Enterococcus faecalis

Multi drug resistance

E. faecalis is usually resistant to many commonly used antimicrobial agents (aminoglycosides, aztreonam and quinolones).^[15] The resistance is mediated by the presence of multiple genes related to drug resistance in the chromosome or plasmid.^[3]

Resistance to vancomycin in E. faecalis is becoming more common.^[16][17] Treatment options for vancomycin-resistant E. faecalis include nitrofurantoin (in the case of uncomplicated UTIs),^[18] linezolid, quinupristin, tigecycline^[15] and daptomycin, although ampicillin is preferred if the bacteria are susceptible.^[19] Quinupristin/dalfopristin can be used to treat Enterococcus faecium but not E. faecalis.^[19]

In root-canal treatments, NaOCl and chlorhexidine (CHX) are used to fight E. faecalis before isolating the canal. However, recent studies determined that NaOCl or CHX showed low ability to eliminate E. faecalis.^[20]

Development of antibiotic resistance

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Combined drug therapies

According to one study combined drug therapy has shown some efficacy in cases of severe infections (e.g. heart valves infections) against susceptible strains of E. faecalis. Ampicillin- and vancomycin-sensitive E. faecalis (lacking high-level resistance to aminoglycosides) strains can be treated by gentamicin and ampicillin antibiotics. A less nephrotoxic combination of ampicillin and ceftriaxone (even though E. faecalis is resistant to cephalosporins, ceftriaxone is working synergistically with ampicillin) may be used alternatively for ampicillin-susceptible E. faecalis.^[21]

Daptomycin or linezolid may also show efficacy in case ampicillin and vancomycin resistance.^[21]

A combination of penicillin and streptomycin therapy was used in the past.^[21]

Tedizolid, telavancin, dalbavancin, and oritavancin antibiotics are FDA approved as treatments against EF.^[15]

DNA repair

In human blood, E. faecalis is subjected to conditions that damage its DNA, but this damage can be tolerated by the use of DNA repair processes.^[26] This damage tolerance depends, in part, on the two protein complex RexAB, encoded by the E. faecalis genome, that is employed in the recombinational repair of DNA double-strand breaks.^[26]

Biofilm formation

The ability of E. faecalis to form biofilms contributes to its capacity to survive in extreme environments, and facilitates its involvement in persistent bacterial infection, particularly in the case of multi-drug resistant strains.^[27] Biofilm formation in E. faecalis is associated with DNA release, and such release has emerged as a fundamental aspect of biofilm formation.^[27] Conjugative plasmid DNA transfer in E. faecalis is enhanced by the release of peptide sex pheromones.^[28]

Indicators of recreational water quality

Because E. faecalis is a common fecal bacterium in humans, recreational water facilities (such as swimming pools and beaches that allow visitors to swim in the ocean) often measure the concentrations of E. faecalis to assess the quality of their water. The higher the concentration, the worse the quality of the water. The practice of using E. faecalis as a quality indicator is recommended by the World Health Organization (WHO) as well as many developed countries after multiple studies have reported that higher concentrations of E. faecalis correlate to greater percentages of swimmer illness. This correlation exists in both freshwater and marine environments, so measuring E. faecalis concentrations to determine water quality applies to all recreational waters. However, the correlation does not imply that E. faecalis is the ultimate cause of swimmer illnesses. One alternative explanation is that higher levels of E. faecalis correspond to higher levels of human viruses, which cause sickness in swimmers. Although this claim may sound plausible, there is currently little evidence that establishes the link between E. faecalis and human virus (or other pathogens) levels. Thus, despite the strong correlation between E. faecalis and water quality, more research is needed to determine the causal relationship of this correlation.^[36]

Human shedding

For recreational waters near or at beaches, E. faecalis can come from multiple sources, such as the sand and human bodies. Determining the sources of E. faecalis is crucial for controlling water contamination, though often the sources are non-point (for example, human bathers). As such, one study looked at how much E. faecalis is shed from bathers at the beach. The first group of participants immersed themselves in a large pool with marine water for 4 cycles of 15 minutes, both with and without contacting sand beforehand. The result shows a decrease in E. faecalis levels for each cycle, suggesting that people shed the most bacteria when they first get into a pool. The second group of participants entered small, individual pools after contact with beach sand, and researchers collected data on how much E. faecalis in the pool came from the sand brought by the participants and how much came from the participants’ shedding. The result shows that E. faecalis from the sand is very small compared to that from human shedding. Although this result may not apply to all sand types, a tentative conclusion is that human shedding is a major non-point source of E. faecalis in recreational waters.^[37]