Beneficial impact of acquired AmpC β-lactamases on bacterial fitness and pathogenicity: a new paradigm.

Unlabelled AmpC-type β-lactamases share structural similarities with penicillin-binding proteins, supporting the view that both enzyme families evolved from a common ancestor. Considering the widespread dissemination of acquired AmpCs in Gram-negative bacteria, we hypothesize that, besides their established role in β-lactam resistance, they may also influence essential bacterial mechanisms. Thus, this study aimed to elucidate whether AmpCs modulate bacterial physiology and fitness. Escherichia coli MG1655 was used as the model organism to assess the impact of clinically relevant acquired AmpCs (CMY, DHA, ACC, and FOX variants) on growth, competition, motility, biofilm formation, and pathogenicity, coupled with transcriptomic profiling. We demonstrated that certain CMY variants (notably CMY-42 and CMY-145) confer an unexpected in vitro growth advantage, whereas CMY-2 enhances pathogenicity in vivo by inducing flagellar production. These findings establish a new paradigm in which some AmpCs act as both resistance determinants and regulators of bacterial fitness and virulence, contributing to the ecological success of Enterobacterales.Importance Although β-lactamases have always been considered enzymes involved in resistance to β-lactam antibiotics, they also have to be considered as playing a role in reshaping bacterial physiology. By studying a series of class C β-lactamases, we uncovered an unexpected dual role. Besides conferring resistance to β-lactams, they also impact bacterial growth, motility, and pathogenicity through modulation of flagellar biosynthesis. Strikingly, while CMY-42- and CMY-145-producing recombinant Escherichia coli strains showed enhanced in vitro fitness growth by reducing energy-costly flagellar production, production of CMY-2 (the most commonly identified acquired AmpC β-lactamase identified in Enterobacterales) restored flagella and increased virulence in vivo. This work reframes β-lactamases as metabolic regulators influencing ecological success, offering new insight into how resistance determinants promote the spread and clinical impact of Enterobacterales.