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Gfa, a new integron-borne fosfomycin resistance family

Kieffer, N.; Prieto, A.; Ortiz-Miravalles, L.; Hipolito, A.; Vergara, E.; Borrero del Pino, J.; Carvalho, A.; Escudero, J.-A.

2026-10-08 microbiology
10.64898/2026.10.07.757220 bioRxiv
Show abstract

Fosfomycin is increasingly used to treat multidrug-resistant Gram-negative infections, yet resistance surveillance largely relies on homology-based detection of known fos genes. Here, we identify Gfa, a previously unrecognized integron-borne fosfomycin resistance determinant uncovered through functional screening of 129 integron gene cassettes. Gfa confers high-level fosfomycin resistance across Enterobacterales and Pseudomonas aeruginosa and is resistant to phosphonoformate inhibition. Structural modelling and mutagenesis reveal a conserved cysteine-rich, zinc-organized architecture essential for activity and distinct from canonical Fos enzymes. Crude-extract assays confirm glutathione-dependent fosfomycin inactivation under metal-chelating conditions. Distant Gfa homologs also conferred resistance, although activity was restricted to a subset of proteins tested. Together, these findings establish Gfa as a previously unrecognized fosfomycin resistance family and highlight the limitations of homology-based mining for discovering novel resistance determinants and predicting which family members are functional.

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