Understanding the ambiguity behind <i>Enterococcus</i> as a probiotic ally or a pathogenic adversary

The genus <i>Enterococcus</i> has been extensively investigated, emphasizing either its potential as a probiotic in fermented foods or its opportunistic pathogenic nature, particularly in hospital settings. This review focuses on the defining characteristics of enterococci species to better understand their dual nature, with the goal of establishing a universal method to safely and effectively characterize probiotic enterococci. Despite harboring genes for potentially harmful enzymes and metabolites, enterococci have traditionally been used to improve the flavor profiles of artisanal dairy products. Additionally, certain strains produce antimicrobial compounds, particularly bacteriocins, which help control the microbial composition of food products. These bacteriocins have been extensively explored as alternatives to antibiotics, driven by the rapid increase in antimicrobial resistance across several bacterial species. However, enterococcal isolates of nosocomial origin are harmful. Recent studies have clarified the divergent lineages that resulted in the emergence of pathogenic and nosocomial strains, thus improving the selection process for determining whether an isolate can be utilized as a probiotic. The insights gathered in this review have important implications for developing regulations on and optimizing the use of enterococci.