AmpC - AmpC cephalosporinase-producing organisms are resistant to penicillins, amoxicillin-clavulanate and cephalosporins.

Piperacillin-tazobactam has variable activity depending on the species. Piperacillin-tazobactam is not recommended for Enterobacter spp or Citrobacter freundii complex, or for complicated infections or if patient is immunocompromised. Piperacillin-tazobactam has marginal activity against Hafnia spp, Providencia spp, and Serratia spp (use only if MIC ≤ 8µg/mL). Most Morganella morganii remain susceptible to piperacillin-tazobactam. Most AmpC cephalosporinase-producing organisms remain susceptible to carbapenems.