In Vitro Evaluation of Fosfomycin Combinations Against Metallo-β-Lactamase-Producing Klebsiella pneumoniae and Pseudomonas aeruginosa Clinical Isolates

Background/Objectives: Metallo-β-lactamase (MBL)-producing Gram-negative bacteria represent a growing global health threat due to their broad resistance to β-lactam antibiotics, including carbapenems, which severely limits treatment options. This study aimed to evaluate the in vitro synergistic activity of fosfomycin (FOS) in combination with selected older and newer antimicrobials against MBL-producing Klebsiella pneumoniae and Pseudomonas aeruginosa. Methods: Synergistic interactions were assessed using agar dilution checkerboard on 42 MBL-producing clinical isolates (22 K. pneumoniae, 20 P. aeruginosa) and confirmed using time-kill assays with selected isolates. FOS was tested in combination with colistin (COL), ceftazidime–avibactam (CAZ-AVI), meropenem (MER), amikacin (AMI), aztreonam (AZT), aztreonam–avibactam (AZT-AVI), or cefiderocol (FDC). Results: Most FOS combinations exhibited additive or synergistic effects against clinical isolates. Synergy rates reached 72.7% for the FOS+CAZ-AVI combination (K. pneumoniae) and 65.0% for the FOS+COL combination (P. aeruginosa). An asymmetric synergistic interaction was identified for FOS+CAZ-AVI, with FOS enhancing the activity of CAZ-AVI more markedly than vice versa, especially in K. pneumoniae. Time-kill assays on selected isolates confirmed synergistic and bactericidal activity of FOS+CAZ-AVI and FOS+COL, and showed that bacterial regrowth observed with FOS, CAZ-AVI, and COL alone was suppressed in combination therapy. Conclusions: FOS-based combinations, particularly with CAZ-AVI and COL, demonstrated potent synergistic activity against MBL-producing K. pneumoniae and P. aeruginosa, supporting their potential utility in rational combination therapies for infections due to MBL-producing bacteria.