fosZ, a novel plasmid-borne fosfomycin resistance gene in Pseudomonas species, especially carbapenem-resistant Pseudomonas aeruginosa isolates.

A novel fosfomycin-resistant glutathione S-transferase (FR-GST) gene, fosZ, was investigated, and its structural characteristics were characterized in Pseudomonas species. The fosZ gene was cloned and expressed in P. aeruginosa 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 fosZ was a transposable passenger gene within ISPa75, likely captured from Pseudomonas species. A total of 159 fosZ-bearing Pseudomonas strains were identified in GenBank over the past 22 years, sharing ten target site duplications (TSDs) associated with ISPa75. Among them, 34 strains were fully sequenced. ISPa75-fosZ was found on at least two chromosomes and 33 plasmids from four incompatibility groups, including the megaplasmids IncP-2 (24/33) and IncpJBCL41 (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 P. aeruginosa of various sequence types, especially ST1076, ST1418, and ST463. Mobilization of ISPa75 between plasmid and chromosome was observed in P. aeruginosa 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, fosZ encodes a distinct FR-GST exhibiting reduced susceptibility to inhibition by PPF and has been acquired by transferable ISPa75 on multidrug-resistant megaplasmids, disseminating fosfomycin resistance in Pseudomonas species, particularly across China.