Discovery and Genomic Context of IMI Variants and Their Relationship to NMC-A

Welcome back. In the previous lesson, you established that IMI enzymes are Ambler class A serine carbapenemases in the IMI/NMC-A family, and that their epidemiological meaning depends strongly on whether $bl{a}_{\mathrm{IMI}}$ lies in a chromosomal integrative element or a plasmid.

This lesson puts that framework into historical order. We will trace what was found first, distinguish the date of isolation from the date an enzyme was characterized or sequenced, and treat each report as a provenance record: organism, place, time, source, allele, and genomic context. This is essential preparation for interpreting claims about the geographical and ecological distribution of IMI-producing Enterobacter.

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A history is not just a list of enzyme names

A concise statement such as “IMI was first found in Enterobacter cloacae” hides several questions:

1. What did “E. cloacae” mean at the time?
   Many early reports predate genome-based resolution of the E. cloacae complex. The historical species label should be retained when describing the original report, but it should not automatically be treated as a modern species assignment.

2. What is the relevant date?
   An isolate can be collected in one year, characterized years later, and have its genome context resolved decades later. These are distinct events.

3. Where did the isolate come from?
   “Clinical,” “river water,” “urine,” and “drain” are not interchangeable source categories. A carbapenemase gene from an aquatic isolate raises a different ecological question from one recovered during bloodstream infection.

4. Was the gene chromosomal, plasmid-borne, or incompletely resolved?
   The same allele has different epidemiological implications in a stable chromosome-associated element than in a self-transmissible plasmid.

For IMI/NMC-A, the central historical lesson is that two related evolutionary stories coexist:

• a chromosomal, Enterobacter-associated story, often involving Xer-dependent integrative elements such as EcloIMEX; and
• a plasmid-associated story, exemplified by IMI-2 and later variants, with clearer potential for transfer between strains and even genera.

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The founding pair: IMI-1 and NMC-A

IMI-1: a carbapenemase detected before clinical imipenem use

IMI-1 was initially reported from an isolate historically identified as Enterobacter cloacae in the United States, collected in 1984. This is striking because imipenem was approved for use in the United States in 1985.

That timing rules out a simplistic explanation in which IMI-1 must have evolved only after widespread therapeutic imipenem exposure. It does not prove that there was no environmental selection pressure: bacteria and their resistance genes encounter many naturally occurring and human-produced $\beta$-lactam-like selective pressures. But it does show that carbapenem-hydrolysing potential in this family was already present before imipenem became a routine clinical selective agent.

For historical precision, report this finding in a form such as:

IMI-1 was initially described from a historically designated E. cloacae isolate collected in the United States in 1984; the original report predates routine genome-based species assignment within the E. cloacae complex.

The curated review identifies the country, organism label, and isolation year, but does not give the original clinical specimen type in its summary. That absence matters. Do not turn “an E. cloacae isolate” into “a urinary isolate,” “a bloodstream isolate,” or “a hospital-drain isolate” without checking the primary report.

NMC-A: related, but not a synonym for IMI-1

NMC-A, short for non-metallo-carbapenemase A, was identified in the historically designated E. cloacae isolate NOR-1, isolated in 1990. It is one of the earliest recognized carbapenemases in Enterobacterales.

NMC-A and IMI-1 are closely related class A carbapenemases, but they are separately named enzymes. Their relationship is best understood as a family relationship:

Feature	IMI-1	NMC-A
Enzyme group	IMI/NMC-A family	IMI/NMC-A family
Ambler class	A	A
Catalytic type	Serine carbapenemase	Serine carbapenemase
Historical host label	E. cloacae	E. cloacae NOR-1
Isolation year in the historical record	1984	1990
Typical genomic association in later studies	EcloIMEX-like chromosomal elements	EcloIMEX-like chromosomal elements

The important correction is this: NMC-A is not “IMI-1 under an old name.” It is a distinct, highly related member of the same enzyme family. Therefore, a report of $bl{a}_{\mathrm{NMC}-\mathrm{A}}$ should not be relabelled $bl{a}_{\mathrm{IMI}-1}$, and vice versa.

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Chromosomal does not mean biologically immobile

Early descriptions encouraged the view that IMI/NMC-A enzymes were mainly chromosomal and restricted to Enterobacter. That generalization contained some truth, but modern sequencing has made it more precise.

Many $bl{a}_{\mathrm{IMI}-1}$-like and $bl{a}_{\mathrm{NMC}-\mathrm{A}}$ loci occur in EcloIMEX-like elements. These are integrative mobile elements associated with the chromosome, commonly inserted at a $dif$-like site between $setB$ and $yeiP$. Their integration is linked to the host XerC/XerD recombination system.

This creates an epidemiological middle ground:

• The locus is not on an obviously free conjugative plasmid.
• Yet it is also not equivalent to an ordinary vertical, housekeeping chromosomal gene.
• The genomic element itself has features consistent with past mobility and recombination.

This distinction is particularly useful for interpreting IMI-positive sink or shower isolates. If multiple isolates have the same carbapenemase allele in the same chromosomal insertion site, that supports a shared type of genomic context. It does not, on its own, establish recent transmission between those isolates or identify the direction of movement.

Taxonomy changes the historical interpretation

This sequencing work also illustrates why historical E. cloacae labels require care. Among Canadian isolates originally received as E. cloacae, E. cloacae complex, or Enterobacter species, genome-informed phylogenetic analysis placed many $bl{a}_{\mathrm{NMC}-\mathrm{A}}$-carrying isolates in an E. ludwigii branch. Other isolates carrying $bl{a}_{\mathrm{IMI}-1}$, $bl{a}_{\mathrm{IMI}-9}$, or newer alleles occurred across several branches and sequence types.

That result does not invalidate the older literature. Instead, it changes the way we formulate its conclusions:

• Historical conclusion: IMI/NMC-A carbapenemases were reported in “E. cloacae.”
• Modern interpretation: IMI/NMC-A genes occur across genetically diverse members of the E. cloacae complex, including lineages now recognized as distinct species.

This is why a careful contemporary report should present both the original designation and the updated genomic assignment where possible.

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IMI-2 changes the ecological story

IMI-2 was first identified in Enterobacter asburiae from United States river samples collected from 1999 to 2001. This report is foundational because it connected an IMI-family carbapenemase to an aquatic environmental reservoir and to a self-transferable plasmid of approximately 66 kb.

The finding has two important implications.

First, IMI-family genes are not solely a clinical phenomenon. River environments can contain bacteria carrying carbapenemase-associated mobile genetic structures. The river finding does not mean that every IMI-positive clinical strain came from a river, nor that rivers are necessarily the immediate source of hospital plumbing isolates. It establishes environmental occurrence, not a direct transmission chain.

Second, a self-transferable plasmid changes the scale at which one should think. A chromosomal EcloIMEX-associated $bl{a}_{\mathrm{IMI}-1}$ locus may move through a constrained integrative mechanism. A conjugative plasmid can potentially transfer between compatible bacterial recipients, depending on plasmid functions, host range, ecological contact, and selection.

Subsequent IMI-2 reports broadened the host and geographical range:

• Klebsiella variicola in the United Kingdom;
• E. asburiae in the Czech Republic;
• historically labelled E. cloacae from Spanish rivers, France, and Canada;
• Enterobacter mori in Austria;
• Escherichia coli in Spain and China.

The recovery of IMI-2 in K. variicola and E. coli is consistent with a plasmid-associated dissemination route. It is not proof that every isolate contained the same plasmid or was descended from one original plasmid-bearing strain. Allele identity alone is lower-resolution evidence than complete plasmid sequence and comparative analysis.

A map is therefore a useful starting point, but a poor endpoint. A point on a map may represent a single isolate, a localized outbreak, an environmental survey, or passive reference-laboratory surveillance. The same apparent geographical pattern could result from genuine biology, uneven testing, differences in reporting, or all three.

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Later IMI variants: diversification in chromosomal and plasmid contexts

The later IMI record is best read as a mixture of new alleles, reclassified isolates, and improved genomic resolution—not as a neat linear sequence of global spread.

Variant or group	Organism and reported setting	Date/source information available in the curated resources	Genomic interpretation
IMI-3	First detected in China and France	Specific original sample type and collection date are not provided in the review excerpt	Reported on an IncFIIY plasmid within composite transposon Tn6306, bounded by two ISEcl1-like copies
IMI-5	Identified in Canadian E. cloacae complex surveillance material	Canadian national reference laboratory submissions, 2010–2015; individual specimen source is not assigned to IMI-5 in the excerpt	Located on the approximately 90-kb IncFII-type pIMI-5 plasmid; IS-rich, but transfer functions appeared incomplete and transfer was not demonstrated
IMI-6	Identified in Canadian E. cloacae complex surveillance material	Canadian reference laboratory surveillance, 2010–2015	Located on an approximately 165-kb IncFIIY plasmid with a functional conjugative transfer region; transfer to an E. coli recipient was demonstrated
IMI-9	Historically labelled E. cloacae in Canada and Norway	One Norwegian comparison isolate was from a patient; the broader Canadian study included urine, blood, rectal swabs, wounds, and bile drainage across all alleles	Associated with EcloIMEX-like chromosomal elements
NMC-A in later Canadian surveillance	Multiple E. cloacae complex lineages, many clustering with E. ludwigii	Canadian submissions, 2010–2015; sources included blood, urine, rectal swabs, wounds, and bile drainage across the complete collection	EcloIMEX-like chromosomal elements, sharing a conserved integration position

Two interpretive habits are crucial here.

Do not assign aggregate data to an individual allele

The Canadian surveillance study contained 19 NMC-A/IMI-positive isolates recovered from a mixture of specimen types, including blood and urine. But unless the isolate-level supplementary data are inspected, one cannot responsibly write “IMI-5 was from blood” or “IMI-6 was urinary” merely because those sources occurred somewhere in the collection.

This principle is especially important in reviews, where tables often compress isolate-level evidence into a sentence or a country list.

Nomenclature must be checked against sequence evidence

The literature also contains historical naming complications. The Canadian genomic study notes that an earlier designation of one plasmid-associated enzyme as IMI-3 was corrected to IMI-6. IMI-6 differs from IMI-3 by a single amino-acid substitution at position 7.

For a modern analysis, record:

• the reported allele name in the source;
• the sequence accession or sequence evidence, if available;
• whether subsequent work revised the allele designation.

This avoids creating artificial “epidemiological links” between isolates that actually encode closely related but distinct proteins.

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A compact historical model

You can now organize the IMI/NMC-A history into four evidence-based phases.

1. Early recognition of related class A carbapenemases
   IMI-1 was associated with a US E. cloacae isolate collected in 1984, followed by NMC-A in E. cloacae NOR-1 isolated in 1990. Their early detection predates the contemporary dominance of KPC, NDM, and OXA-48-like enzymes.

2. Recognition of an environmental, plasmid-associated IMI route
   IMI-2 in E. asburiae from US rivers, collected between 1999 and 2001, established that an environmental aquatic source could contain a self-transmissible IMI plasmid.

3. Increasing geographical and host breadth
   IMI-family alleles have subsequently been reported across Asia, Europe, North America, and the Pacific, including isolates outside Enterobacter. These reports demonstrate broad occurrence, but not necessarily one common clone or one universally shared plasmid.

4. Genome sequencing refines the story
   EcloIMEX-like elements explain why some IMI/NMC-A genes are chromosomal yet associated with mobile-element architecture. IncFII-family plasmids, insertion sequences, and composite transposons explain more direct horizontal-transfer potential for other variants.

For your hospital-water research, this framework leads to a disciplined interpretation. An IMI-positive drain isolate could represent:

• persistence of a lineage with a chromosome-associated EcloIMEX-like locus;
• introduction of a plasmid-bearing organism from a patient, wastewater, or external water source;
• a local plasmid-transfer event within a polymicrobial biofilm;
• or repeated detection of related but independently introduced strains.

The allele name alone cannot discriminate among these explanations.

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Key takeaways

IMI-1 was initially reported from a historically designated E. cloacae isolate collected in the United States in 1984, before routine imipenem use there. NMC-A, found in E. cloacae NOR-1 isolated in 1990, is a closely related but distinct member of the same Ambler class A carbapenemase family.

IMI-2, first found in E. asburiae from US rivers collected between 1999 and 2001, is a pivotal environmental and plasmid-associated discovery. Later IMI variants have been found in diverse countries and organisms, with contexts ranging from EcloIMEX-like chromosomal integrative elements to IncFII-family plasmids and insertion-sequence-bounded regions.

The rigorous way to reconstruct this history is to keep organism assignment, source, geography, isolation date, allele, and genomic context separate. In particular, do not attach collection-wide specimen data to one allele, do not treat historical E. cloacae labels as genome-confirmed species assignments, and do not equate the presence of insertion sequences with demonstrated transfer.

Next, we will widen this historical view into an epidemiological one: the distribution of IMI-producing Enterobacter across species, sequence types, regions, clinical material, and environmental reservoirs.