Abstract
Enterococci, recognized as opportunistic pathogens, are natural inhabitants of the oral cavity, normal intestinal microflora, and female genital tract of both human and animals. They are common nosocomial agents that infect the urinary tract, bloodstream, intra-abdominal and pelvic regions, surgical sites and central nervous system (Murray & Weinstock, 1999; Richards et al., 2000). Enterococcus faecalis is the most common enterococci species, and it is responsible for 80–90 % of human enterococcal infections (Jett et al., 1994; Jones et al., 2004). Enterococcus faecium accounts for the remainder of infections caused by enterococci spp. (Jett et al., 1994).
Biofilm is a population of cells attached irreversibly on various biotic and abiotic surfaces, and encased in a hydrated matrix of exopolymeric substances, proteins, polysaccharides and nucleic acids (Costerton, 2001). Biofilm formation is a complex developmental process involving attachment and immobilization on a surface, cell-to-cell interaction, microcolony formation, formation of a confluent biofilm, and development of a three-dimensional biofilm structure (O'Toole et al., 2000). Bacteria in a biofilm behave differently from their free-floating (planktonic) counterparts. The regulation of bacterial gene expression in response to cell population density, called quorum sensing, is accomplished through the production of extracellular signal molecules called autoinducers (Miller & Bassler, 2001). Biofilm production is regulated by quorum sensing systems in several bacterial pathogens. Biofilms are notoriously difficult to eradicate and are a source of many chronic infections. According to the National Institutes of Health, biofilms are medically important, accounting for over 80 % of microbial infections in the body (Lewis, 2001). A mature biofilm can tolerate antibiotics at concentrations of 10–1000 times more than are required to kill planktonic bacteria. Bacteria in biofilms are resistant to phagocytosis, making biofilms extremely difficult to eradicate from living hosts (Lewis, 2001). Bacteria in biofilms colonize a wide variety of medical devices, such as catheters, artificial cardiac pacemakers, prosthetic heart valves and orthopaedic appliances, and are associated with several human diseases, such as native valve endocarditis, burn wound infections, chronic otitis media with effusion and cystic fibrosis (Costerton et al., 1999). Enterococci in biofilms are more highly resistant to antibiotics than planktonically growing enterococci, thus the potential impact of biofilm formation could be significant.
Enterococci have also been reported as important organisms in periodontal infection (Molander et al., 1998; Peciuliene et al., 2000). The adherence (Joyanes et al., 1999, 2000) and production of a biofilm (Baldassarri et al., 2001; Distel et al., 2002; Mohamed et al., 2003, 2004; Toledo-Arana et al., 2001) by E. faecalis and E. faecium on different biomaterials have been demonstrated, and the capacity of enterococci to bind to various medical devices, such as ureteral stents (Keane et al., 1994), intravascular catheters (Sandoe et al., 2003), biliary stents (Dowidar et al., 1991) and silicone gastrostomy devices (Dautle et al., 2003), has been associated with the ability of enterococci to produce biofilms. Biofilm formation by E. faecalis on ocular lens materials, such as polymethymethacrylate, silicone and acrylic, has been documented (Kobayakawa et al., 2005). In this review, we discuss recent advances in the biology and genetics of biofilm formation by E. faecalis and E. faecium, and the role of the biofilm in enterococci pathogenesis.
The epidemiology of biofilm formation by E. Faecalis and E. Faecium
The prevalence of biofilm production varies worldwide. In Rome, Italy, 80 % of E. faecalis and 48 % of E. faecium isolates from infected patients were able to form biofilms (Baldassarri et al., 2001). In Pamplona, Spain, 57 % of E. faecalis isolates derived from various clinical isolates produced biofilms (Toledo-Arana et al., 2001). In Sardinia, Italy, biofilm production was identified among 87 % of E. faecalis clinical isolates and 16 % of E. faecium clinical isolates (Dupre et al., 2003). In the UK, among 109 enterococcal bloodstream isolates studied, 100 % of E. faecalis and 42 % of E. faecium isolates produced biofilms. E. faecalis isolates from intravascular catheter-related bloodstream infections (CRBI) have been found to produce more biofilm than enterococcal isolates that cause non-CRBI (Sandoe et al., 2003). In the United States, Mohamed et al. (2004) reported that 93 % of E. faecalis strains (51 isolates from outside the United States) identified from clinical and faecal isolates produced biofilms. In the same study, E. faecalis endocarditis isolates were found to produce more biofilm than non-endocarditis isolates (Mohamed et al., 2004). Biofilm-producing enterococcal isolates were characterized by the quantity of biofilm produced (i.e. strong, medium, weak or non-biofilm producer) with an optical density (OD570) classification (Mohamed et al., 2004; Toledo-Arana et al., 2001). In Okayama, Japan, Seno et al. (2005) reported that all of 352 E. faecalis isolates derived from urinary tract infections were capable of producing biofilms. In Poland, 59 % of E. faecalis isolates collected from clinical specimens produced biofilms (Dworniczek et al., 2005). A study from a tertiary care hospital in India showed that 44 of the 171 isolates (26 %) of E. faecalis and none of the 25 E. faecium isolates produced biofilms (Prakash, 2005). In Rome, Italy, among a collection of 52 E. faecalis isolates from orthopaedic infections 96 % produced biofilms (Baldassarri et al., 2006). Other investigators have reported similar results and suggest that E. faecalis (95 %) isolates produce a biofilm more often than E. faecium (29 %) (Di Rosa et al., 2006). Collectively, these data suggest that E. faecalis produces biofilm more often than E. faecium, and that biofilm formation may be an important factor in the pathogenesis of enterococcal infection.