The Enterococcus Cassette Chromosome, a Genomic Variation Enabler in Enterococci.

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• Enterococcus faecium exhibits significant genomic variability due to frequent recombination and horizontal gene transfer.
• Understanding the mobile genetic elements driving this variation is crucial for comprehending bacterial adaptation.

Purpose of the Study:

• To identify and characterize a novel mobile genetic element in Enterococcus faecium.
• To elucidate the mechanism of its transfer and its role in accessory gene acquisition.

Main Methods:

• Identification of a novel genetic island, the enterococcus cassette chromosome (ECC).
• Analysis of insertion locus and mobilization genes (ccrAB).
• Investigation of horizontal transfer via conjugation and homologous recombination, aided by long-read sequencing and PCR.

Main Results:

• A novel ECC element, similar to SCCmec, was identified, encoding ccrAB recombinases and located near the rlmH gene.
• Horizontal transfer of an ECC::cat element was observed, mediated by a conjugative plasmid and IS element homology.
• ECC elements were found in 69/516 E. faecium genomes, with full elements identified in 32, featuring attR/attL sites and diverse gene content.

Conclusions:

• ECC elements act as hotspots for genetic exchange in E. faecium, contributing to accessory gene diversity.
• The ccrAB genes are key for ECC integration into the host genome, similar to staphylococcal SCC elements.
• ECC elements play a significant role in the adaptation of E. faecium to various environments.