Learn About Streptococcus pyogenes — A Pathogenic Bacterium.
Streptococcus pyogenes (S. pyogenes) is a species of Gram-positive, aerotolerant bacteria belonging to the Streptococcus genus. It is also known as group A Streptococcus (GAS) because it possesses the Lancefield group A antigen on its cell wall. It can cause various infections ranging from mild to severe and life-threatening.
Morphology
• S. pyogenes comprises non-motile and non-sporing cocci (round cells) that tend to link in chains.
• It is Gram-positive, meaning it has a thick peptidoglycan layer in its cell wall that retains the purple stain in the Gram staining procedure.
• It is aerotolerant, meaning it can survive in the presence or absence of oxygen.
• It produces beta-hemolysis on blood agar plates, which means it can completely lyse red blood cells and produce clear zones around the colonies.
Cultural Characteristics
• S. pyogenes grows best at 37 °C and pH 7.52.
• It requires enriched media such as blood or chocolate agar for optimal growth.
• It is catalase-negative and does not produce bubbles when exposed to hydrogen peroxide2.
• It is PYR-positive, which produces red colour when incubated with a substrate called L-pyrrolidonyl-beta-naphthylamide.
Incubation Period
• The incubation period of S. pyogenes infections differs depending on the nature and site of the infection.
• For pharyngitis (throat infection), the incubation period is usually 1 to 3 days.
• For skin infections such as impetigo or cellulitis, the incubation period is usually 4 to 10 days.
• For invasive infections such as necrotising fasciitis or toxic shock syndrome, the Incubation period is usually less than 24 hours.
Biochemical Reactions
• S. pyogenes can ferment glucose, lactose, maltose, and sucrose, producing acid but not gas.
• It can hydrolyse esculin and hippurate, producing black and purple, respectively.
• It can produce hyaluronidase, streptokinase, streptolysin O and S, and erythrogenic toxin, which are virulence factors that help the bacteria invade tissues and evade host defences.
Symptoms
• The symptoms of S. pyogenes infections depend on the type and site of infection.
• For pharyngitis, the symptoms may include sore throat, fever, headache, swollen lymph nodes, and white patches on the tonsils.
• For skin infections, the symptoms may include redness, swelling, pain, pus, blisters, and crusts on the affected area.
• For invasive infections, the symptoms may include severe pain, fever, shock, organ failure, tissue necrosis, and death.
Diagnosis
The diagnosis of S. pyogenes infections can be made by various methods such as culture, rapid antigen detection test (RADT), polymerase chain reaction (PCR), serology, and clinical criteria.
· Culture is the standard gold method that isolates and identifies the bacteria from specimens such as throat swabs or wound exudates using selective and differential media such as blood agar or bacitracin disks. S. pyogenes produces beta-haemolytic colonies that are sensitive to bacitracin.
· RADT is a quick method that involves detecting the group A antigen from throat swabs using an enzyme immunoassay or a lateral flow assay. This test has a high specificity but a low sensitivity, so negative results should be confirmed by culture.
· PCR is a sensitive and specific method that involves amplifying and detecting the DNA of the bacteria from specimens such as throat swabs or blood using primers that target specific genes such as emm or spyCEP. This test can also provide information on the bacteria’s strain type and virulence factors.
· Serology is a method that involves measuring the antibody response to the bacteria in serum samples using tests such as anti-streptolysin O (ASO) or anti-DNase B (ADB). This test helps diagnose post-streptococcal diseases such as acute rheumatic fever or post-streptococcal glomerulonephritis but not for acute infections.
Treatment
Treating S. pyogenes infections depends on the type and severity of the infection.
· Oral antibiotics such as penicillin or amoxicillin are usually effective for mild to moderate infections such as pharyngitis or skin infections. The duration of treatment may vary from 7 to 14 days, depending on the clinical response.
· For severe or invasive infections such as necrotising fasciitis or toxic shock syndrome, intravenous antibiotics such as penicillin plus clindamycin or vancomycin are usually required156. The duration of treatment may vary from 14 to 28 days, depending on the clinical response and the extent of tissue damage.
· For patients allergic to penicillin, alternative antibiotics such as erythromycin, azithromycin, cephalexin, or clindamycin may be used depending on the type and site of infection.
· Additional anti-inflammatory drugs, steroids, immunosuppressants, or dialysis may be required for patients with complications such as acute rheumatic fever or post-streptococcal glomerulonephritis.
