Streptococcus pyogenes Diseases
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Direct answer
Streptococcus pyogenes, or group A streptococcus, is a beta-haemolytic, bacitracin-sensitive, PYR-positive Gram-positive coccus that causes a wide range of suppurative and toxin-mediated diseases. Pharyngitis, impetigo and erysipelas are the common presentations, while necrotising fasciitis and streptococcal toxic shock syndrome are rarer and more dangerous. Penicillin remains the drug of choice, with no clinically established resistance.
What you must remember
- Identification: beta-haemolytic colonies on blood agar, bacitracin sensitivity, PYR positivity and Lancefield group A antigen confirm the organism.
- Pharyngitis: sore throat with fever and tender cervical nodes, treated with penicillin V for ten days to prevent acute rheumatic fever.
- Impetigo: superficial infection with honey-coloured crusts, often with Staphylococcus aureus as a co-pathogen, and a recognised trigger of post-streptococcal glomerulonephritis.
- Erysipelas: a sharply demarcated, raised, red plaque with fever, typically on the face or legs.
- Necrotising fasciitis: pain out of proportion to the visible signs, spread along fascial planes and rapid tissue necrosis that needs urgent surgical debridement.
- Streptococcal toxic shock syndrome: streptococcal pyrogenic exotoxins act as superantigens, causing hypotension and multi-organ failure, often with a necrotising focus.
- Virulence: M protein resists phagocytosis and defines the serotype, the hyaluronic acid capsule resembles human connective tissue, and streptolysins O and S, streptokinase and DNase B aid spread.
Sorting the diseases by hallmark
Group A streptococcal disease ranges from a mild throat infection to a fulminant necrotising illness, and the hallmark of each disease helps place it correctly. The table summarises the usual presentations and the key lesson for each.
| Disease | Hallmark | Key point |
|---|---|---|
| Pharyngitis | Sore throat, tender nodes | Penicillin for 10 days |
| Impetigo | Honey-coloured crusts | Post-streptococcal nephritis risk |
| Erysipelas | Raised, sharp-edged plaque | Facial or leg involvement |
| Scarlet fever | Sandpaper rash, strawberry tongue | Erythrogenic toxin |
| Necrotising fasciitis | Pain out of proportion | Surgical debridement |
| Streptococcal toxic shock | Shock with organ failure | Superantigen, clindamycin used |
The table shows that the same organism causes both superficial and deep disease. Recognising the hallmark quickly matters, because the need for surgery and the duration of antibiotic therapy depend on which presentation is in front of the clinician. Superficial disease usually settles with penicillin alone, whereas a deep or rapidly spreading lesion needs prompt surgical assessment as well as antibiotics.
Where the exam tries to catch you
Group A streptococcal questions often test the duration of therapy and the need for surgery.
- Trap: assuming every red throat with a sore throat needs antibiotics Correct: only a confirmed group A infection, by rapid antigen test or culture, justifies penicillin, since viral pharyngitis is much more common.
- Trap: giving a short course for group A pharyngitis Correct: a full ten-day course of penicillin is required to eradicate the organism and prevent acute rheumatic fever.
- Trap: treating necrotising fasciitis with antibiotics alone Correct: urgent surgical debridement is the key intervention, with antibiotics as an adjunct that covers the organism.
Frequently asked questions
What is the drug of choice for group A streptococcal pharyngitis?
Penicillin is the drug of choice for group A streptococcal pharyngitis, usually given as penicillin V for ten days. A first-generation cephalosporin suits non-anaphylactic penicillin allergy, while clindamycin or a macrolide is used in severe allergy, and macrolide resistance varies by region.
Which tests distinguish Streptococcus pyogenes from other beta-haemolytic streptococci?
Bacitracin sensitivity together with a positive PYR test and Lancefield grouping distinguishes Streptococcus pyogenes from other beta-haemolytic streptococci. The organism is bacitracin sensitive and PYR positive, and a rapid antigen test can detect the Lancefield group A antigen directly.
Why does necrotising fasciitis need surgery as well as antibiotics?
Necrotising fasciitis needs surgery because the infection spreads along fascial planes with tissue death that antibiotics cannot reach. Early debridement removes dead tissue and limits toxin spread, while antibiotics cover the organism and help suppress toxin production.
What causes the rash of scarlet fever?
The rash of scarlet fever is caused by erythrogenic toxins released by group A streptococci, which produce a diffuse, sandpaper-like erythema. The rash is accompanied by fever, and the tongue often becomes strawberry red as the illness progresses.