Genomic Island-Mediated Horizontal Transfer of the Erythromycin Resistance Gene (X) among Bifidobacteria.

Antibiotic resistance is a serious medical issue driven by antibiotic misuse. Bifidobacteria may serve as a reservoir for antibiotic resistance genes (ARGs) that have the potential risk of transfer to pathogens. The erythromycin resistance gene (X) is an ARG with high abundance in bifidobacteria, especially in Bifidobacterium longum species. However, the characteristics of the spread and integration of the gene (X) into the bifidobacteria genome are poorly understood. In this study, 10 W-positive bifidobacterial strains and 1 (X)-positive bifidobacterial strain were used to investigate the transfer of ARGs. Conjugation assays found that the (X) gene could transfer to five other bifidobacterial strains. Dimethyl sulfoxide (DMSO) and vorinostat significantly promoted the transfer of the (X) from strain Bifidobacterium catenulatum subsp. DSM 21854 to Bifidobacterium longum subsp. DSM 20211. Whole-genome sequencing and comparative genomic analysis revealed that the (X) gene was located on the genomic island BKGI1 and that BKGI1 was conjugally mobile and transferable. To our knowledge, this is the first report that a genomic island-mediated gene (X) transfer in bifidobacteria. Additionally, BKGI1 is very unstable in B. catenulatum subsp. DSM 21854 and transconjugant D2TC and is highly excisable and has an intermediate circular formation. analysis showed that the BKGI1 homologs were also present in other bifidobacterial strains and were especially abundant in B. longum strains. Thus, our results confirmed that genomic island BKGI1 was one of the vehicles for (X) spread. These findings suggest that genomic islands play an important role in the dissemination of the gene (X) among species. Bifidobacteria are a very important group of gut microbiota, and the presence of these bacteria has many beneficial effects for the host. Thus, bifidobacteria have attracted growing interest owing to their potential probiotic properties. Bifidobacteria have been widely exploited by the food industry as probiotic microorganisms, and some species have a long history of safe use in food and feed production. However, the presence of antibiotic resistance raises the risk of its application. In this study, we analyzed the transfer of the erythromycin resistance gene (X) and revealed that the molecular mechanism behind the spread of the gene (X) was mediated by genomic island BKGI1. To the best of our knowledge this is the first report to describe the transfer of the gene (X) via genomic islands among bifidobacteria. This may be an important way to disseminate the gene (X) among bifidobacteria.