# Dissemination of Enterococcuscal Genetic Lineages: A One Health Perspective
## 1 _Enterococcus_ spp.-An Overview  
_Enterococcus_ spp. are ubiquitous gram-positive cocc that inhabit the normal intestinal microbiota of humans and animals and colonize a variety of environmental sources such as soils, water, and fresh and fermented foods. These facultative anaerobes, catalase-negative, non-spore-forming bacteria can tolerate high salt concentrations (6.5% NaCl), wide pH values (4.5-10.0), and growth temperatures ranging from 10 to 45 \({}^{\circ}\)C (optimal at 36 \({}^{\circ}\)C) and present variable resistance to chemical disinfectants, such as alcohol and chlorheximide [1; 2; 3]. In fact, enterococci possess remarkable stress resilience, which can be enhanced by exposure to sublethal harsh conditions, surviving temperatures as high as 60 \({}^{\circ}\)C, desiccation in the environment, and starvation [4]. In addition, enterococci are known to hydrolyze esculin in the presence of 40% bile salts [1; 2]. As homofermutative bacteria, they produce lactic acid from glucose fermentation without gas production [2]. Likewise, enterococci play a significant role in the organoleptic properties of fermented foods, due to their natural occurrence as contaminants in raw and dairy food products, resistance to food processing conditions (thernotolerance), low toxicity, and ability to acidify the environment [2]. These attributes are crucial for successful use in food fermentation processes, either as starter or non-starter bacteria, and inclusion in probiotic formulations [5]. However, enterococci have emerged as relevant opportunistic pathogens in the last decades, representing a major cause of healthcare-associated infections due to rapid adaptation to the host and ability to acquire a diversity of virulence and antibiotic resistance genes (ARGs; resistance [6]) through horizontal gene transfer (HGT), largely mediated by mobilegenetic elements (MGEs; composing the mobilome) [7]. These bacteria can cause a variety of infections in humans (urinary tract, intra-abdominal, and tissue infections; bacteremia, and endocarditis [8]) and animals (urinary tract infection, otitis externa, peritonitis, endocarditis, and mastitis in dairy animals [9]). Among the growing number of enterococcal species identified, _Enterococcus faecalis_ and _E. faecium_ are the most common infection-related species, with _E. faecalis_ accounting for the majority of infections [10]. Yet, the proportion of _E. faecium_ infections has been progressively increasing, due to the spread of antimicrobial resistance (AMR), particularly vancomycin-resistant _E. faecium_ (VREfm), which has been responsible for the majority of vancomycin-resistant enterococci (VRE) outbreaks worldwide [11; 12]. According to recent evidence, the percentage of clinical VREfm varies between 1 and 46.3% in Europe, 75 and 80% in the USA, and 11.3 and 75% in Australia [11]. These increased levels of resistance have led to the inclusion of VREfm in the high-priority pathogens list, defined by the World Health Organization (WHO) and the ESKAPE pathogen group formed by _E. faecium_, _Staphylococcus aureus_, _Klebsiella pneumoniae_, _Acinetobacter baumantii_, _Pseudomonas aeruginosa_, and _Enterobacter_ spp. [13; 14]. Other relevant species of the enterococcal genus include _E. avium_, _E. gallinarum_, _E. casselflavus_, _E. hirae_, _E. mundutii_, and _E. raffinosus_, _E. gallinarum_ and _E. casselflavus_ being of special interest, due to their intrinsic resistance to vancomycin [2]. In general, it has been estimated that enterococci represent 6.1-17.5% of the total number of isolates obtained from European patients carrying hospital-acquired infections between 2010 and 2020 [15]. The aim of this literature review is to describe enterococci's most relevant features, focusing on the overall dissemination of virulence and antimicrobial resistance genetic traits across the One Health sectors. A literature search was mainly conducted on PubMed and ScienceDirect databases (including cross-references).