Mobile element-mediated carbapenem resistance in Enterobacter hormaechei in a Nigerian intensive care unit

This study reveals that carbapenem resistance in a Nigerian ICU is driven by the horizontal spread of a conserved mobile resistance island carrying *bla*NDM-5 across diverse *Enterobacter hormaechei* and *Klebsiella pneumoniae* strains, highlighting the critical need to track mobile genetic elements alongside bacterial lineages for effective infection control in low-resource settings.

The Gist

The Story: A Case of Mistaken Identity and a Sneaky "Super-Plasmid"

Imagine a hospital in Nigeria (University College Hospital, Ibadan) where the doctors noticed something scary happening in the Intensive Care Unit (ICU). Patients were getting sick with infections that weren't getting better with their strongest antibiotics.

1. The Red Herring: The Wrong Suspect
At first, the hospital staff thought the culprit was a specific bacteria called Acinetobacter baumannii (let's call it "The Badger"). They saw a spike in infections and found a few "Badgers" in the patients. They suspected a "Badger Outbreak" was spreading through the ICU.

However, the researchers decided to use a high-tech microscope called Whole Genome Sequencing (think of it as reading the bacteria's entire instruction manual) to investigate.

• The Twist: When they read the manuals, they realized the "Badgers" weren't actually related to each other. They were just different strangers who happened to show up at the same time. The "Badger Outbreak" was a false alarm.

2. The Real Culprit: The Shape-Shifter
While ruling out the Badger, the researchers found something even more interesting hiding in the data. They discovered a different bacteria, Enterobacter hormaechei (let's call it "The Chameleon"), that was causing serious bloodstream infections.

Here is the scary part:

• They found the exact same "Chameleon" bacteria in a patient's blood.
• They found the exact same "Chameleon" bacteria on the hospital environment (like bed rails or sinks).
• They even found the exact same "Chameleon" in a different type of bacteria (Klebsiella) in the same room.

This meant the bacteria weren't just spreading from patient to patient; they were hopping between patients, the environment, and even different species of bacteria.

3. The Secret Weapon: The "Swiss Army Knife" Plasmid
Why was this bacteria so dangerous? It had a superpower. Inside its cell, it carried a tiny, circular piece of DNA called a plasmid.

Think of this plasmid as a Swiss Army Knife or a USB drive loaded with dangerous software.

• The Software: This USB drive contained the code to make a weapon called blaNDM-5. This weapon destroys the hospital's "last-resort" antibiotics (carbapenems).
• The Extra Gear: But it didn't just have one weapon. The USB drive was packed with codes to resist almost every other type of antibiotic too (painkillers, fever reducers, etc.). It was a "Super-Resistance Kit."

4. The "Russian Doll" Effect
The most fascinating discovery was how this "Super-Resistance Kit" moved around.

• The researchers found this exact same USB drive in the "Chameleon" bacteria from 2022.
• But they also found the same USB drive in a different strain of "Chameleon" bacteria from 2020 (two years earlier).
• Even more surprisingly, they found this same USB drive inside a Klebsiella bacteria in the hospital environment.

It's like finding the same stolen wallet in the pockets of three different people, in two different years, in the same building. The bacteria didn't just clone themselves; they were sharing the USB drive.

5. The "Movable Island"
The researchers realized that the dangerous part of this USB drive (the part that kills antibiotics) was built like a Lego block or a modular island.

• This "Resistance Island" could detach itself from one bacteria and jump onto another.
• It could jump from a "Chameleon" to a "Klebsiella."
• It could jump from a patient to a sink.
• It could even jump from a 2020 bacteria to a 2022 bacteria.

This is called Horizontal Gene Transfer. Instead of waiting for a baby bacteria to inherit the weapon from its parent (vertical), the bacteria are just swapping weapons with their neighbors.

The Big Lesson

What does this mean for us?

1. Don't just look at the bacteria: If you only look at the bacteria species (like "Is it a Badger or a Chameleon?"), you might miss the real danger. You have to look at the weapons (the plasmids) they are carrying.
2. The Environment Matters: The hospital environment (sinks, beds) is a "battleground" where these bacteria swap their super-weapons. If you don't clean the room, the bacteria can swap weapons even if no patient is there.
3. Low-Resource Settings Need High-Tech Help: In places with fewer resources, it's hard to track these invisible swaps. This study shows that using advanced DNA sequencing is crucial to stop outbreaks before they spread.

In a nutshell:
The hospital thought they were fighting a war against a specific enemy (the Badger), but they were actually fighting a war against a super-weapon (the Plasmid) that was being passed around like a hot potato between different bacteria, patients, and even the furniture in the ICU. To win, they need to track the weapon, not just the carrier.

Technical Summary

1. Problem Statement

Carbapenem-resistant Gram-negative bacteria represent a critical public health threat, particularly in low- and middle-income countries (LMICs) where genomic surveillance resources are limited. In May 2022, the Adult Intensive Care Unit (ICU) at University College Hospital (UCH) in Ibadan, Nigeria, experienced a sharp surge in Hospital-Acquired Infections (HAIs). Initial clinical microbiology data suggested a potential outbreak of Carbapenem-Resistant Acinetobacter baumannii (CRAB). However, the etiology was unclear, and standard phenotypic identification methods were insufficient to confirm an outbreak or identify the true drivers of resistance. There was a need to determine whether the surge was due to clonal expansion of a specific pathogen or the horizontal spread of mobile resistance elements.

2. Methodology

The study employed a comprehensive genomic epidemiology approach combining short-read and long-read sequencing:

• Sample Collection: Blood cultures from ICU patients and environmental swabs were collected during the HAI surge (May–November 2022).
• Phenotypic Characterization: Isolates were initially identified using API-20E and VITEK2 systems. Susceptibility testing was performed via Kirby-Bauer disc diffusion.
• Whole-Genome Sequencing (WGS):
  • Short-Read: Illumina MiSeq sequencing was performed on all clinical and environmental isolates.
  • Long-Read: Oxford Nanopore (MinION) sequencing was performed on select Enterobacter strains to generate complete genome assemblies.
  • Hybrid Assembly: Illumina and Nanopore reads were combined using Unicycler to resolve complex plasmid structures and mobile genetic elements (MGEs).
• Bioinformatic Analysis:
  • Species/ST Identification: Bactinspector and Pathogenwatch were used for species confirmation and Multilocus Sequence Typing (MLST).
  • Resistance Gene Detection: ABRicate and NCBI AMRFinder were used to identify Antimicrobial Resistance Genes (ARGs).
  • Plasmid & MGE Analysis: PlasmidFinder, MOB-suite, MobileElementFinder, and VRprofile2 were used to characterize plasmid replicons, transposons, and integrons.
  • Phylogenetics: SNP-based phylogenetic analysis (Snippy, snp-dists) was used to assess clonal relationships.
  • Comparative Genomics: Clinker and Clustermap.js were used to visualize synteny and conserved resistance islands across different isolates and species.
• Retrospective Analysis: Genomic data from a 2020 bloodstream isolate cluster (ST109) from the same facility was re-interrogated for comparison.

3. Key Results

A. Ruling Out the Suspected Outbreak

• WGS revealed that the suspected CRAB outbreak was a misdiagnosis. The two Acinetobacter baumannii isolates recovered belonged to distinct Sequence Types (ST2833 and ST919) and differed by over 52,000 SNPs, confirming they were not part of a single outbreak.
• Two other presumptive Acinetobacter isolates were re-identified as Proteus mirabilis and Enterobacter hormaechei.

B. Discovery of the E. hormaechei ST114 Clone

• A carbapenem-resistant Enterobacter hormaechei bloodstream isolate (UCH_778) was found to be genetically indistinguishable (0–2 SNPs) from two environmental E. hormaechei isolates (UCH_710, UCH_770).
• All three ST114 isolates carried 24 resistance genes conferring resistance to nine antimicrobial classes, including the carbapenemase blaNDM-5.
• These isolates also carried high-level aminoglycoside resistance genes (armA, rmtB).

C. Characterization of Mobile Genetic Elements

• Hybrid Assembly: Revealed a conserved ~19 kb resistance island containing the blaNDM-5 cassette, flanked by insertion sequences (IS) and located within a Class 1 integron.
• Plasmid Architecture:
  • The resistance island was located on a 46,176 bp IncR plasmid (designated pCRE-NDM).
  • A second, larger, multi-replicon plasmid (~253 kb) containing IncHI2, IncHI2A, and RepA_pKPC-CAV1321 replicons was also identified, though it could not be fully circularized in all strains due to assembly fragmentation.
• Horizontal Gene Transfer (HGT):
  • The same 46 kb IncR plasmid carrying the blaNDM-5 resistance island was identified in a Klebsiella pneumoniae ST15 environmental isolate from the same ICU.
  • Retrospective analysis of 2020 data revealed a clonal cluster of E. hormaechei ST109 bloodstream isolates carrying a nearly identical resistance island on a much larger (267 kb) conjugative plasmid (IncFIB/IncHI1B).

D. Resistance Profile

• The E. hormaechei ST114 isolates exhibited a "Russian doll" containment of resistance, where the blaNDM-5 gene was embedded in a stable MDR island capable of integrating into diverse plasmid backbones (IncR, IncHI2, IncFIB).
• The resistance profile included ESBLs (blaCTX-M-15, blaTEM), carbapenemases (blaNDM-5), and genes conferring resistance to sulfonamides, tetracyclines, and macrolides.

4. Key Contributions

1. Correction of Outbreak Hypothesis: Demonstrated the critical value of WGS in ruling out false-positive outbreak alerts (CRAB) and identifying the true pathogen (E. hormaechei).
2. Mobile Element Tracking: Shifted the focus from purely clonal expansion to the horizontal dissemination of mobile resistance elements. The study proved that the blaNDM-5 resistance island is a modular unit capable of jumping between different plasmid backbones and bacterial species (E. hormaechei and K. pneumoniae).
3. Environmental Reservoirs: Highlighted the role of the ICU environment as a reservoir for carbapenem-resistant clones and plasmids, showing genetic identity between clinical and environmental isolates.
4. One Health Insight: Provided evidence that resistance is maintained not just by carbapenem use but by the co-selection of plasmids carrying resistance to multiple antibiotic classes (e.g., aminoglycosides, sulfonamides).

5. Significance

• Public Health Impact: The findings underscore that in resource-limited settings, carbapenem resistance is driven by the rapid spread of highly stable, mobile genetic elements rather than just the spread of specific bacterial clones.
• Surveillance Strategy: The study argues for a paradigm shift in genomic surveillance: tracking mobile elements alongside bacterial lineages is essential for effective infection control. Relying solely on strain typing may miss the transmission of resistance genes between different species.
• Clinical Implications: The presence of blaNDM-5 and high-level aminoglycoside resistance (armA, rmtB) in the ICU poses a severe threat to patient survival, as these genes render last-resort antibiotics and supportive therapies ineffective.
• Policy Recommendation: The authors advocate for the implementation of blood culture-based diagnostics and enhanced environmental surveillance in ICUs, supported by genomic capabilities, to detect and contain such outbreaks early.

In conclusion, this study reveals a complex epidemiological scenario in a Nigerian ICU where a suspected Acinetobacter outbreak was actually a manifestation of a highly adaptable Enterobacter clone and its mobile resistance plasmids, which had spread horizontally to other species within the hospital environment.