Antimicrobial susceptibility of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Enterobacteriaceae isolates to fosfomycin

The advancing antimicrobial drug resistance among Enterobacteriaceae renders the evaluation of potential novel therapeutic options necessary. We sought to evaluate the in vitro antimicrobial activity of fosfomycin against multidrug-resistant (MDR) Enterobacteriaceae isolates. Antimicrobial susceptibility to fosfomycin and 12 additional antibiotics of MDR Enterobacteriaceae isolates collected between November 2007 and April 2009 at the University Hospital of Heraklion, Crete, Greece, was examined using the Etest method. A total of 152 MDR Enterobacteriaceae isolates were studied, including Klebsiella pneumoniae (76.3%), Escherichia coli (17.1%), Proteus mirabilis (4.6%) and other species (2.0%). Antimicrobial susceptibility rates were highest for fosfomycin (92.8%), tigecycline (92.1%) and colistin (73.0%) followed by imipenem (35.5%), tetracycline (20.4%), gentamicin (19.7%), trimethoprim/sulfamethoxazole (12.5%) and ciprofloxacin (10.5%). Of the 152 isolates, 85 (55.9%) were extensively drug-resistant (XDR), of which 78 (91.8%) remained susceptible to fosfomycin. Susceptibility to fosfomycin of the 79 carbapenemase-producing, 34 extended-spectrum beta-lactamase-producing and 24 metallo-beta-lactamase-producing isolates was 94.9%, 94.1% and 83.3%, respectively. In conclusion, in this study fosfomycin exhibited good in vitro antimicrobial activity against MDR and XDR Enterobacteriaceae. We suggest further evaluation of the potential clinical utility of fosfomycin against infections caused by these pathogens.