FIG. 1. Double-disk synergy test on Mueller-Hinton agar plates with broad-spectrum cephalosporins and either cloxacillin (upper set of disks)
or cefoxitin (lower set of disks) applied to three AmpC-carrying strains (CAZ, ceftazidime; CTX, cefotaxime; FOX, cefoxitin; CL500, cloxacillin
at 500 g). The distances between the disks were optimized according to the inhibition diameters, as described in the text. Cloxacillin is synergistic
with both ceftazidime and cefotaxime for the three strains. (A) Citrobacter freundii harboring ACC-1 (strain Cf1) for which cefoxitin is synergistic
with cefotaxime and ceftazidime; (B) Citrobacter freundii with chromosomal ampC derepression for which no synergy is observed between cefoxitin
and either cefotaxime or ceftazidime; (C) Klebsiella pneumoniae harboring DHA-1 (36) for which cefoxitin induces resistance to cefotaxime and
ceftazidime.