Evolutionary paths to macrolide resistance in a Neisseria commensal converge on ribosomal genes through short...

Area of Science:

• Microbiology
• Genetics
• Evolutionary Biology

Background:

• Commensal Neisseria species are known reservoirs of antibiotic resistance genes that can transfer to pathogenic relatives.
• Understanding the evolutionary mechanisms of reduced antibiotic susceptibility in commensal Neisseria is crucial for predicting resistance spread.

Purpose of the Study:

• To identify genetic adaptations conferring azithromycin resistance in the commensal Neisseria elongata using in vitro selection.
• To investigate the evolutionary pathways and potential fitness implications of these resistance mutations.

Main Methods:

• In vitro selection was employed to generate azithromycin-resistant mutants of Neisseria elongata.
• Mutations in ribosomal protein genes (rpmH and rpsC) were identified using genetic sequencing.
• Causality of identified mutations was confirmed through genetic transformation experiments.

Main Results:

• Multiple independent lineages of N. elongata acquired azithromycin resistance (≥ 2 μg/mL) via short tandem duplications.
• Mutations converged on the rpmH and rpsC loci, affecting ribosomal protein L34 and S3.
• Some identified rpmH mutations were identical or highly similar to those found in azithromycin-resistant Neisseria gonorrhoeae.
• While most resistant mutants exhibited fitness costs, one variant showed no significant growth defect.

Conclusions:

• Genetic adaptations in rpmH and rpsC are key drivers of azithromycin resistance in N. elongata.
• Convergent evolution of resistance mechanisms highlights potential shared pathways between commensal and pathogenic Neisseria.
• The identification of a fit azithromycin-resistant variant suggests a potential for natural selection and dissemination in microbial populations.