Cryptic β-Lactamase Evolution Is Driven by Low β-Lactam Concentrations

Area of Science:

• Microbiology
• Evolutionary Biology
• Biochemistry

Background:

• Understanding how low antibiotic concentrations influence antimicrobial resistance evolution is crucial.
• The role of sub-inhibitory concentrations of beta-lactams in driving resistance, particularly for beta-lactamases like OXA-48, remains understudied.

Purpose of the Study:

• To investigate the evolutionary impact of low ceftazidime concentrations on the carbapenemase OXA-48.
• To test the hypothesis that low antibiotic levels promote genetic diversity that can lead to clinical resistance.

Main Methods:

• Experimental evolution of *Escherichia coli* expressing *bla*_OXA-48 at sub-minimum inhibitory concentrations (sub-MICs) of ceftazidime.
• Identification and characterization of OXA-48 variants.
• Assessment of susceptibility profiles, dose-response curves, and catalytic efficiencies.

Main Results:

• Seven single variants of OXA-48 were identified, conferring only marginal increases in resistance.
• These variants exhibited significant selectable fitness benefits in competition experiments at sub-MICs.
• Variants showed enhanced catalytic efficiencies toward ceftazidime, linked to structural changes in key loops and active site residues.

Conclusions:

• Low-level beta-lactam concentrations can drive beta-lactamase evolution through cryptic phenotypes.
• This process generates standing genetic diversity that can be selected for under increased antibiotic pressure, contributing to clinical resistance.