Comprehensive investigation of antibiotic resistance gene content in cfiA-harboring Bacteroides fragilis isolates of human and animal origins by whole genome sequencing.

Introduction: The emergence of multidrug resistance in Bacteroides fragilis, especially the phylogenetic lineage carrying the carbapenemase gene cfiA, represents an increasing threat to human health. However, knowledge on the diversity of the multidrug-resistant strains and the genetic elements carrying the antibiotic resistance genes (ARGs) remains limited.

Aim: The objective of the study was to describe the resistome in cfiA-positive B. fragilis.

Methods: A collection of cfiA-positive B. fragilis from diverse human (8 bacteremias, 15 wound infections) and animal (2 chickens, 2 pigs, 6 dogs, 3 cats) sources in Hong Kong, 2015-2017 was analysed by whole genome sequencing.

Results: In the 36 isolates, 13 distinct ARGs (total number 83, median 2, range 0-7 per isolate) other than cfiA were detected. ARGs encoding resistance to aminoglycosides, β-lactams, macrolides, sulphonamides and tetracyclines were carried by CTn341-like, CTnHyb-like, Tn5220-like, Tn4555-like and Tn613-like transposons and were detected in phylogenetically diverse isolates of different host sources. Only few ARGs encoding resistance to metronidazole and tetracyclines were localized on plasmids. In two chicken isolates, a novel transposon (designated as Tn6994) was found to be involved in the dissemination of multiple ARGs mediating resistance to multiple antibiotics, including metronidazole and linezolid that are critically important for treatment of anaerobic infections. In mating experiments, Tn6994 and the associated phenotypic resistance could be transferred to Bacteroides nordii recipient.

Conclusion: This study illustrates the importance of transposons in the dissemination of ARGs in the cfiA-positive division of B. fragilis. One Health approach is necessary to track the dissemination of ARGs.