Microbiology
Pathogens
A 5 year old child is brought to ED with a widespread sandpaper-like rash and a sore tongue shown below. She has been off school over the last couple of days with a sore throat. What is the most likely causative pathogen:
(Image by Afag Azizova (Own work) [CC BY-SA 3.0 , via Wikimedia Commons)
Answer:
The image shows the classical (white) strawberry tongue seen in scarlet fever. Scarlet fever is an exotoxin-mediated reaction caused by Streptococcus pyogenes, characterised by fever, pharyngotonsillitis, an extensive red sandpaper-like rash, facial flushing with perioral sparing, strawberry tongue and cervical lymphadenopathy. Scarlet fever is a notifiable disease.Streptococcus Pyogenes
Microbiology / Pathogens / Streptococci and Staphylococci
Last Updated: 21st April 2019
| Microorganism | Streptococcus Pyogenes |
|---|---|
| Gram stain | Gram positive |
| Shape | Cocci (chains) |
| Catalase | Negative |
| Oxygen requirements | Facultative anaerobe |
| Haemolysis | Beta-haemolytic (Group A) |
| Additional features | Produces haemolytic exotoxin |
| Reservoir | Skin and nasopharynx |
| Common diseases | Acute tonsillitis, skin & soft tissue infections, necrotising fasciitis |
| Toxin-mediated reactions | Scarlet fever, toxic shock syndrome |
| Post-infectious immune-mediated reactions | Rheumatic fever, erythema nodosum, acute glomerulonephritis |
Transmission
Streptococcus pyogenes (Group A beta-haemolytic streptococci) is carried asymptomatically in the nasopharynx of 5 - 30% of the population, more commonly in children. Spread occurs via respiratory droplets or by direct contact with infected wounds on the skin.
Clinical Disease
Streptococcus pyogenes causes a wide range of infectious disease including:
- Local nasopharyngeal spread
- Pharyngitis
- Tonsillitis
- Peritonsillar abscess
- Sinusitis
- Otitis media
- Pneumonia (rarely)
- Meningitis
- Skin and soft tissue infection
- Impetigo
- Erysipelas
- Wound infections
- Necrotising fasciitis
- Septic arthritis
- Osteomyelitis
- Toxin-mediated reactions
- Scarlet fever
- Streptococcal toxic shock syndrome
- Puerperal fever
- Postinfectious immune-mediated sequelae
- Rheumatic fever
- Post-streptococcal glomerulonephritis
- Erythema nodosum
Treatment
Benzylpenicillin is usually the treatment of choice. Amoxicillin or phenoxymethylpenicillin may be used for oral therapy in less severe infections. Macrolides are an alternative for patients with allergy.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
