Disk diffusion susceptibility test on Mueller-Hinton agar. Strains of Enterobacter cloacae that have not acquired any resistance determinant are naturally resistant to aminopenicillins (e.g., ampicillin) and first-generation cephalosporins (e.g., cefalexin, cefazolin) due to the production of an inducible chromosomal cephalosporinase (AmpC-type β-lactamases).

The trouble is that E.cloacae is often an ESBL-producing organism resistant to extended-spectrum (third generation) cephalosporins (e.g., ceftazidime, cefotaxime, and ceftriaxone) and monobactams (e.g., aztreonam). See CDC website.