🦠 microbiology

Post-transfer instability, not initial transfer, limits dissemination of IncI1-blaCTX-M-1 plasmids between chicken and human Escherichia coli.

Although IncI1-blaCTX-M-1 plasmids transfer readily from chicken to human *E. coli*, their dissemination is primarily limited by significantly higher post-transfer plasmid loss rates and host-specific evolutionary instability in human strains rather than by the initial conjugation efficiency.

Original authors: Buffoni, M., Fischer, E. A. J., DelaFuente, J., San Millan, A., Schurch, A. C., Willems, R. J. L. C., de Visser, A. J. G. M.

Published 2026-02-21

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Buffoni, M., Fischer, E. A. J., DelaFuente, J., San Millan, A., Schurch, A. C., Willems, R. J. L. C., de Visser, A. J. G. M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine a bustling airport where bacteria are the travelers and plasmids (small, circular rings of DNA) are the luggage they carry. Some of this luggage is dangerous because it contains "keys" that unlock antibiotic drugs, making the bacteria immune to medicine.

This paper investigates a specific type of dangerous luggage called IncI1-blaCTX-M-1. Scientists wanted to know: If this luggage is picked up by a chicken's bacteria, can it easily travel to a human's bacteria? And if it does, does it stay there, or does it get lost?

Here is the story of their findings, broken down into simple steps:

1. The Boarding Process (Transfer)

The Analogy: Imagine the bacteria are trying to board a plane (the transfer of DNA).
The Finding: The scientists found that getting the luggage onto the human bacteria was actually quite easy. It didn't matter if the human bacteria came from a human or a chicken, and it didn't matter if the room was hot (like a chicken's body) or cool (like a human's body).
The Catch: The only thing that made boarding difficult was the specific personality of the bacteria. Some bacteria had "bouncers" (defensive systems) that stopped the luggage from entering, while others were very welcoming. But generally, the luggage could get on the plane.

2. The Hotel Stay (Stability)

The Analogy: Once the luggage is on the plane, does the passenger keep it for the whole trip, or do they throw it out the window?
The Finding: This is where things got interesting. While the luggage got on the human bacteria easily, it was very unstable once it arrived.

In Chicken Bacteria: The luggage stayed put. The bacteria were comfortable with it.
In Human Bacteria: The luggage was treated like a nuisance. The human bacteria tried to get rid of it four times faster than the chicken bacteria did. It was like the human bacteria were saying, "I don't know this guest; get out!"

3. The Renovation (Evolution)

The Analogy: If the luggage is annoying, the bacteria might try to remodel the house to make it fit better, or they might try to break the luggage itself.
The Finding: When the bacteria tried to adapt to this new, unwanted luggage, they started making changes to their DNA.

The "Human" Reaction: Human bacteria went into a panic mode. They started deleting parts of the luggage to make it smaller and less annoying.
The "Chicken" Reaction: Chicken bacteria were more calm. They only made changes if the luggage was very different from what they were used to.

The Tragic Twist: In their rush to make the luggage smaller and easier to carry, the human bacteria accidentally cut off the safety straps (genes that keep the luggage attached to the bacteria).

Result: The bacteria became happier (less stressed), but the luggage fell off completely. The bacteria lost the antibiotic resistance because they threw away the very thing that kept the resistance alive.

4. The "Veteran" Travelers

The Analogy: What if a human bacteria had already carried this dangerous luggage before? Would it be easier this time?
The Finding: Yes and no.

Yes: The bacteria were less stressed by the new luggage because they were used to the weight. They grew faster.
No: Even though they were less stressed, they still threw the luggage away just as fast as the "naive" bacteria. Being familiar with the luggage didn't stop them from losing it.

The Big Picture: Why This Matters

For a long time, scientists worried that because antibiotic resistance moves easily from chickens to humans, we are in big trouble.

This paper says: "Wait a minute."

While the resistance can jump from chicken to human easily (the boarding is easy), it often fails to stick once it arrives. The human body is a hostile environment for this specific type of genetic luggage. The bacteria try to fix the problem by deleting the resistance genes, which actually stops the spread.

The Takeaway:
To stop antibiotic resistance, we shouldn't just look at how fast it spreads. We also need to look at how well it survives in the new host. Sometimes, the enemy is so uncomfortable in our bodies that it defeats itself. However, the authors warn us not to get too comfortable; if the bacteria figure out how to keep the luggage without deleting it, the spread could become unstoppable.

1. Problem Statement

Antimicrobial resistance (AMR), particularly mediated by extended-spectrum beta-lactamases (ESBLs) like blaCTX-M-1, poses a critical threat to global health. The IncI1 plasmid group is a primary vector for these genes, frequently detected in both poultry and human E. coli. While epidemiological data confirms the presence of similar plasmids in both hosts, the biological mechanisms governing their successful cross-species transmission remain unclear. Specifically, it is unknown whether the barrier to zoonotic spread lies in the initial conjugative transfer (entry) or in the post-transfer persistence (maintenance and evolution) within the new host. This study aims to deconstruct the dissemination process to determine if host origin (chicken vs. human) and temperature act as primary filters for plasmid spread.

2. Methodology

The researchers employed a comprehensive "One Health" experimental design combining microbiological assays, fitness measurements, and high-resolution genomics.

Strains and Plasmids:
- Donors: Two chicken-derived E. coli strains harboring IncI1 plasmids: pMLST3 (ST3) and pMLST7 (ST7), both carrying blaCTX-M-1.
- Recipients: A panel of 20 antibiotic-susceptible E. coli strains (10 human, 10 chicken origin), selected to represent diverse phylogenetic backgrounds.
- Selection: Recipients were engineered to carry a non-mobilizable chloramphenicol-resistance plasmid (pBGC) to allow selection of transconjugants (resistant to both chloramphenicol and cefotaxime).
- Curing Subgroup: Three human strains naturally carrying blaCTX-M-1 plasmids were cured using a CRISPR-Cas9 system to test the effect of prior exposure.
Experimental Assays:
- Conjugation Rates: Measured via mating assays at 37°C and 42°C. Rates were calculated using the "approximate extended Simonsen" model, accounting for differential growth rates of donors, recipients, and transconjugants.
- Fitness Costs: Quantified by comparing maximum growth rates ( $\mu_{max}$ ) of transconjugants versus their isogenic recipients in monoculture.
- Post-Transfer Stability: Plasmid loss rates were measured by growing transconjugants in non-selective media for 1 hour and calculating the decline in plasmid-carrying fractions.
- Evolutionary Analysis: Whole-genome sequencing (Illumina + Oxford Nanopore) was performed on 31 transconjugant lineages after ~60 generations. Bioinformatic pipelines (breseq, SVIM) identified SNPs, indels, and large structural deletions.
Statistical Analysis:
- Nested ANOVA, Spearman correlations, Poisson Generalized Linear Models (GLMs), and Wilcoxon tests were used to analyze the effects of host origin, temperature, plasmid identity, and genomic distance.

3. Key Contributions

This study provides a mechanistic deconstruction of plasmid dissemination, moving beyond simple transfer rate measurements to include:

Decoupling Transfer from Persistence: Demonstrating that high transfer rates do not guarantee successful dissemination if post-transfer stability is low.
Host-Specific Evolutionary Trajectories: Identifying distinct, convergent evolutionary paths (parallel deletions) taken by plasmids in human vs. chicken hosts.
The "Pyrrhic Victory" of Adaptation: Revealing that adaptive deletions to reduce metabolic burden in new hosts often inadvertently remove essential stability modules (partitioning/toxin-antitoxin systems), leading to long-term plasmid loss.
Role of Prior Exposure: Showing that while prior carriage of similar plasmids reduces metabolic fitness costs, it does not resolve vertical incompatibility issues regarding plasmid stability.

4. Key Results

A. Transfer Dynamics

Host Origin & Temperature: Neither the recipient's host origin (human vs. chicken) nor the incubation temperature (37°C vs. 42°C) significantly affected conjugation rates.
Genotype is King: Transfer efficiency was primarily determined by the recipient's specific genotype.
- In chicken recipients, transfer was negatively correlated with the accessory genome distance and the number of large resident plasmids (>10 kbp).
- Chicken strains possessed significantly more plasmid-encoded defense/anti-defense systems (mean 3.2 vs. 1.2 in humans), acting as a "gatekeeper" against new plasmid entry.

B. Post-Transfer Stability (The Critical Bottleneck)

Human Instability: Human-derived transconjugants exhibited a four-fold higher plasmid loss rate compared to chicken-derived transconjugants (mean loss rate: 0.97 h⁻¹ vs. 0.22 h⁻¹).
Decoupling: This instability was not correlated with transfer rates or immediate fitness costs, suggesting a specific incompatibility between the plasmid replication/partitioning systems and the human host background.

C. Fitness and Trade-offs

Temperature Stress: At 42°C (avian physiological temperature), a trade-off was observed in human strains: higher transfer rates correlated with higher fitness costs. This suggests that thermal stress exacerbates the metabolic burden of plasmid carriage in non-native hosts.
Prior Exposure: Human strains previously cured of blaCTX-M-1 plasmids showed reduced fitness costs when acquiring new plasmids (better tolerance), but loss rates remained high. This indicates that while hosts can adapt metabolically, they cannot easily overcome structural incompatibilities (e.g., partitioning system mismatches).

D. Evolutionary Adaptation (Convergent Deletions)

Post-acquisition evolution followed two distinct, plasmid-specific trajectories involving large parallel deletions:

pMLST3::D1 Trajectory: Deletions occurred in the "leading region," removing stress-response genes (SOS inhibitors psiA/psiB).
- Observation: These deletions were more frequent in human strains.
- Consequence: In human strains, extensive deletions often removed essential partitioning (parM/parB) and toxin-antitoxin (relE/impCAB) systems, directly causing the observed high instability.
pMLST7::D2 Trajectory: Deletions targeted the conjugation machinery (trbA/B, nikB) and specific toxin-antitoxin systems (sok, pndC).
- Observation: This occurred in 50% of transconjugants regardless of host, suggesting a compensatory mechanism to shed costly conjugation functions when not under strong selection.

Mutational Patterns: Human transconjugants showed higher mutation rates in regulatory proteins and mobile genetic elements (phages/transposons), indicative of a generalized stress response. Chicken transconjugants showed mutations more tightly correlated with genetic distance between donor and recipient.

5. Significance and Conclusion

The study concludes that the initial transfer of IncI1-blaCTX-M-1 plasmids from chicken to human E. coli is not the limiting factor; these plasmids transfer efficiently across species barriers. However, their long-term dissemination is severely restricted by post-transfer instability in the human host.

Mechanism of Restriction: The human host environment triggers a stress response that drives the plasmid to undergo adaptive deletions to reduce metabolic cost. Unfortunately, these deletions often remove the very genetic modules (partitioning and addiction systems) required for the plasmid's vertical stability, leading to rapid loss.
One Health Implication: Risk assessments for AMR transmission cannot rely solely on conjugation rates or the presence of resistance genes in food chains. They must account for host-specific evolutionary dynamics and plasmid stability. A plasmid may successfully jump species but fail to establish a persistent reservoir if it cannot maintain itself in the new host without selective pressure.
Future Directions: Understanding these "downstream dynamics" is crucial for predicting the true risk of zoonotic AMR spread and designing interventions that target plasmid stability rather than just transfer.