Enterococci are gram positive, catalase/oxidase negative, non-spore forming, facultative anaerobic bacteria which appear either as single or diplococci under the light microscope sometimes in short chains. Despite naturally occurring in the colon/bowel, oral cavity and the skin Enterococci has rapidly emerged from what used to be described as a harmless, medically unimportant microorganism to the third most common nosocomial pathogen with the potential to cause life threatening infections. There are currently more than thirty five known species, however according to the UK Health Protection Agency (HPA) the two most common isolated from almost 80% of clinical samples are E.faecalis and E.faecium (HPA 2009).
Enterococci are rather rugged and when outside the mammalian body can survive relatively harsh conditions therefore can be easily transmitted on inanimate surfaces. Despite their optimum growth temperature being 35°c the majority of species are able to grow between 10 and 45°c. In addition they can grow in up to pH9.6, in relatively saline conditions (6.5% NaCl) and can hydrolyse esculin in the presence of 40% bile salts. The ability to survive such conditions explains why it has become such a problem in healthcare settings. As it naturally inhabits the bowel, it can be easily transferred to toilet seats and door handles used by multiple patients, many of which are likely to be immunocompromised therefore susceptible to bacterial infection meaning hospitals can act as a 'hub'. As many more strains become resistant to antibiotics, detergents and other cleaning agent's even strict hospital cleaning and sanitizing regimes may not be sufficient to kill them, further hindering the problem and allowing transmission to and from surgical instruments and medical staff.
Their presence has also been noted in foodstuffs in particular those derived from animal origin such as milk, cheese, sausage and minced beef which has been partially attributed to their ability withstand heating at 60°c for 30 minutes, thus surviving the heating process (Foulquie et al 2006). For years Enterococcus spp. have been used in the food industry, both in the manufacturing process and as probiotics. Kuhn et al (2003) reported that agricultural crops treated with pig manure tested 100% positive for Enterococci, whilst those grown in the absence of animal fertilisers reduced numbers by approximately 77%. However one should be aware that not all strains isolated from foodstuffs possess the virulent nature which can lead to disease.
Enterococci are associated with a wide range of clinical manifestations including Bacteraemia, surgical site and soft issue wound infections, Urinary Tract Infections (UTI), neonatal sepsis, endocarditis, intra-abdominal/pelvic infections and meningitis (Butler 2006).
Virulence factors can be defined as molecules expressed or secreted by pathogenic bacteria to facilitate host colonization, entry into cells, evasion of the host immune system and the obtaining of nutrients key for growth and survival. In Enterococci some virulence genes are located on a 150kb pathogenicity island (PAI) first described by Huyche et al (1991), whilst others can be found on bacterial plasmids. Over the last decade much research has been carried out on the virulent properties of Enterococci however many of the mechanism are still incompletely defined.
Enterococcal Surface Protein (ESP)
ESP is a 200 kDa cell wall anchored protein found predominantly in clinical isolates of E.faecalis. It is thought to promote adhesion and colonization of host tissues as well as playing a role in the evasion of the host immune response. Shanker et al (2001) demonstrated the role of ESP in the adhesion and colonization of uroepithilial cells in ascending urinary tract infection (UTI). Several studies have also a role on biofilm formation (Arme et al 2001).
Aggregation Substance (AS)
AS is a 137 kDa surface glycoprotein which when expressed promotes clumping of bacterial cells. The gene is located on a plasmid regulated by a pheromone secreted from cells which do not possess the plasmid themselves. This promotes conjugative plasmid transfer to plasmid free cells via pili, which in addition have been shown to play a role in biofilm formation (Singh et al 2007). AS also promotes adherence to eukaryotic cells including renal epithelium, enterocytes and neutrophils. Ref 31 and 33 suggest aggregation of cells increases the hydrophobicity of the surface thereby assisting the bacteria in evasion of the host immune system.
BUTLER, Karina M (2006). Enterococcal Infection in Children. Seminars in Paediatric Infectious Diseases.
FOULQUIE MORENO, MR, SARANTINOPOULOS, P, TSAKALIDOU, E and DE VUYST, L (2006). The role and application of enterococci in food and health. Journal of International of Food Microbiology, 106 (1), 1-24.