Carbapenemases

Background

• Enzymes that hydrolyze carbapenems
• Includes members of most Ambler classes
  • Class A: KPC
  • Class B: NDM, IMI, VIM
  • Class D: OXA-48

Identification

• Screening is done by identifying isolates with decreased susceptibility to one or more carbapenems
  • Testing "not susceptible" to ertapenem is the most sensitive indicator
  • Typically also resistant to third-generation cephalosporins, as well, with the exception of SME in Serratia marcescens and IMI
• A modified Hodge test can confirm the presence of a carbapenemase, but is no longer routinely recommended
• Modified carbapenem inactivation method (mCIM)
  • Place meropenem disc in a suspension of the test isolate, and incubate at 35ºC for 4 hours
  • Remove disc, squeezing out excess fluid, and place on a lawn of susceptible Escherichia coli
  • Positive for carbapenemase if zone diameter is clear ≤15 mm (or pinpoint colonies up to 18 mm)
  • Negative if zone diameter is clear ≥19 mm
  • Indeterminate if clear diameter 16 to 18 mm, or pinpoint colonies ≥19 mm
• EDTA-modified carbapenem inactivation method (eCIM)
  • Only done if mCIM is positive, to test for metallo-β-lactamases
  • As above for mCIM, but adds EDTA to the tube
  • Positive for metallo-β-lactamase if zone diameter increases ≥5 mm from mCIM
    • Ignore pinpoint colonies within any zones of inhibition in the eCIM

Management

• Ceftazidime-avibactam (not active against metallo-β-lactamases)
• Meropenem-vaborbactam (limited activity against metallo-β-lactamases and oxacillinases)
• Colistin
• Tigecycline