In vivo de-amplification of a multi-resistance pseudo-compound transposon in Escherichia coli

1. Problem Statement

The rapid expansion of antimicrobial resistance (AMR) in Gram-negative pathogens, particularly in vulnerable populations like infants, poses a critical clinical challenge. Mobile Genetic Elements (MGEs), specifically Insertion Sequence 26 (IS26), are known to drive the dissemination of Antimicrobial Resistance Genes (ARGs) through replicative transposition. IS26 can form Pseudo-Compound Transposons (PCTs) that facilitate the creation of Translocatable Units (TUs). Through a mechanism called Targeted Conservative Co-integration (TCC), these TUs can amplify into tandem arrays, simultaneously increasing the copy number of multiple ARGs and potentially expanding resistance to new antibiotic generations. While amplification is well-documented, the dynamics of de-amplification (reduction in copy number) within a living host (in vivo) and its impact on bacterial fitness and phenotypic resistance remain poorly understood.

2. Methodology

The study utilized a longitudinal approach tracking a community-acquired, multi-drug resistant (MDR) Escherichia coli strain colonizing an infant in Tanzania over an 18-week period.

• Sample Collection: Two isolates were obtained from the same healthy infant (part of the ProRIDE clinical trial):
  • 1532WA: Collected at 6 weeks of age.
  • 1532MA: Collected at 6 months of age.
  • Context: The infant had no hospitalization or antibiotic exposure during the observation period.
• Sequencing Strategy:
  • Hybrid Assembly: Both strains were sequenced using Illumina NovaSeq (short reads) and Oxford Nanopore Technologies (ONT) (long reads).
  • Assembly: Reads were processed using the Hybracter pipeline to generate high-quality hybrid assemblies.
  • Analysis:
    • Variant Calling: Breseq was used to identify SNPs and indels by mapping 1532MA reads against the 1532WA assembly.
    • Copy Number Estimation: Gene copy numbers were estimated using ARIBA (mapping Illumina reads to ARG databases) and ONT read coverage analysis, normalized against the single-copy rpoB gene.
    • Phenotypic Testing: Antimicrobial Susceptibility Testing (AST) was performed via disk diffusion (EUCAST guidelines) for ciprofloxacin, gentamicin, meropenem, piperacillin-tazobactam (TZP), tetracycline, and chloramphenicol.
    • Fitness Assay: Growth curves were measured in LB broth to calculate the Area Under the Curve (AUC) as a proxy for fitness.

3. Key Contributions

• First In Vivo Observation of De-amplification: The study documents the natural reduction of a complex IS26-mediated tandem ARG array within a human host without antibiotic selection pressure.
• Mechanistic Insight: It provides evidence supporting Targeted Conservative Co-integration (TCC) as the driver for both the initial amplification and subsequent de-amplification of the PCT.
• Evolutionary Rate Calculation: It establishes a high evolutionary rate (10.22 SNPs/genome/year) for E. coli in the rapidly maturing neonatal gut, challenging standard clonality thresholds used in adult epidemiology.
• Phenotype-Genotype Disconnect: It highlights that significant genomic de-amplification does not always result in immediate, detectable changes in phenotypic resistance or fitness.

4. Key Results

A. Genomic Evolution and Clonality

• The two isolates were confirmed as highly related clones (99.9983% Average Nucleotide Identity).
• SNP Analysis: Only 7 chromosomal SNPs and 2 indels were found between the 18-week interval, equating to an evolutionary rate of 10.22 SNPs per genome per year. This rate is higher than typical adult-derived estimates, likely due to the rapid maturation of the infant microbiome and increased selective pressures.
• Structural Variation: The primary difference was the presence of a massive tandem array in the ancestral strain (1532WA) that was absent in the descendant (1532MA).

B. The Pseudo-Compound Transposon (PCT) and De-amplification

• Structure: The ancestral strain (1532WA) contained a PCT flanked by two directly oriented IS26 elements. This structure allowed for the amplification of a Translocatable Unit (TU):
  • TU Sequence: Tn3-like(tnpA)-tetR-tetA-yedA-ΔTn1721(tnpA)-IS26-aac(6')-Ib-cr-blaOXA-1-ΔcatB3-IS26.
• Amplification: 1532WA possessed six tandem copies of this TU (approx. 63.5 kbp total expansion), whereas 1532MA retained only a single copy (the core PCT).
• Mechanism: The absence of Target Site Duplications (TSDs) and the specific IS26 orientation suggest the amplification occurred via TCC, a replicative mechanism unique to IS26. The de-amplification likely occurred via a reverse recombination event or excision.

C. Phenotypic and Fitness Outcomes

• Fitness: Despite the loss of ~48 kbp of DNA (5 TU copies), there was no significant difference in fitness between the two strains (p = 0.275).
• Antibiotic Susceptibility:
  • Piperacillin-Tazobactam (TZP): No change in susceptibility was observed, despite the reduction in blaOXA-1 copies. This suggests blaOXA-1 alone (even in high copy numbers) does not confer clinical resistance to TZP in this context.
  • Gentamicin: The descendant strain (1532MA) showed increased susceptibility to gentamicin compared to the ancestor. This correlates with the de-amplification of aac(6')-Ib-cr. While one copy usually confers resistance, the study suggests that high copy numbers (5x in 1532WA) may provide a "buffer" or saturation effect that enhances resistance levels, a phenomenon not previously well-characterized for this specific gene.
  • Other Drugs: No changes were detected for ciprofloxacin, tetracycline, or chloramphenicol (the latter due to a plasmid-borne catA1, not the truncated catB3 in the PCT).

5. Significance

• Dynamic Nature of AMR: The study demonstrates that ARG amplification is not a permanent state; bacteria can spontaneously de-amplify complex resistance arrays in vivo, even in the absence of antibiotic pressure.
• Clinical Implications: The finding that high gene copy numbers can subtly modulate resistance levels (e.g., gentamicin) suggests that genomic surveillance must account for copy number variation, not just gene presence/absence.
• Neonatal Microbiome: The high mutation rate observed underscores the infant gut as a "hotspot" for rapid bacterial evolution, necessitating tailored surveillance and treatment strategies for neonatal sepsis in low-resource settings.
• IS26 Dynamics: The research reinforces the role of IS26-mediated TCC in the rapid expansion and contraction of MDR clusters, highlighting the plasticity of the bacterial genome in response to environmental pressures.