Enterococcus faecium: Introduction, Morphology, Pathogenicity

Introduction

Table of Contents

Enterococcus faecium is a bacterium that belongs to the Enterococcus genus, which is a part of the Firmicutes phylum. It is a Gram-positive, facultative anaerobic coccus, typically occurring in pairs or short chains. Enterococci are commensal bacteria commonly found in the gastrointestinal tract of humans and animals.

While E. faecium is a normal inhabitant of the human intestinal flora, it has also gained importance as an opportunistic pathogen associated with various infections, especially in hospital settings. In recent years, it has become a major concern due to its increasing resistance to multiple antibiotics, making it challenging to treat infections caused by this bacterium.

Some key characteristics of Enterococcus faecium include:

Clinical Significance: E. faecium can cause various infections, such as urinary tract infections, bloodstream infections (bacteremia), endocarditis, and intra-abdominal infections. It is a common cause of hospital-acquired infections, particularly in patients with compromised immune systems or those undergoing invasive medical procedures.
Antibiotic Resistance: It has developed resistance to multiple antibiotics, including vancomycin, which is often considered a last-resort drug for treating serious bacterial infections. The emergence of vancomycin-resistant Enterococcus (VRE) strains is a significant public health concern.
Healthcare-Associated Infections: It is frequently associated with healthcare settings, where it can spread easily between patients due to its ability to survive on surfaces and medical equipment.
Nosocomial Outbreaks: In hospitals and long-term care facilities, E. faecium outbreaks have been reported, especially in intensive care units (ICUs) and surgical wards.
Intrinsic and Acquired Resistance: E. faecium possesses intrinsic resistance to many antibiotics, and it can acquire additional resistance genes through horizontal gene transfer, contributing to its multidrug resistance profile.
Biofilm Formation: Enterococcus faecium can form biofilms on medical devices, such as catheters and prosthetic heart valves, making it challenging to eradicate infections associated with these devices.

Morphology

Enterococcus faecium is a Gram-positive bacterium with a characteristic morphology. Here are some key features of the morphology of E. faecium:

Shape: It appears as small, spherical or ovoid-shaped cells. It is a coccus, meaning it has a round shape.
Arrangement: In microscopic examination, E. faecium cells are often seen as single cells, occurring in pairs (diplococci), or short chains (streptococci).
Gram Staining: Enterococcus faecium stains purple in the Gram stain procedure, indicating that it is Gram-positive. This means it has a thick peptidoglycan layer in its cell wall.
Capsule: Some strains of E. faecium may produce a slimy capsule around the cell, which aids in protection and adherence to surfaces.
Motility: It is generally non-motile, lacking flagella for movement.
Spore Formation: E. faecium is non-spore-forming, meaning it does not produce endospores.
Size: The size of Enterococcus faecium cells typically ranges from 0.5 to 1.0 micrometers in diameter.
Growth on Agar: On nutrient agar or blood agar plates, Enterococcus faecium colonies appear smooth, convex, and off-white to light gray in color.
Catalase Test: Enterococcus faecium is catalase-negative, meaning it does not produce the enzyme catalase.

Fig. Gram positive cocci in singles, pairs and chains of Enterococcus faecium in Gram staining of culture microscopy

Pathogenicity

Enterococcus faecium is considered an opportunistic pathogen, meaning it generally does not cause disease in healthy individuals but can lead to infections in patients with compromised immune systems or other predisposing factors. The pathogenicity of E. faecium is primarily attributed to its ability to:

Antibiotic Resistance: One of the most significant aspects of E. faecium‘s pathogenicity is its increasing resistance to multiple antibiotics. This resistance can limit treatment options and lead to difficult-to-treat infections, especially in healthcare settings where antibiotic use is prevalent.
Intrinsic Virulence Factors: It possesses intrinsic virulence factors that contribute to its ability to cause infections. These factors may include adherence mechanisms, colonization factors, and biofilm formation, which allow the bacterium to persist and establish infections on various surfaces and medical devices.
Nosocomial Infections: It is frequently associated with healthcare-associated infections, particularly in hospitals and long-term care facilities. It can easily spread between patients through contaminated hands, medical equipment, and environmental surfaces.
Invasive Infections: E. faecium is capable of causing various infections, such as urinary tract infections, bloodstream infections (bacteremia), intra-abdominal infections, and endocarditis. In immunocompromised individuals, these infections can be severe and life-threatening.
Biofilm Formation: It can form biofilms on medical devices, such as catheters and prosthetic heart valves. Biofilms provide protection from the host’s immune system and antimicrobial agents, leading to persistent infections.
Acquisition of Resistance Genes: E. faecium has the ability to acquire resistance genes through horizontal gene transfer, allowing it to develop multidrug resistance and challenge the effectiveness of antibiotic therapy.
Immunosuppression: In patients with weakened immune systems or underlying medical conditions, Enterococcus faecium can take advantage of the host’s reduced ability to fend off infections.
Association with Surgery: E. faecium infections have been linked to surgical procedures, particularly in patients with surgical site infections and postoperative complications.

Lab Diagnosis

The laboratory diagnosis of Enterococcus faecium involves several steps to accurately identify the bacterium and determine its antibiotic susceptibility. Here is an overview of the typical laboratory procedures used for the diagnosis:

Enterococcus faecium in wet mount of Tryptone soy broth microscopy at a magnification of 1600X — Fig. *Enterococcus faecium* in wet mount of Tryptone soy broth microscopy at a magnification of 1600X

Sample Collection: The first step is to collect the clinical specimen, such as blood, urine, wound swabs, or other body fluids, from the patient suspected to have an E. faecium infection.
Gram Stain: A Gram stain is performed on the clinical specimen to provide initial information about the bacterial morphology and help narrow down the possible pathogens.
Culture: The collected specimen is then streaked onto appropriate culture media, such as blood agar or MacConkey agar, and incubated at the appropriate temperature (usually 35-37°C) to promote the growth of bacteria.
Identification: Once colonies have grown, they are examined for specific characteristics indicative of Enterococcus faecium. These include Gram-positive, catalase-negative, and positive reactions for certain biochemical tests like pyrrolidonyl arylamidase (PYR) and esculin hydrolysis.
Bile Esculin Test: Enterococcus faecium typically hydrolyzes esculin in the presence of bile, producing a black-brown color. This test helps differentiate Enterococcus species from other bacteria.
Antibiotic Susceptibility Testing: The antibiotic susceptibility of E. faecium is determined using methods such as disk diffusion or broth microdilution. Commonly tested antibiotics include vancomycin, ampicillin, gentamicin, and others. This is crucial in detecting potential antibiotic-resistant strains, such as Vancomycin-resistant Enterococcus (VRE).
Molecular Testing: In some cases, further confirmation and characterization of Enterococcus faecium may involve molecular testing, such as polymerase chain reaction (PCR) or DNA sequencing. This can be useful in detecting specific virulence or resistance genes.
Serotyping: For research or epidemiological purposes, serotyping methods may be used to identify specific surface antigens on Enterococcus faecium strains.

Treatment

The treatment of Enterococcus faecium infections can be challenging due to the bacterium’s increasing resistance to multiple antibiotics, including those considered as last-resort options. The choice of treatment depends on the specific infection site, severity of the infection, and the susceptibility profile of the isolated E. faecium strain. Here are some general considerations for the treatment:

Antibiotic Susceptibility Testing: It is crucial to perform antibiotic susceptibility testing on the isolated E. faecium strain to guide appropriate antibiotic therapy. This helps identify which antibiotics are effective against the specific infection.
Vancomycin-Susceptible Strains: In cases where Enterococcus faecium is susceptible to vancomycin, this antibiotic remains one of the primary treatment options. The dosage and administration of vancomycin will depend on the severity of the infection and the patient’s individual characteristics.
Linezolid: Linezolid is an alternative antibiotic option for the treatment of Enterococcus faecium infections, particularly in cases where the bacterium is resistant to vancomycin.
Daptomycin: Daptomycin is another antibiotic that can be considered for the treatment of certain Enterococcus faecium infections, especially if the strain is susceptible to this drug.
Combination Therapy: In severe or complicated infections, combination therapy with multiple antibiotics may be necessary to improve treatment efficacy. However, the use of combination therapy should be carefully evaluated, considering potential adverse effects and drug interactions.
Ampicillin and Ceftriaxone: In some cases, E. faecium may still be susceptible to ampicillin or ceftriaxone, and these antibiotics may be used as part of the treatment regimen.
Avoid Aminoglycosides Alone: It is generally not recommended to use aminoglycoside antibiotics (e.g., gentamicin) alone for the treatment of Enterococcus faecium infections due to increasing resistance and concerns about efficacy.
Tigecycline and Quinupristin/Dalfopristin: In certain situations, tigecycline or quinupristin/dalfopristin may be considered as salvage therapy for E. faecium infections, particularly if other treatment options are limited.

Prevention

Preventing the spread of Enterococcus faecium, particularly antibiotic-resistant strains, is essential to reduce healthcare-associated infections and protect public health. Here are some preventive measures:

Infection Control Practices: Implement and adhere to rigorous infection control practices in healthcare settings. This includes proper hand hygiene, environmental cleaning, and the use of personal protective equipment (e.g., gloves and gowns) when caring for patients with E. faecium infections.
Antibiotic Stewardship: Promote judicious use of antibiotics to minimize the development of antibiotic-resistant strains. Use antibiotics only when necessary, follow evidence-based guidelines for treatment, and avoid inappropriate use or overuse of broad-spectrum antibiotics.
Isolation Precautions: If a patient is colonized or infected with a multidrug-resistant strain of E. faecium, consider placing the patient on contact precautions to prevent transmission to other patients.
Screening and Surveillance: Consider active surveillance for high-risk patients, such as those admitted to intensive care units (ICUs), long-term care facilities, or patients with a history of E. faecium infection. This helps identify carriers and implement appropriate infection control measures.
Environmental Cleaning: Regularly clean and disinfect frequently-touched surfaces and medical equipment in healthcare settings to prevent cross-contamination.
Patient Education: Educate patients and their families about the importance of hand hygiene and infection prevention practices, especially for patients with weakened immune systems or those undergoing invasive medical procedures.
Catheter Care: Properly insert and care for indwelling medical devices, such as urinary catheters and central venous catheters, to reduce the risk of infection.
Hospital Design: Consider hospital design and engineering practices that minimize the potential for cross-contamination and facilitate proper infection control measures.
Decolonization: In some cases, decolonization procedures may be considered for patients at high risk of infection or transmission. This may involve using antimicrobial agents or antiseptic washes to reduce the number of bacteria present in the patient’s body.
Surveillance of Antimicrobial Resistance: Regularly monitor and analyze data on antimicrobial resistance patterns to detect trends and identify outbreaks of antibiotic-resistant E. faecium strains.
Research and Development: Invest in research and development efforts to identify new antibiotics and alternative treatment options for E. faecium infections.

Keynotes

Here are some keynotes on Enterococcus faecium:

Bacterium: E. faecium is a Gram-positive bacterium belonging to the Enterococcus genus, commonly found in the human gastrointestinal tract.
Opportunistic Pathogen: It is an opportunistic pathogen, causing infections primarily in immunocompromised individuals or those with underlying health conditions.
Healthcare-Associated Infections: It is a significant cause of healthcare-associated infections, often associated with hospital settings and long-term care facilities.
Infections: It can cause various infections, including urinary tract infections, bloodstream infections (bacteremia), intra-abdominal infections, and endocarditis.
Antibiotic Resistance: It is known for its increasing resistance to multiple antibiotics, including vancomycin, making treatment challenging.
Vancomycin-Resistant Enterococcus (VRE): Some strains of E. faecium are classified as VRE due to their resistance to vancomycin, posing a serious public health concern.
Biofilm Formation: It has the ability to form biofilms on medical devices, contributing to its persistence and resistance to antimicrobial agents.
Infection Control: Strict infection control measures are crucial to prevent the spread of E. faecium in healthcare settings.
Antibiotic Stewardship: Promoting appropriate antibiotic use and antibiotic stewardship programs are essential to combat antibiotic resistance.
Laboratory Diagnosis: Identification and antibiotic susceptibility testing are critical in managing Enterococcus faecium infections effectively.
Research and Development: Continuous research efforts are needed to develop new antibiotics and alternative treatment strategies to combat infections caused by E. faecium.
Public Health Concern: The emergence of antibiotic-resistant strains of E. faecium poses a significant public health challenge, warranting comprehensive surveillance and control efforts.

Enterococcus faecium: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes