Doxycycline PEP can induce doxycycline resistance in in a model of PEP.

Background: Four randomized controlled trials have now established that doxycycline post exposure (sex) prophylaxis (PEP) can reduce the incidence of chlamydia and syphilis in men who have sex with men. These studies have concluded that the risk of selecting for antimicrobial resistance is low. We evaluated this risk and using a infection model.

Methods: We evaluated how long it took for doxycycline resistance to emerge during passage on doxycycline containing agar plates in 4 species - , , and . We then assessed if could acquire resistance to doxycycline (and cross resistance to other antimicrobials) during intermittent exposure to doxycycline in a model of doxycycline PEP.

Results: In our passage experiments, we found that resistance first emerged in . By day 7 the MIC had increased from 2 mg/L to a median of 96 mg/L (IQR 64-96). Under various simulations of doxycycline PEP in the G. mellonella model, the doxycycline MIC of increased from 2 mg/L to 48 mg/L (IQR 48-84). Ceftriaxone and ciprofloxacin MICs increased over ten-fold. Whole genome sequencing revealed acquired mutations in ramR which regulates the expression of the AcrAB-TolC efflux pump.

Conclusion: Doxycycline PEP can select for doxycycline, ceftriaxone and ciprofloxacin resistance in in a G. mellonella model. The emergent ramR mutations were similar to those seen in circulating strains of . These findings suggest that we need to assess the effect of doxycycline PEP on resistance induction on a broader range of bacterial species than has hitherto been the case.