Gfa, a new integron-borne fosfomycin resistance family

Fosfomycin, a widely used antibiotic for treating multidrug-resistant Gram-negative infections, is increasingly facing resistance challenges. Current resistance surveillance methods heavily depend on detecting known fos genes through homology-based techniques. However, a novel fosfomycin resistance determinant, Gfa, has been identified through functional screening of 129 integron gene cassettes, marking a significant discovery.

Gfa exhibits its resistance prowess across Enterobacterales and Pseudomonas aeruginosa, conferring high-level fosfomycin resistance. Distinct from the typical Fos enzymes, Gfa possesses a conserved cysteine-rich, zinc-organized architecture, which remains resilient even under phosphonoformate inhibition. This architectural feature is crucial for Gfa's activity and sets it apart from the conventional enzymes.

Using crude-extract assays, researchers have confirmed that Gfa inactivates fosfomycin under metal-chelating conditions, reliant on the presence of glutathione. Remarkably, distant Gfa homologs also display resistance, although their activity is limited, confining the resistance to a specific subset of tested proteins.

These findings collectively reveal Gfa as a previously unknown fosfomycin resistance family, emphasizing the need for more comprehensive methods beyond homology-based mining. This approach uncovers novel resistance determinants and sheds light on predicting the functional capabilities of various family members.