Microbiology & infectious diseases

Gram positive bacteria

Enterococcus



Last author update: 19 June 2025
Last staff update: 19 June 2025

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PubMed Search: Enterococcus

Megan Amerson-Brown, M.L.S., Ph.D.
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Cite this page: Amerson-Brown M. Enterococcus. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/microbiologyenterococcus.html. Accessed August 31st, 2025.
Definition / general
  • Common Gram positive opportunistic bacterium
  • Taxonomy (NCBI: Taxonomy Browser - Enterococcus [Accessed 25 February 2025])
    • Order: Lactobacillales
    • Family: Enterococcaceae
    • Genus: Enterococcus
    • > 60 species
      • Examples of clinically relevant species include E. faecalis, E. faecium, E. casseliflavus, E. gallinarum, E. avium, E. raffinosus
Essential features
  • Catalase negative, Gram positive cocci in pairs and chains
  • Opportunistic pathogen in immunocompromised persons and indwelling devices
  • Vancomycin resistance can be intrinsic or acquired and is species dependent
  • Surveillance testing for vancomycin resistant Enterococcus is recommended in high risk patient populations (e.g., transplant) (Antimicrob Steward Healthc Epidemiol 2021;1:e41)
Epidemiology
Sites
Pathophysiology
Clinical features
  • vanA transferable gene, confers resistance to vancomycin and teicoplanin
    • Most often associated with E. faecium
  • vanB transferable gene, confers resistance to vancomycin
    • Most often associated with E. faecalis, found in other bacteria
  • vanC chromosomally encoded, confers low level resistance to vancomycin
    • E. casseliflavus and E. gallinarum
  • vanA / vanB are transferrable and are therefore an infection prevention concern, unlike vanC, which is not transferrable
  • vanA / vanB gene clusters act by altering the bacterial cell wall structure and lowering the affinity for vancomycin in VRE
  • Reference: StatPearls: Vancomycin-Resistant Enterococci [Accessed 19 February 2025]
Diagnosis
  • Culture based detection methods
  • Nucleic acid amplification testing and 16S sequencing
  • vanA and vanB can be detected by molecular methods, e.g., PCR (polymerase chain reaction) and LAMP (loop mediated isothermal amplification)
  • Reference: Biomed Res Int 2022;2022:4384196
Laboratory
  • Aerobic growth in routine culture conditions 37 °C
  • Small, gray, round, wet colonies
  • Catalase negative
  • Leucine aminopeptidase (LAP) positive
  • Pyrrolidonyl arylamidase (PYR) positive
  • Motility positive (only E. casselifalvus and E. gallinarum)
  • Yellow pigment (e.g., E. casseliflavus, E. gilvus, E. plantarum, E. ureilyticus)
  • Tellurite positive (E. faecalis only)
  • Variable hemolysis on blood agar: alpha, beta or gamma
  • Esculin hydrolysis positive
  • Will grow in the presence of bile and 6.5% NaCl
  • Some species express streptococcal group D antigen
  • Chromogenic agar for surveillance screening
  • Identification by matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry
  • Reference: Carroll: Manual of Clinical Microbiology, 13th Edition, 2023
Case reports
Treatment
  • Enterococcus can harbor beta lactamases and altered penicillin biding proteins (PBP) that render penicillin and ampicillin ineffective
  • Enterococci are intrinsically resistant to aminoglycosides, trimethoprim sulfamethoxazole and cephalosporins
  • E. gallinarum and E. casseliflavus are intrinsically resistant to vancomycin
  • Vancomycin resistant enterococci are frequently treated with linezolid, dalbavancin, oritavancin, new generation tetracyclines or combination therapy with beta lactams and high level aminoglycosides
  • For urinary tract infections, nitrofurantoin susceptibility is variable
  • E. faecium is more likely to be resistant to ampicillin and vancomycin than E. faecalis
  • Penicillin, ampicillin or amoxicillin can be used as monotherapy, if susceptible
  • References: UpToDate: Treatment of Enterococcal Infections [Accessed 19 February 2025], CLSI: Performance Standards for Antimicrobial Susceptibility Testing [Accessed 19 February 2025]
Clinical images

Contributed by Megan Amerson-Brown, M.L.S., Ph.D.
E. casseliflavus on blood agar

E. casseliflavus on blood agar

E. faecalis on blood agar E. faecalis on blood agar

E. faecalis on blood agar

Microscopic (histologic) description
  • Gram positive cocci in pairs and chains
Microscopic (histologic) images

Contributed by Megan Amerson-Brown, M.L.S., Ph.D.
Acridine orange stain of Enterococcus species

Acridine orange stain of Enterococcus species

Enterococcus species in blood culture

Enterococcus species in blood culture

E. faecium Gram stain from a colony

E. faecium Gram stain from a colony

Gram positive cocci

Gram positive cocci

Molecular / cytogenetics description
  • 16S gene sequencing
    • May not be sufficient to differentiate between Enterococcus species
  • Enterococcus species and resistance genes are identified by some rapid molecular based syndromic panels
  • Nucleic acid amplification testing (PCR or LAMP) for vanA and vanB genes from rectal swabs for surveillance testing
Differential diagnosis
Practice question #1

The image above shows Gram positive cocci in pairs and chains growing on 5% sheep blood agar. The organism is catalase negative, leucine aminopeptidase (LAP) positive, pyrrolidonyl arylamidase (PYR) positive, grows in the presence of bile in culture media and is tellurite positive. Which organism is this?

  1. Aerococcus urinae
  2. Enterococcus faecalis
  3. Streptococcus pyogenes
  4. Viridans group streptococci
Practice answer #1
B. Enterococcus faecalis. While all the other organisms are Gram positive cocci in pairs and chains and catalase negative, E. faecalis is the only one that is LAP, PYR and tellurite positive and grows in the presence of bile in culture media. Answer A is incorrect because Aerococcus urinae is Gram positive cocci in clusters. Answer C is incorrect because S. pyogenes is beta hemolytic. The image indicates an alpha hemolytic organism. Answer D is incorrect because viridans group streptococci is PYR negative.

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Reference: Enterococcus
Practice question #2
Which Enterococcus species listed below is considered to be intrinsically resistant to vancomycin?

  1. Enterococcus avium
  2. Enterococcus faecalis
  3. Enterococcus faecium
  4. Enterococcus gallinarum
Practice answer #2
D. Enterococcus gallinarum. E. gallinarum carries vanC on its chromosome that encodes for low level constitutive resistance. Answer A is incorrect because E. avium is rarely vancomycin resistant. Answers B and C are incorrect because E. faecalis and E. faecium can carry vanA and vanB genes; however, these are carried on mobile genetic elements and not always present.

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Reference: Enterococcus
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