Tn125-borne blaNDM-1 is decoupled from clonal background in a transcontinental Acinetobacter baumannii ST126/KL14
Nguyen Si-Tuan1, Hua My Ngoc2, Le Duy Nhat2
1Faculty of Clinical Laboratory, Hong Bang International University, Ho Chi Minh City, Vietnam.
Background/Objectives:
Carbapenem-resistant A. baumannii (CRAB) is a WHO Critical Priority pathogen. The blaNDM-1-carrying ST126/KL14 lineage has been independently reported from Vietnam (2015), Malaysia (2016), the USA (2023-2026), and Costa Rica (2024). Whether these geographically distinct reports represent a single transcontinental clone and through what mechanism blaNDM-1 disseminates has not been formally tested.
Methods:
We performed comprehensive whole-genome reanalysis of the Vietnamese sentinel isolate DMS06669_L1 using three nested panels (n = 19, n = 138, and n = 609 Vietnamese A. baumannii genomes) and surveyed 429 plasmids extracted from 99 NDM-1 A. baumannii genomes retrieved from NCBI Pathogen Detection.
Results:
Four ST126/KL14 isolates share high inter-regional average nucleotide identity (ANI; 99.77-99.92%) but wide intra-clade core-SNP distances (41-731 SNPs, well above the ∼20-40-SNP range typical of single-outbreak transmission clusters) and lack a significant molecular clock, consistent with a related transcontinental lineage rather than a single recent clone. NDM-1 plasmid evolution is statistically uncorrelated with chromosomal sequence type (Spearman ρ = 0.131, P = 0.573). At 609-strain population scale, under a fragmentation-aware detection criterion, all 41 blaNDM-1-carrying Vietnamese strains also carry ISAba125, with none carrying blaNDM-1 without it (Fisher exact test; Haldane-Anscombe-corrected OR ≈2.0 × 10³, 95% CI 1.2 × 10² to 3.4 × 10⁴; P = 4.74×10⁻⁴⁸; φ = 0.79).
Conclusions:
The blaNDM-1 dissemination pattern in this ST126/KL14 lineage is primarily consistent with Tn125 transposition acting alongside plasmid-borne spread. Standard MLST-based surveillance is insufficient; multi-level genomic monitoring - including chromosomal and plasmid-level detection of the Tn125/ISAba125 unit - is required to track this resistance threat.
