<i>fosZ</i> , a novel plasmid-borne fosfomycin resistance gene in <i>Pseudomonas</i> species, especially carbapenem-resistant <i>Pseudomonas aeruginosa</i> isolates

A novel fosfomycin-resistant glutathione S-transferase (FR-GST) gene, <i>fosZ</i>, was investigated, and its structural characteristics were characterized in <i>Pseudomonas</i> species. The <i>fosZ</i> gene was cloned and expressed in <i>P. aeruginosa</i> PAO1 and HS355, where it displayed reduced susceptibility to fosfomycin (8- to 64-fold) and its inhibitor sodium phosphonoformate (PPF). FosZ shares less than 70% amino acid identity with known FosA proteins. Bioinformatics analyzes revealed that <i>fosZ</i> was a transposable passenger gene within IS<i>Pa75</i>, likely captured from <i>Pseudomonas</i> species. A total of 159 <i>fosZ</i>-bearing <i>Pseudomonas</i> strains were identified in GenBank over the past 22 years, sharing ten target site duplications (TSDs) associated with IS<i>Pa75</i>. Among them, 34 strains were fully sequenced. IS<i>Pa75-fosZ</i> was found on at least two chromosomes and 33 plasmids from four incompatibility groups, including the megaplasmids IncP-2 (24/33) and Inc_pJBCL41 (8/33), which also carried carbapenem and other antibiotic resistance genes. The most common TSDs were TSD6 (82/156) and TSD10 (48/156), predominantly in carbapenem-resistant <i>P. aeruginosa</i> of various sequence types, especially ST1076, ST1418, and ST463. Mobilization of IS<i>Pa75</i> between plasmid and chromosome was observed in <i>P. aeruginosa</i> MAS152. Structure prediction and analysis of FR-GST proteins revealed distinct features in the dimer interface loop. In the FosZ structure, an expanded K⁺-binding loop caused a deviation at residue S95, which participated in binding both fosfomycin and PPF. In conclusion, <i>fosZ</i> encodes a distinct FR-GST exhibiting reduced susceptibility to inhibition by PPF and has been acquired by transferable IS<i>Pa75</i> on multidrug-resistant megaplasmids, disseminating fosfomycin resistance in <i>Pseudomonas</i> species, particularly across China.