needed to kill 99% of cells (MDK99) across ceftriaxone concentrations, and a Galleria mellonella infection model. Whole-genome sequencing with in silico typing compared strain type between episodes.

Results:
Tolerance disc testing was positive for isolates 22073 and 25355. Across the growth trajectory, tolerant variants showed consistently lower bacterial counts than non-tolerant variants. Tolerant variants tended to exhibit longer MDK99 values, and in Galleria they declined more slowly under ceftriaxone exposure. Sequencing revealed distinct sequence types across episodes, supporting reinfection rather than within-host persistence.

Conclusions:
Assays that capture killing kinetics detected ceftriaxone tolerance that was not captured by MIC-based testing. Genomic analysis distinguished reinfection from persistence. This integrated workflow may improve the evaluation of suspected treatment failure in gonorrhoea.