Fosfomycin susceptibility of carbapenem-resistant Enterobacterales and methicillin-resistant Staphylococcus aureus bloodstream isolates in a tertiary care hospital, South India

Background: The emergence of multidrug-resistant organisms has limited the choice of therapeutic options to treat infections. The lack of development of new antimicrobials paved the way for considering the reassessment of older antibiotics like fosfomycin. In this context, we assessed the in-vitro effect of fosfomycin against carbapenem-resistant Enterobacterales and methicillin-resistant Staphylococcus aureus (MRSA) bloodstream isolates by agar dilution, disk diffusion, and screen agar. Methods: In this study, 141 consecutive blood isolates resistant to carbapenem and 62 MRSA blood culture isolates were collected over a period of 8 months. The methods used were fosfomycin agar dilution (0.25 µg/ml to 512 µg/ml), Kirby-Bauer disk diffusion (150 µg of fosfomycin + 50 µg of glucose-6-phosphate), and fosfomycin screen agar (32 µg/ml, 48 µg/ml, and 64 µg/ml). All three methods were interpreted using the European Committee on Antimicrobial Susceptibility Testing guidelines. The agreement between the new method and the reference method was calculated. Results: Among the tested isolates, 100% of MRSA, followed by Escherichia coli (E. coli) (86.4%), Klebsiella pneumonia (K. pneumonia) (65.2%), and E. cloacae (50%) were susceptible to fosfomycin. The MIC50 and MIC90 of fosfomycin were 0.5 µg/ml and 2 µg/ml for MRSA, 16 µg/ml and 32 µg/ml for K. pneumoniae, 4 µg/ml and 16 µg/ml for E. coli, and 8 µg/ml and 32 µg/ml for E. cloacae, respectively. Conclusion: Fosfomycin demonstrated a good in-vitro effect on most of the carbapenem-resistant Enterobacterales and MRSA isolates tested.