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

Carbapenem-resistant Gram-negative bacteria pose a critical public health threat. The role of mobile genetic elements in driving their transmission and persistence remains poorly defined. In 2022, we investigated a suspected outbreak of carbapenem-resistant Acinetobacter baumannii (CRAB) in a Nigerian adult intensive care unit (ICU), using short-read whole genome sequencing (WGS) of carbapenem-resistant clinical and environmental isolates during the cluster period. Mobile element dynamics were then inferred from hybrid assemblies of Illumina and Oxford Nanopore reads. The suspected CRAB outbreak was ruled out by WGS but a carbapenem-resistant Enterobacter hormaechei ST114 bloodstream isolate was found to be indistinguishable from two environmental isolates, all recovered during the Acinetobacter surge. Hybrid assemblies revealed a strikingly conserved [~]19 Kb resistance island shared across all ST114 genomes. The island contained a blaNDM-5 cassette alongside many other antimicrobial resistance genes, within class 1 integronns and flanked by insertions sequences, located on a 46,176 bp plasmid. Using the ST114 plasmids hybrid assembly as scaffold, the same plasmid was identified in the genome of a Klebsiella pneumoniae ST15 isolate from the ICU environment during the same period. Additionally, re-interrogation of genomic surveillance data uncovered four clonal 2020 ST109 Enterobacter bloodstream isolates from the same facility that carried the resistance genes in the same context on a large 267,242 bp plasmid. Carbapenem resistance in hospital Enterobacterales is driven by both clonal expansion and horizontal spread of mobile resistance elements. These findings underscore the need to track mobile elements alongside bacterial lineages to inform evidence-based infection control, especially in low-resource settings. Impact StatementCarbapenem resistance among Enterobacterales remains a major public health threat, yet how mobile genetic elements contribute to their persistence and spread in hospital settings is still poorly understood. In this study, we investigated a suspected outbreak of carbapenem-resistant Acinetobacter baumannii in an adult intensive care unit in Nigeria. Although the outbreak was eventually ruled out, genomic analysis has shown the importance of careful interpretation of suspected outbreak cases in hospital settings. Our findings highlight the importance of close monitoring of ICU environments, the implementation of blood culture-based diagnostics, and the value of genomic support in outbreak investigations. These findings demonstrate that carbapenem resistance in hospital Enterobacterales is driven not only by clonal expansion but also by the horizontal dissemination of a highly stable blaNDM-5-associated MDR island capable of integrating into diverse plasmid backbones. This study emphasizes the need for genomic surveillance that tracks both mobile elements and bacterial lineages to strengthen outbreak investigations, especially in low-resource settings. It further underscores the links between clinical and environmental AMR reservoirs and reinforces the value of a One Health approach to controlling carbapenem resistance. Data summaryFASTQ sequences were deposited in the NCBI BioSample database under accession numbers SAMN55915584 - SAMN55915597.