Dina Zheng

Multidrug-resistant (MDR) Pseudomonas aeruginosa presents a serious threat to public health due to its widespread resistance to numerous antibiotics. P. aeruginosa commonly causes nosocomial infections including urinary tract infections (UTI), which have become increasingly difficult to treat. The lack of effective therapeutic agents has renewed interest in fosfomycin, an old drug discovered in the 1960s and approved prior to the rigorous standards now required for drug approval. Fosfomycin has a unique structure and mechanism of action, making it a favorable therapeutic alternative for MDR pathogens that are resistant to other classes of antibiotics. The absence of susceptibility breakpoints for fosfomycin against P. aeruginosa limits its clinical use and interpretation due to extrapolation of breakpoints established for Escherichia coli or Enterobacterales without supporting evidence. Furthermore, fosfomycin use and efficacy for treatment of P. aeruginosa are also limited by both inherent and acquired resistance mechanisms. This narrative review provides an update on currently identified resistance mechanisms to fosfomycin, with a focus among those mediated by P. aeruginosa such as peptidoglycan recycling enzymes, chromosomal Fos enzymes, and outer membrane porin channel modification. Additional fosfomycin resistance mechanisms mediated by Enterobacterales, including mutations in transporters and associated regulators, plasmid-mediated Fos enzymes, kinases, and murA modification, are also summarized and contrasted. These data highlight that differences in fosfomycin resistance mechanisms may be associated with elevated MIC values in P. aeruginosa compared to Enterobacterales, emphasizing the need for breakpoints specific to P. aeruginosa.