Enterococci and VRE
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Direct answer
Enterococci are catalase-negative Gram-positive cocci that grow on bile-aesculin agar and in 6.5 per cent sodium chloride, and they cause hospital-acquired urinary, wound, bloodstream and endocardial infections. They are intrinsically resistant to cephalosporins and clindamycin. Vancomycin-resistant enterococci (VRE) carry vanA or vanB gene clusters, and the gene type decides whether teicoplanin remains usable.
What you must remember
- Identification: catalase negative, PYR and leucine aminopeptidase positive, growing on bile-aesculin agar and in 6.5 per cent sodium chloride, with Lancefield group D antigen in most isolates.
- Common species: Enterococcus faecalis is the commoner urinary isolate and is usually ampicillin susceptible, whereas E. faecium is more often ampicillin and vancomycin resistant.
- Intrinsic resistance: cephalosporins, clindamycin and low-level aminoglycosides, so a cephalosporin result is expected rather than alarming.
- High-level aminoglycoside resistance: it abolishes the synergy of cell-wall agents with gentamicin, which changes the plan for endocarditis and other serious infection.
- vanA: high-level resistance to vancomycin and teicoplanin, inducible and carried on mobile elements.
- vanB: vancomycin resistance with teicoplanin usually preserved, so teicoplanin can remain an option for vanB strains.
- Alternative drugs: linezolid and daptomycin are used for VRE, and nitrofurantoin is reserved for uncomplicated urinary isolates.
A stepwise route from isolate to therapy
A urine isolate from a catheterised patient in the medical ward raises questions about species, resistance pattern and infection control. Each step narrows the diagnosis and the drug choice.
- Confirm the genus: catalase negativity, bile-aesculin growth and salt tolerance support the identification, and MALDI-TOF or a biochemical panel confirms the species.
- Read the intrinsic pattern: cephalosporin resistance is expected in any enterococcus, so the team does not treat it as an unusual finding.
- Test ampicillin and vancomycin: ampicillin resistance is common in E. faecium, and a raised vancomycin MIC triggers a check for VRE.
- Identify the vancomycin gene: PCR for vanA and vanB separates high-level resistance from vanB resistance, which decides whether teicoplanin can be used.
- Match the site to the drug: uncomplicated cystitis is treated with nitrofurantoin, while endocarditis calls for ampicillin with ceftriaxone for E. faecalis, per current guidance.
- Apply infection control: contact precautions, dedicated equipment and hand hygiene are used, since VRE colonises the gut and survives on hospital surfaces.
Where the exam tries to catch you
Enterococci look like streptococci on the bench, and the resistance questions turn on a few precise facts.
- Trap: choosing a cephalosporin for an enterococcal infection because it is broad spectrum Correct: enterococci are intrinsically cephalosporin resistant, so the choice fails whatever the dose.
- Trap: assuming teicoplanin always works against VRE Correct: vanA strains resist teicoplanin as well, so the gene type matters before the drug is chosen.
- Trap: reading PYR positivity as proof of group A streptococcus Correct: enterococci are also PYR positive, so catalase testing must come first.
Frequently asked questions
Which tests identify enterococci in the laboratory?
Enterococci are identified by catalase negativity, growth on bile-aesculin agar and in 6.5 per cent sodium chloride, and positive PYR and leucine aminopeptidase reactions. Automated systems or MALDI-TOF mass spectrometry then confirm the species.
Why are enterococci resistant to cephalosporins?
Enterococci are resistant to cephalosporins because their penicillin-binding proteins have low affinity for these drugs. This intrinsic resistance is present whatever the dose, so cephalosporins should not be selected for enterococcal infection.
What is the difference between vanA and vanB in enterococci?
VanA gives high-level resistance to both vancomycin and teicoplanin, while vanB gives vancomycin resistance with teicoplanin usually remaining active. Both gene types alter the peptidoglycan target to D-alanine-D-lactate, which reduces glycopeptide binding.
Which drugs are used against vancomycin-resistant enterococci?
Linezolid and daptomycin are the main drugs used against vancomycin-resistant enterococci, and the choice depends on the site of infection and susceptibility results. Nitrofurantoin is reserved for uncomplicated urinary tract infection, where it reaches adequate urinary concentrations.