In vitro and in vivo efficacy of minocycline-based therapy for Elizabethkingia anophelis and the impact of reduced minocycline susceptibility

Objectives: Elizabethkingia anophelis is inherently resistant to multiple antibiotics, except minocycline. This study aimed to determine the in vitro and in vivo efficacy of minocycline monotherapy and combi-nation therapy against susceptible strains and the impact of reduced minocycline susceptibility. Methods: Three clinical isolates and one laboratory-induced mutant with reduced minocycline suscepti-bility were included. Time-kill and checkerboard assays were used to assess in vitro efficacy and synergy, respectively. Galleria mellonella infection and mouse pneumonia models were used to assess in vivo effi-cacy, and a mouse thigh infection model was used to determine the bacterial load. Results: Minocycline monotherapy exerted a modest inhibitory effect on three clinical minocycline-susceptible E. anophelis isolates in vitro , but delayed G. mellonella death and improved infected mouse survival; it also significantly reduced the in vivo bacterial load. Minocycline had decreased efficacy on G. mellonella and mice infected by the mutant with reduced minocycline susceptibility. Genome compari-son revealed several spontaneous mutations associated with reduced minocycline susceptibility. Among eight antibiotics tested in combination with minocycline, rifampin consistently showed in vitro synergy. The addition of rifampin (1 mg/L) reduced the mutant prevention concentration of minocycline from 2-4 mg/L to < 0.5 mg/L. However, compared with monotherapy, the combination of rifampin and minocycline did not further reduce the bacterial load or improve the survival of G. mellonella or mice. Conclusion: Minocycline monotherapy was in vivo effective against susceptible E. anophelis. Reduced minocycline susceptibility due to spontaneous mutation decreased its therapeutic efficacy. In combina-tion with rifampin, it prevented the in vitro emergence of reduced susceptibility but did not provide additional in vivo survival benefit. (c) 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

The study aimed to evaluate the efficacy of minocycline monotherapy and combination therapy against Elizabethkingia anophelis and the impact of reduced minocycline susceptibility. The results showed that minocycline monotherapy had a modest inhibitory effect on susceptible strains and improved survival rates in infected models, but reduced susceptibility decreased its efficacy. The addition of rifampin showed in vitro synergy, but did not provide additional survival benefits in combination with minocycline.