# Dissemination of Enterococcuscal Genetic Lineages: A One Health Perspective
## 3 Enterococcal Fitness Factors, Virulence Traits, and Antimicrobial Resistance
### Antimicrobial Resistance and Associated Mechanisms  
AMR refers to the ability of bacterial cells to withstand and avoid the harmful effects of antimicrobial agents [36], leading to difficulties in treating infections caused by resistant bacteria. As opportunistic pathogens, enterococci initially caused infections in immunocompromised patients, and later gained relevance as a major pathogen in hospitals with the introduction of third-generation cephalospororus (\(\beta\)-lactams class), to which these bacteria are naturally resistant [3]. As resistance to ampicillin and ciprofloxacin emerged a decade  
Figure 1: _Enterococcus_ spp. most relevant virulence factors. Created with BioRender.com. Adapted from references [1; 30]. later [3; 5], the glycopeptide vancomycin started being used as an alternative therapeutic option [3]. Moreover, the incorrect and prophylactic overuse of antimicrobials, both in healthcare environments and production animals, led to a progressive increase of resistance among enterococci, even to last-resort antimicrobials [5]. Thus, there is an urgent need for active biosurveillance of the dissemination of ARGs between bacteria present in each sector of the One Health triad. Briefly, enterococci are known to be intrinsically resistant to a variety of antimicrobials, including cephalosporporins, trimethoprim-sulfamethoxazole, and present low-level resistance to \(\beta\)-lactams and aminoglycosides [27]. Healthcare-associated strains belonging to clade A are generally resistant to ampicillin [37] and quinolones [24]. In addition, the animal-associated sub-clade A2, and the community-associated clade B, rarely lead to invasive infection or present vancomycin resistance [22; 23]. Multidrug resistance to macrolides, tetracyclines, streptogramins, and glycopeptides has been observed in both clinical and animal/veterinary enterococci [38]. A summary of the main antimicrobial classes, mechanisms of action in enterococcal species, and genes responsible for antimicrobial resistance is presented in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  
VRE, particularly VREfm, has become a growing concern in modern medicine, especially considering that vancomycin is a last-resort antimicrobial to treat hospital-acquired life-threatening infections (HAIs). Accordingly, VREfm, defined as an ESKAPE pathogen, is identified by the WHO as a high-priority pathogen, emphasizing the urgent need for the development of novel antimicrobial treatment options [40]. Glycopeptide resistance (e.g., vancomycin and teicoplanin) occurs through the substitution of N-terminal D-Alanine-D-Alanine in the peptidoglycan synthesis pathway, to either D-Alanine-D-Lactate (D-Ala-D-Lac) or D-Alanine-D-Serine (D-Ala-D-Ser) [41], leading to a lower binding affinity of glycopeptides in the cell. To date, various gene clusters responsible for glycopeptide resistance have been described in enterococci, named _vanA_ to _vanN_[42]. Vancomycin resistance is more common in the species _E. facetum_, _vanA_ and _vanB_ operons being the most frequently detected [1; 42; 43]. _vanA_ is plasmid-located and mediated by Tn1576 transposon, being characterized by a high degree of acquired inducible resistance to both vancomycin and teicoplanin [5; 42; 44]. While predominantly observed in _E. facetum_ strains, it has also been detected in _E. faecalis_ and, to a lesser extent, in _E. durans_, _E. raffinosus_, _E. hirae_, _E. avium_, and _E. gallinarum_[5]. In contrast, the _vanB_ resistance phenotype, encoded on the chromosome and carried by Tn1549-Tn5382, is characterized by variable levels of vancomycin resistance and susceptibility to teicoplanin [42; 44]. A recently described variant of VRE, vancomycin-variable enterococci (VVE), are susceptible to vancomycin while carrying the _vanA_ genotype (VVE-S), with the ability of becoming resistant to vancomycin upon exposure to glycopeptides [42; 45]. Several outbreaks of VVE have been reported around the globe [42; 45; 46; 47].