Ambler Classification of β-Lactamases
The Ambler classification divides β-lactamases into four molecular classes (A, B, C, and D) based on amino acid sequence homology, with classes A, C, and D containing active-site serine residues while class B enzymes are metallo-β-lactamases requiring zinc ions for catalytic activity 1, 2.
Class A β-Lactamases (Serine-Based)
Class A represents the most commonly encountered β-lactamases in clinical practice, including both chromosomal and plasmid-mediated enzymes 3.
Key Enzymes in Class A:
- Extended-spectrum β-lactamases (ESBLs): Hydrolyze cephalosporins and aztreonam but remain susceptible to carbapenems 4
- KPC carbapenemases: The most prevalent carbapenemase globally, accounting for 47.4% of meropenem-resistant Enterobacterales 5, 4
- SME-1, NMC-A, IMI-1: Rare carbapenem-hydrolyzing enzymes 6
Clinical Characteristics:
- Inhibited by clavulanic acid, sulbactam, and tazobactam 3
- Critical amino acid positions for inhibitor resistance include Met69, Ser130, Arg244, Arg275, and Asn276 3
- For KPC-producing organisms, use ceftazidime/avibactam or meropenem/vaborbactam as first-line therapy 5, 4
Class B β-Lactamases (Metallo-β-Lactamases/MBLs)
Class B enzymes are particularly concerning because they hydrolyze all β-lactams except monobactams and are not inhibited by classic β-lactamase inhibitors 5.
Major MBL Families:
- IMP variants (IMP-1 through IMP-9): First reported in Japan, now detected in Europe, with 85-99% amino acid identity 6
- VIM variants (VIM-1 through VIM-3): First described in Europe (Italy, France, Greece), now in Korea, sharing 30% amino acid identity with IMP series 6
- NDM enzymes: Represent 20.6% of carbapenem-resistant isolates 5, 4
Clinical Characteristics:
- Require one or two zinc ions for catalytic activity 1
- Resistant to clavulanic acid 6
- Retain susceptibility to aztreonam because MBLs cannot hydrolyze monobactams 5
- For MBL-producing organisms, use ceftazidime/avibactam PLUS aztreonam or cefiderocol 4
Class C β-Lactamases (AmpC Enzymes)
Class C enzymes confer resistance to oxiimino-β-lactams but critically remain susceptible to carbapenems 5, 7.
Two Major Types:
- Chromosomal AmpC (cAmpC): Found in Enterobacter, Citrobacter, and Serratia species with inducible expression 5, 7
- Plasmid-mediated AmpC (pAmpC): Occurs in E. coli, Klebsiella, and Proteus species 7
Critical Clinical Pitfall:
- Never use third-generation cephalosporins for serious infections caused by Enterobacter, Citrobacter, or Serratia, even when susceptibility testing shows sensitivity 7
- Strains initially appear susceptible but develop resistance during therapy through mutation, causing clinical failure 7
Class D β-Lactamases (OXA Enzymes)
Class D includes oxacillinases, with OXA-48-like enzymes accounting for approximately 19% of carbapenemases in Enterobacterales 5, 4.
Clinical Characteristics:
- OXA-23 through OXA-27 variants have been reported 6
- OXA-48 producers typically show susceptibility to ceftazidime/avibactam, similar to KPC producers 5
- Genes are often integron- and plasmid-located, facilitating horizontal transfer 6
Practical Clinical Algorithm for Treatment Selection
Step 1: Identify Resistance Pattern
- Carbapenem-resistant + ceftazidime/avibactam susceptible → Likely Class A (KPC) or Class D (OXA-48) 5
- Carbapenem-resistant + aztreonam susceptible → Likely Class B (MBL) 5
- Cephalosporin-resistant + carbapenem susceptible → Likely Class C (AmpC) 7
Step 2: Select Targeted Therapy
- KPC (Class A): Ceftazidime/avibactam or meropenem/vaborbactam 5, 4
- MBL (Class B): Ceftazidime/avibactam PLUS aztreonam or cefiderocol 4
- AmpC (Class C): Carbapenems remain effective 7
- OXA-48 (Class D): Ceftazidime/avibactam 5
Historical Context and Evolution
The Ambler classification originated when amino acid sequences began to be reported for β-lactamases, initially dividing enzymes into those with active-site serine residues versus metallo-enzymes 2. This molecular classification system has proven more durable than earlier functional naming schemes (penicillinase, cephalosporinase) as enzyme diversity expanded 2. The system now encompasses almost 2,800 unique proteins, with nomenclature currently under the purview of NCBI 1.