AmpC
One-Sentence Definition
AmpC β-lactamases are class C enzymes (chromosomal or plasmid-mediated) that hydrolyze many penicillins and cephalosporins — including cephamycins — and are poorly inhibited by clavulanate, causing distinctive resistance patterns in Enterobacterales and other Gram-negatives.
Simple Explanation
Another cephalosporin-cutting enzyme, but unlike typical ESBLs it shrugs off clavulanate and can “wake up” during therapy in organisms that keep the gene on their chromosome.
Detailed Scientific Explanation
| Form | Classic hosts | Clinical trap |
|---|---|---|
| Inducible chromosomal AmpC | Enterobacter cloacae complex, Klebsiella aerogenes, Citrobacter freundii, Serratia, Morganella (“ESCPM/MYSPACE” teaching sets) | May look susceptible to 3rd-gen cephalosporins in vitro then fail on therapy after derepression |
| Stable derepressed mutants | Same | High-level constitutive AmpC |
| Plasmid AmpC (pAmpC) | E. coli, K. pneumoniae (CMY, DHA, FOX…) | Spreads like ESBL; clavulanate synergy absent |
Lab clues vs ESBL:
- Cefoxitin resistance common with AmpC
- Clavulanate does not restore cephalosporin activity (contrast ESBL)
- Molecular: blaCMY, blaDHA, chromosomal ampC variants; Acinetobacter baumannii ADC is related cephalosporinase logic
Mechanism
Serine class C β-lactamase hydrolyzes β-lactams; induction via cell-wall fragment / AmpD–AmpR regulatory circuit (Bacterial Operons and Sigma Factors, Two-Component Regulatory Systems-adjacent envelope stress). Derepression → high enzyme → cephalosporin failure.
Clinical Importance
- Avoid relying on 3rd-generation cephalosporins for serious infections by inducible AmpC producers even if “S” on report — prefer cefepime (often) or carbapenems per syndrome/guidelines.
- Plasmid AmpC in E. coli/Klebsiella complicates UTI/bacteremia therapy like ESBL but inhibitor profile differs.
- Stewardship: don’t treat colonization; know organism identity (MALDI-TOF MS).
Research Importance
- ampC regulation genetics; plasmid CMY-2 epidemiology in One Health
- Feature in AMR Gene Databases and Machine Learning for AMR Prediction
Diagnostic Relevance
- Phenotypic algorithms + cefoxitin screen; confirm with molecular when available
- WGS annotates pAmpC vs chromosomal context (Plasmid and Mobile Element Analysis)
AMR Relevance
Major interpretive AMR — mechanism where genotype/organism ID changes drug choice beyond a simple “cephalosporin R” flag. Distinct from ESBL and Carbapenemases.
Related Organisms
- Escherichia coli · Klebsiella pneumoniae · Enterobacter spp. · Pseudomonas aeruginosa (chromosomal AmpC-like)
Related Methods
Related MOCs
Learning Aids
Clinical Example
Example
Case: Enterobacter cloacae bacteremia; day-1 ceftriaxone MIC susceptible; day-5 clinical failure; repeat isolate ceftriaxone R, cefoxitin R, clavulanate no synergy.
Question: ESBL or AmpC derepression?
Answer: Classic inducible AmpC selection — switch therapy per guidelines (often cefepime or carbapenem).
Active Recall Questions
- How does AmpC differ from ESBL regarding clavulanate?
- Why is cefoxitin a useful clue?
- Name a risk of treating Enterobacter bacteremia with ceftriaxone based on initial “S”.