ESBL (extended spectrum β-lactamases) – ESBL-producing organisms are typically resistant to penicillins and cephalosporins. β-lactamase inhibitor combination drugs have variable/unreliable activity. Carbapenems remain susceptible.

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.

Class A cephalosporinase - Class A cephalosporinase-producing organisms are typically resistant to penicillins and cephalosporins but remain susceptible to β-lactamase inhibitor combination drugs (amoxicillin-clavulanate and piperacillin-tazobactam).

CPO - carbapenemase producing organisms are resistant to all penicillins, cephalosporins, carbapenems and β-lactamase inhibitor combination drugs