Non-clinical isolates bring new findings on enterococcal virulence

Dissertation presented to obtain the PhD degree in Biology

Enterococci are Gram-positive lactic acid bacteria, widespread in the environment, present in water, soil, plants and animals, including humans. They typically colonize the skin and mucous membranes, namely the gastrointestinal tract. However, enterococci, and most notably Enterococcus faecalis and Enterococcus faecium, have become problematic causative agents of several nosocomial infections, including urinary tract infections, bacteraemia, surgical sight infections, and endocarditis. Besides being opportunistic pathogens, the resilient bacteria of the genus Enterococcus are key factors contributing to the ripening, flavour, and the organoleptic properties of fermented food products. The ubiquitous nature of enterococci derives from a number of features, which can be intrinsic to the genus or specific to some species or even strains. These traits allow probing the environment in order to adapt, enabling a survival and fitness advantage. They are encoded in numerous genes that can be easily transferable due to the high genomic promiscuity of enterococci. These genes have been ascribed a role in virulence as they are relevant to different stages of the bacterial infection process, including adhesion, colonization, invasion, evasion of the immune system and spread through the hostʼs tissues. Enterococcal virulence factors can be either secreted (cytolysin, proteases, hyaluronidase, superoxide), surface associated (enterococcal surface protein, aggregation substance, extracellular polymeric substances, pilin gene clusters, enterococcal microbial surface component recognizing adhesive matrix molecules), or intracellular. At the time this thesis work began, researchers were starting to realize that virulence factors in enterococcal clinical isolates were also present in isolates from other environments, in particular, where enterococci play beneficial roles, namely food. Since dissemination of virulence factors among food isolates was no longer crucial, other issues started to become relevant in the still debated enterococcal virulence.(...)