Enterococcus faecalis and VRE Infection Models

Enterococcus faecalis (E. faecalis) is a Gram-positive coccus and a normal member of the gastrointestinal flora of humans and animals. Hardy and adaptable, it tolerates a wide range of temperature, pH, and salt, and survives on surfaces and through disinfection. It persists readily in the healthcare environment, where it turns opportunistic.  E. faecalis is a common cause of urinary tract infection, bacteremia, wound infection, and intra-abdominal infection. It is a leading cause of enterococcal infective endocarditis, and an occasional cause of meningitis. 

Its clinical difficulty comes from resistance: enterococci are intrinsically resistant to several antibiotic classes, and the acquisition of vancomycin resistance – the vancomycin-resistant enterococci, or VRE – removes one of the drugs of last resort. The CDC classifies vancomycin-resistant Enterococcus as a serious threat, and vancomycin-resistant Enterococcus faecium, the closely related species, sits in the high-priority tier of the WHO’s 2024 Bacterial Priority Pathogens List. 

TransPharm Preclinical Solutions evaluates candidates against E. faecalis in vivo across four validated infection models and produces the quantitative CFU burden and survival data a development program needs.

The scientific problem in an E. faecalis study is matching the challenge to a pathogen defined by its resistance. The gut is both the commensal niche and the launching point for infection – vancomycin-resistant enterococci colonize the intestine before they invade – so a colonization model and a systemic model answer different, complementary questions. Strain choice is central: enterococci range from susceptible isolates, for which ampicillin remains effective, to vancomycin-resistant strains that require last-line agents, and E. faecalis differs from E. faecium in that it more often retains ampicillin susceptibility, so the species and resistance profile of the challenge strain both matter.

The validated reference strain, ATCC 700802 (V583), is itself a vancomycin-resistant clinical isolate, which anchors the models to the resistant disease that drives the need for new agents. TransPharm Preclinical Solutions maintains an on-site library of hundreds of validated pathogens and can source or validate characterized E. faecalis isolates, including defined resistant phenotypes, to a program’s specifications.

Every study parameter is configurable to the scientific question: challenge strain, host species and strain, route of administration, comparator agents, and dosing schedule. The core readouts are quantitative bacterial burden (CFU) in target tissues, organs, and blood, with survival and mortality tracking where applicable.

The following are available as add-ons to any study:

• Pharmacokinetic profiling and PK/PD analysis
• Blood chemistry
• Additional tissue harvests
• Histopathology

The gut is the enterococcal reservoir, and intestinal colonization by vancomycin-resistant E. faecalis is the step that precedes invasive disease and enables spread – which makes the gastrointestinal model directly relevant to strategies aimed at decolonization or at preventing invasion. The resident microbiota is disrupted, typically with an antibiotic regimen, and the animal is then challenged by oral gavage. Test article may be administered orally or perirectally, by subcutaneous, intravenous, or intraperitoneal injection, or via diet or drinking water. Endpoints include mortality tracking, clinical observations, weight tracking, and CFU burden in feces, organs, and/or blood. Validated in mice. E. faecalis isolates can be validated on request.

E. faecalis is a frequent component of intra-abdominal infection, and the peritonitis model supports both survival- and burden-based readouts. Animals are challenged by intraperitoneal injection, often with gastric mucin to help establish infection. Test article may be administered by oral gavage or by subcutaneous, intravenous, or intraperitoneal injection. Endpoints include mortality tracking and CFU burden in harvested organs and/or blood. Validated in mice and rats. Validated E. faecalis isolates ATCC 700802.

E. faecalis is a common cause of wound and surgical-site infection, and the soft tissue (thigh) model establishes a localized infection in the muscle by intramuscular injection into one or both quadriceps, giving a quantifiable burden well suited to PK/PD and dose-ranging work. Test article may be administered by oral gavage or by subcutaneous, intravenous, or intraperitoneal injection. Endpoints include CFU burden at the infected site and in harvested organs and/or blood, with mortality tracking. Validated in mice. Validated E. faecalis isolates ATCC 700802.

Enterococcal bacteremia is a common healthcare-associated bloodstream infection and the gateway to endocarditis and deep-seated disease. The bacteremia model screens systemic activity and ranks candidates on survival and on bacterial burden in blood and organs. Animals are challenged by intravenous injection in the lateral tail vein. Test article may be administered by oral gavage or by subcutaneous, intravenous, or intraperitoneal injection. Endpoints include survival (mortality tracking) and CFU burden in harvested organs and/or blood. Validated in mice, with rats available on request. Validated E. faecalis isolate: ATCC 700802.

In vitro susceptibility testing complements the in vivo models. E. faecalis MIC is determined by broth microdilution per CLSI, with vancomycin resistance and high-level aminoglycoside resistance characterized, and can be paired with MBC, time-kill, and checkerboard analysis – the last of these relevant to the combination regimens (for example, a cell-wall agent with an aminoglycoside) that enterococcal disease often requires.

Enterococci are difficult to treat because two kinds of resistance stack on top of each other. The first is intrinsic: E. faecalis is inherently poorly susceptible to cephalosporins, to clindamycin, and to low concentrations of aminoglycosides, which narrows the useful drugs before any resistance is acquired. The second is acquired, and the headline is vancomycin resistance – the vanA and vanB operons remodel the peptidoglycan precursor from its normal D-alanyl-D-alanine terminus to D-alanyl-D-lactate, so that vancomycin can no longer bind.

High-level aminoglycoside resistance, which abolishes the synergy that cell-wall agents and aminoglycosides otherwise provide, compounds the problem, and resistance to the last-line agents daptomycin and linezolid has emerged. E. faecalis more often remains ampicillin-susceptible than E. faecium does, so ampicillin-based therapy is frequently still an option – which is exactly why the species and susceptibility profile of the challenge strain determine what an efficacy study can claim.