IMI Carbapenemases: Classification, Catalytic Biology, Naming, and Significance in Enterobacter

Welcome. This module turns from general carbapenem resistance to a family especially relevant to Enterobacter: IMI carbapenemases. IMI enzymes are less frequently discussed than KPC, NDM, or OXA-48-like carbapenemases, but they matter precisely because their phenotype can be subtle, their history predates clinical imipenem use, and some variants occupy mobile genomic contexts that connect environmental and clinical populations.

By the end of this lesson, you should be able to state exactly what an IMI carbapenemase is; distinguish the enzyme from its gene; place it in Ambler class A; explain its serine-based catalytic cycle; and recognize why an IMI-positive Enterobacter isolate cannot be interpreted from a carbapenem result alone.

---

IMI: a precise vocabulary

A carbapenemase is a $\beta$-lactamase capable of hydrolysing one or more carbapenem antibiotics. Hydrolysis opens the antibiotic’s $\beta$-lactam ring, preventing it from efficiently binding its penicillin-binding-protein targets and thereby protecting cell-wall synthesis.

IMI stands for imipenemase. IMI enzymes belong to the IMI/NMC-A family:

• IMI denotes the imipenemase family and its numbered variants, such as IMI-1, IMI-2, and IMI-3.
• NMC-A means non-metallo-carbapenemase A. It is a closely related, but distinct, enzyme within the same family grouping.
• IMI-1 and NMC-A are very closely related proteins, sharing about $97\%$ amino-acid identity. They should not, however, be treated as identical names for the same enzyme.

The genetic and protein nomenclature must also remain separate:

Term	Meaning	Example of appropriate wording
$bl{a}_{\mathrm{IMI}-2}$	The DNA gene encoding IMI-2	“The isolate carries $bl{a}_{\mathrm{IMI}-2}$.”
IMI-2	The encoded $\beta$-lactamase protein	“IMI-2 hydrolyses carbapenems.”
IMI-like	A sequence related to the IMI family, possibly without a fully assigned allele	“An IMI-like gene was detected.”
IMI-producing isolate	An isolate in which an IMI enzyme is inferred or demonstrated to be expressed	Best used cautiously when genomic, phenotypic, or biochemical evidence supports expression

In genomic reporting, “IMI-positive” is often used after PCR or sequence detection. A more rigorous interpretation is to state what is directly known: whether the gene sequence is present, whether its allele is resolved, whether the surrounding regulatory region is intact, and whether the resistance phenotype fits.

Ambler class is a molecular classification

IMI carbapenemases are Ambler class A serine $\beta$-lactamases. The Ambler system classifies $\beta$-lactamases primarily by amino-acid sequence and catalytic architecture:

• Classes A, C, and D are serine $\beta$-lactamases: a serine residue in the active site forms a temporary covalent bond with the antibiotic.
• Class B enzymes are metallo-$\beta$-lactamases: they use metal ions, usually zinc, to activate water for hydrolysis and do not form the same covalent acyl-enzyme intermediate.

Thus, IMI is fundamentally different from an NDM or VIM enzyme, even though all can be called carbapenemases. IMI is also traditionally placed in functional group 2f, a category for class A enzymes with carbapenem-hydrolysing activity and variable inhibition by older class A inhibitors.

---

How an IMI enzyme destroys a carbapenem

A carbapenem normally kills susceptible bacteria by acting as a structural mimic of the peptide substrate handled by penicillin-binding proteins. When a carbapenem acylates a penicillin-binding protein, the target enzyme is trapped and peptidoglycan cross-linking fails.

IMI intercepts the antibiotic first. Like other class A carbapenemases, it uses a catalytic serine at Ambler position 70, conventionally written Ser70. The reaction has two essential phases: acylation and deacylation.

1. Acylation: the enzyme captures the antibiotic

The $\beta$-lactam substrate enters the active site and is positioned so that the hydroxyl group of Ser70 can attack its carbonyl carbon. Lys73 helps organize the proton transfers that make this nucleophilic attack possible. The $\beta$-lactam ring opens, and the antibiotic becomes covalently attached to Ser70 as an acyl-enzyme intermediate.

At this stage, the antibiotic is no longer an intact, target-active carbapenem. But for resistance to be effective, the enzyme must do more than bind and open the drug once: it must be regenerated to attack additional molecules.

2. Deacylation: the enzyme is regenerated

A water molecule enters the active site. Glu166 is central to activating this water so it can attack the acyl-enzyme bond. Hydrolysis releases an inactive, ring-opened antibiotic product and restores the free enzyme.

The central biological distinction is therefore not merely “can Ser70 bind a carbapenem?” Many serine enzymes can form an acyl complex with a carbapenem. A successful carbapenemase must also efficiently deacylate that complex. IMI-family enzymes have an active-site architecture that permits this complete catalytic cycle.

For enzyme kinetics, three quantities are useful:

• $k_{\mathrm{cat}}$: the turnover number, describing how rapidly an enzyme processes substrate under saturating conditions.
• $K_m$: a composite kinetic parameter related to substrate handling under the assay conditions.
• $k_{\mathrm{cat}}/K_m$: catalytic efficiency, which combines productive substrate interaction and turnover.

These values describe purified-enzyme behaviour, not the full susceptibility phenotype of an isolate. In a living Enterobacter cell, expression level, outer-membrane permeability, other $\beta$-lactamases, inoculum, and the testing method also influence the measured MIC.

---

The characteristic IMI phenotype: informative, but not diagnostic by itself

IMI enzymes have a distinctive tendency, not an invariant antibiogram. IMI-1 and related enzymes commonly hydrolyse:

• penicillins;
• early-generation cephalosporins;
• aztreonam;
• carbapenems, particularly imipenem.

In contrast, IMI-1 generally has weak or undetectable hydrolysis of some expanded-spectrum cephalosporins, especially ceftazidime. Biochemical studies show markedly greater turnover of imipenem than meropenem for IMI-1. This can produce an unusual pattern in which an isolate has reduced susceptibility or resistance to imipenem while appearing less strikingly resistant to certain later-generation cephalosporins.

That pattern is a clue, not confirmation. It is unsafe to reason as follows: “ceftazidime is susceptible, therefore no carbapenemase is present,” or “a carbapenem MIC is low, therefore the IMI gene is irrelevant.”

Three reasons explain why phenotype alone can mislead:

1. Expression is variable. IMI-family loci often include an upstream LysR-family transcriptional regulator, commonly termed ImiR. In reported IMI/NMC-A contexts, expression can be inducible by $\beta$-lactam exposure rather than constitutively high.

2. The cell envelope modifies exposure. A carbapenemase only acts on drug molecules that reach the periplasm. Altered porins, efflux, and the basal permeability of the host can amplify or mask the gene’s apparent effect.

3. The rest of the resistome matters. Enterobacter can possess inducible chromosomal AmpC, acquired ESBLs, aminoglycoside-resistance genes, quinolone-resistance determinants, and other resistance mechanisms. Early descriptions of IMI/NMC-A producers sometimes found susceptibility to non-$\beta$-lactam drugs, but that is not a defining feature of IMI itself and must not be assumed for contemporary isolates.

IMI is a class A enzyme, and IMI-1 is inhibited biochemically by clavulanate and tazobactam. However, biochemical inhibition is not equivalent to a treatment recommendation. In an Enterobacter isolate, concurrent AmpC expression, enzyme abundance, inoculum effects, and the pharmacology of the antibiotic–inhibitor combination all matter. We will later build a systematic framework for interpreting these combinations.

---

Why IMI matters in Enterobacter

IMI is often called a “minor” carbapenemase because it is reported far less often than KPC, NDM, VIM, IMP, or OXA-48-like enzymes. “Minor” describes recognized epidemiological frequency, not clinical importance. An IMI-producing Enterobacter from blood, urine, respiratory samples, or a high-risk hospital ward can still complicate therapy, surveillance, and outbreak interpretation.

There are four reasons it deserves special attention in Enterobacter research.

It is historically important

The original IMI-1-associated E. cloacae isolate was collected in the United States in 1984, before imipenem entered routine clinical use there in 1985. The landmark description of the enzyme was published later. The distinction between isolation date and publication date matters when reconstructing history.

The key implication is that IMI-family carbapenem-hydrolysing capacity did not simply arise as a new mutation in response to therapeutic imipenem. It likely reflects a pre-existing environmental evolutionary reservoir that was subsequently selected, detected, and sometimes mobilized in the antibiotic era.

It is not an intrinsic property of the whole species

An Enterobacter isolate should not be assumed to produce IMI merely because it belongs to the Enterobacter cloacae complex. IMI genes occur in a subset of isolates and can reside on mobile or potentially mobile genomic structures.

This is also why historical labels such as “E. cloacae” deserve scrutiny. Some earlier reports predate routine whole-genome species assignment, so the reported species may represent E. cloacae sensu stricto or another member of the complex. For a modern epidemiological claim, species assignment should be checked independently from the carbapenemase call.

Its genomic location shapes its epidemiological meaning

A useful first approximation is:

Gene context	Common reported association	Epidemiological interpretation
Chromosomal $bl{a}_{\mathrm{IMI}-1}$-like locus	Enterobacter isolates, often associated with EcloIMEX-like integrative elements	Can persist within a lineage and may not spread as readily as a conjugative plasmid
Plasmid-borne $bl{a}_{\mathrm{IMI}-2}$ and several later variants	Environmental and clinical Enterobacter, with occasional occurrence beyond the genus	Creates a plausible route for horizontal transfer across strains and species
IS-flanked locus or composite transposon	Some plasmid-associated IMI regions	Suggests a mobility mechanism, but gene flanking alone does not prove active transfer

Chromosomal IMI-1-like genes have been associated with EcloIMEX elements, meaning Xer-dependent integrative mobile elements. Reported EcloIMEX structures insert at a chromosomal $dif$ site, between the setB and yeiP genes. This is not the same as a freely conjugative plasmid, but it is also not simply an immutable housekeeping-gene location.

By contrast, IMI-2 was first identified in E. asburiae from United States river samples on a self-transferable plasmid. This environmental observation is significant for hospital ecology: aquatic environments can contain carbapenemase-associated genetic contexts. It does not establish that a particular hospital sink or shower is the source of a clinical isolate. That claim requires dense sampling, genomic comparison, and a transmission model.

Reading a gene neighbourhood cautiously

The comparative plasmid map illustrates two important observations. First, the regulatory arrangement is often conserved: $imiR$ lies upstream of $bl{a}_{\mathrm{IMI}}$. Second, the gene neighbourhood can differ among plasmids, with insertion sequences or their remnants appearing near the locus.

The appropriate inference is not “an insertion sequence is present, so the gene definitely moved yesterday.” Instead, use a graded interpretation:

• A conserved $imiR$–$bl{a}_{\mathrm{IMI}}$ arrangement supports a recognizable IMI regulatory module.
• Flanking insertion sequences can provide a plausible mechanism for past movement or rearrangement.
• A gene embedded in a demonstrably conjugative plasmid provides stronger evidence for intercellular transfer potential.
• Demonstrating actual movement requires experimental transfer data or carefully resolved comparative genomic evidence.

For environmental Enterobacter work, this distinction prevents a frequent overstatement: a gene may be present in a drain isolate, a plasmid may be predicted, and insertion sequences may be visible, yet the direction, timing, and route of transmission remain unknown.

---

A practical interpretation framework

When you encounter a putative IMI-producing Enterobacter, keep four questions separate:

1. What organism is this?
   Establish genus and, where possible, genome-resolved species within the E. cloacae complex.

2. What exact resistance determinant is present?
   Report the allele, such as $bl{a}_{\mathrm{IMI}-1}$ or $bl{a}_{\mathrm{IMI}-2}$, rather than only “carbapenemase positive.”

3. What is its genetic context?
   Ask whether the locus appears chromosomal or plasmid-associated, whether $imiR$ is present, and whether mobile elements flank the region.

4. What does the isolate actually do phenotypically?
   Interpret carbapenem MICs with the complete $\beta$-lactam profile and with possible contributions from AmpC, ESBLs, and permeability changes.

This framework is especially useful for isolates from sinks, shower drains, or wastewater. A single environmental IMI-positive isolate is biologically interesting. It becomes epidemiologically meaningful only when integrated with species assignment, allele/context resolution, temporal and spatial sampling, and comparison with patient or other environmental isolates.

---

Key takeaways

IMI carbapenemases are Ambler class A serine carbapenemases in the IMI/NMC-A family, historically linked most closely to Enterobacter. Their catalytic Ser70 forms an acyl-enzyme intermediate with a carbapenem, while Glu166 helps activate water to complete deacylation and regenerate the enzyme.

IMI is a protein name, whereas $bl{a}_{\mathrm{IMI}}$ denotes its gene. IMI-family enzymes commonly show strong activity against imipenem and variable activity across other $\beta$-lactams, so an apparently non-dramatic antibiogram does not exclude their presence. Their epidemiological significance depends heavily on genomic context: IMI-1-like loci are often chromosomal and EcloIMEX-associated, whereas IMI-2 and some later variants have been found on plasmids with greater potential for horizontal dissemination.

Next, we will trace IMI-1, NMC-A, and later IMI variants through their discovery history, organisms, locations, sources, and genomic contexts.