First detection of the Ambler class C 1 AmpC beta-lactamase in Citrobacter freundii by a new, simple double-disk synergy test.

We report on the first detection of an AmpC-type Ambler class C 1 (ACC-1) beta-lactamase in Citrobacter freundi isolated from a patient also harboring ACC-1-producing Escherichia coli and Klebsiella pneumoniae. We propose a simple cefoxitin-based double-disk synergy test (DDST) for the specific detection of ACC-1 in members of the family Enterobacteriaceae, including natural AmpC producers, in association with a cloxacillin-based DDST as a first-line AmpC-type beta-lactamase screening test.