A Small Multihost Plasmid Carrying erm(T) Identified in Enterococcus faecalis

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Horizontal gene transfer (HGT) is a major mechanism for the spread of antimicrobial resistance.
• The erm(T) gene confers resistance to macrolide antibiotics, including erythromycin and clindamycin.
• Enterococcus faecalis is a significant opportunistic pathogen, and understanding its resistance mechanisms is crucial.

Purpose of the Study:

• To identify mobile genetic elements responsible for the horizontal transfer of the erm(T) gene in Enterococcus faecalis.
• To investigate the transmissibility of the erm(T) gene in various heterologous bacterial hosts.
• To assess the fitness cost associated with the erm(T)-carrying plasmid in recipient strains.

Main Methods:

• Screening of 159 erythromycin-resistant enterococci isolates for macrolide resistance genes using PCR.
• Whole genome sequencing of the erm(T)-carrying Enterococcus faecalis E165 isolate.
• Conjugation, natural transformation, and electrotransformation experiments to assess plasmid transmissibility.
• Growth curve and competition experiments to evaluate fitness costs.

Main Results:

• The erm(T) gene was detected in a single Enterococcus faecalis isolate (E165), located on a 4,244-bp plasmid designated pE165.
• Plasmid pE165 was successfully transferred to heterologous hosts including Enterococcus faecalis, Streptococcus suis, Streptococcus mutans, and Staphylococcus aureus.
• The erm(T)-carrying plasmid conferred erythromycin and clindamycin resistance to all tested heterologous hosts.
• While no growth disadvantage was observed in standard growth curves, pE165-carrying recipients exhibited a fitness cost in competition experiments.

Conclusions:

• A novel erm(T)-carrying plasmid, pE165, has been identified in Enterococcus faecalis.
• This plasmid is transferable to a range of Gram-positive bacteria, highlighting a potential route for macrolide resistance dissemination.
• The erm(T) gene on pE165 contributes to macrolide resistance in diverse hosts, albeit with a potential fitness cost.