Streptococci and enterococci !
- Bacteria
Gram-positive cocci in chains or pairs that cause anything from sore throat and pneumonia to newborn sepsis and endocarditis.
Updated 2026-11-01
What they are
These are catalase-negative Gram-positive cocci arranged in pairs or chains. They are sorted by haemolysis on blood agar (alpha, beta or gamma) and by Lancefield groups (A, B, D). Enterococci, once group D streptococci, are hardier: they grow in bile and salt and are intrinsically resistant to cephalosporins.
What they share
- Cocci in chains or pairs, catalase negative
- Beta-haemolytic: S. pyogenes and S. agalactiae
- Alpha-haemolytic: S. pneumoniae and viridans
- Enterococcus: grows on bile esculin and in 6.5 % NaCl
- Enterococcus: intrinsically resistant to cephalosporins
Microorganisms in this group
- Streptococcus pyogenesA throat and skin bacterium that causes strep throat, impetigo and necrotising fasciitis, and can leave rheumatic fever or kidney damage.Exam priority: high
- Streptococcus pneumoniaeA nose and throat bacterium behind most community pneumonia, ear infections, sinusitis and meningitis; a vaccine is in Colombia's PAI.Exam priority: high
- Streptococcus agalactiaeA bacterium that lives in the gut and vagina of many pregnant women and can cause sepsis, pneumonia and meningitis in the newborn.Exam priority: medium
- Viridans streptococciMouth streptococci that cause tooth decay and subacute endocarditis; S. gallolyticus is linked to colon cancer.Exam priority: medium
- EnterococcusGut cocci that cause urinary infection, endocarditis and hospital infections; some strains resist vancomycin (VRE).Exam priority: medium
Test yourself
Think of the answer, then tap the question to see it.
Which two tests separate the alpha-haemolytic streptococci?
Optochin and bile solubility: pneumococcus is sensitive and soluble; viridans streptococci are resistant and insoluble.
Which beta-haemolytic streptococcus is bacitracin sensitive?
Streptococcus pyogenes (group A); group B (S. agalactiae) is resistant.
Why do cephalosporins fail against enterococci?
Their penicillin-binding proteins have low affinity for cephalosporins: an intrinsic resistance.
What is the mechanism of enterococcal vancomycin resistance?
van genes (such as vanA) change the peptidoglycan end from D-Ala-D-Ala to D-Ala-D-Lac, which vancomycin binds poorly.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: About Group A Strep Infection (CDC, 2024); Clinical Overview of Pneumococcal Disease (CDC, 2026); Clinical Overview of Group B Strep Disease (CDC, 2024); Vancomycin-resistant Enterococci (VRE) Basics (CDC, 2024).
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Representative model, for educational purposes only.