Susceptibility testing.

A total of 649 unique cephalosporin-resistant E. coli clinical isolates collected at the University of Pittsburgh Medical Center between 2011 and 2015 were subjected to susceptibility testing with fosfomycin using the standard disk diffusion method (15). The fosfomycin disks (containing 200 μg of fosfomycin and 50 μg of G6P) and BBL MH agar II were purchased from BD (Franklin Lakes, NJ). When colonies were observed inside the inhibition zone, the test was repeated for reproducibility, and strains that consistently produced inner colonies in the nonsusceptible range (i.e., ≤15 mm) were included in the further studies. Fosfomycin MICs were determined by the agar dilution method with the addition of 25 μg/ml of G6P (12).