Adaptation and compensation in a bacterial gene regulatory network evolving under antibiotic selection

Area of Science:

• Microbial evolution
• Bacterial genetics
• Antibiotic resistance

Background:

• Gene regulatory networks enable bacterial adaptation to environmental changes.
• Evolutionary rewiring of these networks under selection pressure is not well understood.
• The antibiotic trimethoprim targets dihydrofolate reductase (DHFR) in bacteria.

Purpose of the Study:

• To investigate the early evolutionary response of *Escherichia coli* to trimethoprim.
• To elucidate the mechanisms linking gene regulatory network changes to antibiotic adaptation.
• To understand how selection pressure shapes evolutionary trajectories.

Main Methods:

• Laboratory evolution experiments with *Escherichia coli*.
• Whole-genome sequencing to identify mutations.
• Mutation reconstruction to validate functional effects.

Main Results:

• Trimethoprim exposure led to MgrB inactivation, derepressing the Mg2+-sensitive PhoPQ system.
• Derepressed PhoPQ upregulated DHFR, conferring trimethoprim tolerance.
• Mutations in *mgrB* preceded DHFR mutations, facilitating resistance evolution.
• Bacteria evolved to either develop DHFR-mediated resistance or compensate for fitness costs via RpoS inactivation, depending on selection strength.

Conclusions:

• Early bacterial adaptation to antibiotics involves specific regulatory network changes.
• The strength of antibiotic selection dictates the evolutionary path towards resistance or compensation.
• This study provides mechanistic insights into how selection shapes bacterial evolution.