# Dissemination of Enterococcuscal Genetic Lineages: A One Health Perspective
## 3 Enterococcal Fitness Factors, Virulence Traits, and Antimicrobial Resistance
### Virulence Traits  
Virulence factors are bacterial traits that result in host infection [3]. These determinants are encoded by virulence genes, some of which can be found in pathogenesis islands (PAIs) or plasmids, thus being important to consider in the dissemination of specific genetic lineages across the One Health continuum. Previous reports have shown that clinical enterococcal isolates present the highest virulence, with _E. faecalis_ having the leading role [1], followed by food isolates and starter strains [27]. Virulence factors can be divided into two main groups: (a) externally secreted factors, e.g., cytolysin, gelatinase, secreted antigen A, serine protease, and glycosyl hydrolase; and (b) cell-surface factors, e.g., extracellular surface proteins (Esp), aggregation substances, adhesins, pili, and others [1; 5]. Cytolysin is responsible for a bactericidal effect against other gram-positive bacteria and hemolytic disruption of blood erythrocytes [8; 28]. It is commonly encoded on plasmids [29], also being found on the chromosome [3], and is regulated by a quorum-sensing mechanism [27]. Gelatinase is a member of the metalloproteinase extracellular zinc-endopeptidase family, able to hydrolyze gelatin, collagen, casein, and other peptides [30]. Encoded by the _gelE_ gene, located on the chromosome, it is co-transcribed with the serine protease SprE, by the FSR system [1; 8]. Both cytolysin and gelatinase have been mostly studied for _E. faecalis_[30]. Secreted antigen A (SagA) is a protein exclusively present in _E. ficeum_, with a role in stress activation and cell growth, being putatively involved in cell wall metabolism [31]. In addition, SagA containing a repeated region in clade A1 enterococci appears to improve biofilm formation, in contrast to SagA present in strains from clade B [32]. Glycosyl hydrolase, mistakenly named hyaluronidase in _E. ficeum_, is a degradative enzyme encoded by the chromosomal _hyl_ gene, acting on hyaluronic acid present in cellular tissue and facilitating the spread of enterococci and secreted toxins through the host tissues [27; 30]. However, there is still no proof that it contributes to enterococcal colonization and pathogenesis [33]. Regarding cell-surface virulence factors, these are important for bacterial defense mechanisms, such as biofilm formation and immune system evasion [1]. The enterococcal surface protein (Esp), mostly present in _E. faecalis_ and _E. ficeum_ clinical isolates, is an adhesin associated with colonization and persistence in urinary tract infections [8] and has an important role in biofilm formation and antimicrobial resistance [34; 35]. The _esp_ gene can be encoded in PAIs and may be transmitted by conjugation [5]. Aggregation substances (known as AS or Agg) are pheromone-inducible surface proteins, responsible for the adhesion to both prokaryotic and eukaryotic cells [5; 8], also aiding in colonization. Aggregation substances are mediated by plasmids--pPD1 contains genes encoding the Asp1 protein, pCF10 contains genes encoding the Asc10 protein, and pAD1 contains genes encoding the Asa1 protein [5]. Microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) are a subfamily of adhesions that are able to bind to extracellular matrix elements. Three types of MSCRAMMs have been described so far in _Enterococcus_: (1) Ace (adhesion of collagen from _E. faecalis_), which plays a role in endocarditis pathogenesis together with (2) Acm (adhesion of collagen from _E. ficeum_), and, lastly, (3) Scm (second collagen from _E. ficeum_) [1; 27]. Pili are composed of multimeric fibers made of piliu subunits. In _E. faecalis_, pili are encoded by two distinct gene clusters: _ebp_ (endocarditis- and biofilm-associated pili) and _bee_ (biofilm formation) [1].