Beta-Lactamase Classification: Ambler vs. Bush-Jacoby, and Why an Enzyme Has Both

Beta-lactamases are the single most important cause of beta-lactam resistance in Gram-negative bacteria. They are enzymes that hydrolyze the beta-lactam ring, destroying the antibiotic before it can reach its target. Hundreds of these enzymes exist, and making sense of them requires a classification system. Two systems are in universal use, and the source of most student confusion is that they do not compete. They describe the same enzymes from two different angles, so every beta-lactamase carries a label in each system at once.

This article explains both systems, the Ambler molecular classification and the Bush-Jacoby-Medeiros functional classification, and how they map onto each other. The three enzyme groups that matter most at the bedside, ESBLs, AmpC enzymes, and carbapenemases, each have a dedicated detection article; this page is the framework that tells you what they are.

Classification of Beta lactamases

The Ambler molecular classification and the Bush-Jacoby-Medeiros functional classification are the two most commonly useful classification systems for β-lactamases.

Two Systems, the Same Enzymes

These aren't competing classifications, they're two different lenses on the same enzymes. Ambler groups beta-lactamases by molecular structure, what the enzyme's amino acid sequence looks like. Bush-Jacoby-Medeiros groups them by function, what substrate they hydrolyze and which inhibitors block them. A single enzyme carries a label in both systems at once.

Quick hook: A, C, and D all use serine for hydrolysis, think of them as the "standard" mechanism. B is the outlier. It needs a zinc ion instead, which is exactly why metal chelators like EDTA inhibit it while clavulanic acid, the inhibitor that works on the serine classes, does nothing against it.

Ambler classification

The most widely used classification of β-lactamases is the Ambler classification which divides β-lactamases into four classes (A, C, D, and B) based on their amino acid sequences. Ambler originally specified two classes (A and B) but later added C and D as new β-lactamases bore no resemblance to the existing class.

Class A, C, and D enzymes utilize serine for β-lactam hydrolysis, and class B metalloenzymes require divalent zinc ions for substrate hydrolysis.

Class A

contained the active-site serine β-lactamases

Class C

a member of serine β-lactamases, known as the ‘AmpC’ β-lactamases.

Class D

Another class of serine β-lactamases, commonly known as the OXA β-lactamases that had no resemblance to either class A or class C.

Class B

The metallo-β-lactamases that require a bivalent metal ion, usually zinc for activity.

Bush-Jacoby-Medeiros classification scheme

The Bush-Jacoby-Medeiros classification scheme groups β-lactamases according to functional similarities (substrate and inhibitor profile). There were four groups described by Jacoby and Bush, but the fourth group has now been omitted as the enzyme properties were similar to that of the previous 3 groups, so currently, there are three main groups and multiple subgroups in this system.

Group I cephalosporinases

Ambler Class C beta-lactamases (also known as AmpC enzymes) fall in this group. Cephalosporin is the substrate. Distinctive characters are

• Beta-lactamase inhibitors like clavulanic acid or EDTA does not inhibit this group.
• Greater hydrolysis of cephalosporin than penicillinase hydrolyzes cephamycins
• Often chromosomal enzymes in gram-negatives, but some are plasmid-coded
• In this class, the enzyme is inducible. Thus any exposure of bacteria to beta-lactam antibiotics leads to an increase in enzyme production.
• The Group I producer beta-lactamases are resistant to beta-lactam/beta-lactamase inhibitor combinations, penicillins, cephamycins, and 1st, 2nd, and 3rd generation cephalosporins but sensitive to cefepime and carbapenems.
• The enzymes in group I are present in the Enterobacteriaceae family as well as Pseudomonas aeruginosa. Examples include E. coli AmpC, P99, ACT-1, CMY-2, FOX-1, MIR-1 enzymes.

Group I contains a subgroup 1e that can hydrolyze ceftazidime and often other oxyimino-β-Lactams. Examples include GC1, CMY-37.

Group II serine β-lactamases

Functional group 2 β-lactamases, including molecular classes A and D (ambler classification), represent the largest group of β-lactamases. There are various subgroups, each with a different property.

Group III MBLs

Group III β lactamases include Metallo-β -lactamases (MBLs) in class B of Amblers classification. They differ structurally from the other β-lactamases by their requirement for a zinc ion at the active site.

Distinguishing characters include

• Ability to hydrolyze carbapenems, but not monobactams.
• Clavulanic acid or tazobactam does not inhibit it. However, metal ion chelators such as EDTA, dipicolinic acid, etc inhibit it.
• Originally were identified as chromosomal enzymes in Gram-positive or occasional Gram-negative bacilli, such as Bacteroides fragilis or Stenotrophomonas maltophilia but now are plasmid-mediated hence can be detected on a wide variety of bacteria.

Only two functional subgroups are described.

1. Subgroup 3a includes the major plasmid-encoded MBL families, such as the IMP and VIM enzymes that have appeared globally, most frequently in non-fermentative bacteria but also in Enterobacteriaceae.
2. Subgroup 3b contains a smaller group of MBLs that preferentially hydrolyze carbapenems in contrast to penicillins and cephalosporins. Examples include L1, CAU-1, GOB-1, FEZ-1 enzymes.

Knowing an isolate produces an MBL or another carbapenemase only matters if the lab can detect it. Two phenotypic methods are commonly used: the older, still widely used Modified Hodge Test, and the faster, more specific Carba NP test.

How to Remember

The one distinction that unlocks the whole table: Ambler asks what does the enzyme look like (structure, its amino acid sequence). Bush-Jacoby asks what does the enzyme do (which drugs it destroys, which inhibitors stop it). Structure versus function. Once you hold those two questions apart, the fact that AmpC is "Class C and Group 1 at the same time" stops being a contradiction and becomes obvious: it is one enzyme answered by two different questions.

Serine vs. zinc, the split that predicts the inhibitor: Classes A, C, and D all cut with an active-site serine. Class B cuts with a zinc ion. That single structural fact tells you the treatment-relevant behavior: clavulanic acid (a serine-trap) blocks A, C, and D but does nothing to B, which needs a metal chelator like EDTA instead. If you remember only one thing from Ambler, remember that B is the metallo-outlier.

Key exam facts

Where Students Get Confused

"Ambler and Bush-Jacoby are rival systems and I have to pick one." No. They classify the same enzymes by different criteria. Every enzyme has both a class and a group simultaneously. AmpC is Class C and Group 1.

"All OXA enzymes are the same thing." No. OXA is Ambler Class D, but the group spans ordinary penicillinases (2d), ESBLs (2de), and carbapenemases (2df, such as OXA-48). The Class D label alone does not tell you what an OXA enzyme hydrolyzes.

"Clavulanic acid inhibits all beta-lactamases." No. It inhibits the serine classes (A, C, D) to varying degrees but not Class B metallo-enzymes, and some serine enzymes (the 2br "inhibitor-resistant" TEMs) have specifically evolved to resist it.

"Class B is rare so it doesn't matter." Clinically it matters most. Class B metallo-enzymes (IMP, VIM, NDM) hydrolyze carbapenems and are not stopped by the usual inhibitors, which is exactly why they are the hardest to treat.

References

• Ambler RP. The structure of beta-lactamases. Philos Trans R Soc Lond B Biol Sci. 1980;289(1036):321-331.
• Bush K, Jacoby GA. Updated functional classification of beta-lactamases. Antimicrob Agents Chemother. 2010;54(3):969-976. doi:10.1128/AAC.01009-09
• Hall BG, Barlow M. Revised Ambler classification of beta-lactamases. J Antimicrob Chemother. 2005;55(6):1050-1051. doi:10.1093/jac/dki130
• Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.