The genus Streptococcus describes a large group of Gram-positive, spherical bacteria common in the human microbial community, yet responsible for a wide range of infections. They characteristically arrange themselves in pairs or chains, giving the genus its name, which derives from the Greek word for “twisted chain.” While many species live harmlessly on the skin, in the mouth, or in the digestive tract, certain strains cause significant disease. Understanding the specific types of these bacteria and the infections they cause is fundamental to managing the health risks they present.
Categorizing Streptococcus
Classification relies on two primary systems used by health professionals to identify species and predict disease potential. The first is the Lancefield grouping, based on carbohydrate antigens on the bacterial cell wall. This system assigns letter codes, with Group A, Group B, and Group C/G being the most important in human medicine.
The second method uses hemolytic properties, describing the bacteria’s ability to break down red blood cells on a blood agar plate. Beta-hemolysis indicates the complete destruction of red cells, leaving a clear zone. Alpha-hemolysis is a partial breakdown producing a greenish discoloration, while gamma-hemolysis means no breakdown occurs.
The most clinically relevant species include Group A Streptococcus (GAS), or Streptococcus pyogenes, a beta-hemolytic organism responsible for many common infections. Group B Streptococcus (GBS), or Streptococcus agalactiae, is also beta-hemolytic and causes severe infection in newborns. Streptococcus pneumoniae (pneumococcus) is an alpha-hemolytic bacterium not assigned a Lancefield group, but it is a leading cause of various respiratory and systemic infections.
Common Acute Infections and Symptoms
Group A Streptococcus (GAS) causes common, non-invasive illnesses, most notably streptococcal pharyngitis (strep throat). Symptoms typically have a sudden onset and include a sore throat, fever, and sometimes white patches on the tonsils. GAS also causes skin infections such as impetigo (crusty, scabbing sores) and scarlet fever, which is characterized by a distinctive sandpaper-like red rash.
GAS can also cause severe, invasive infections when the bacteria enter the bloodstream or deep tissues. These invasive diseases include necrotizing fasciitis, a rapidly spreading infection that destroys muscle and fat tissue. Streptococcal Toxic Shock Syndrome (STSS) is another acute emergency, presenting with fever, a sudden drop in blood pressure, and organ failure.
Streptococcus pneumoniae is the primary bacterial cause of community-acquired pneumonia, leading to fever, cough, and difficulty breathing. Pneumococcus also frequently causes acute sinusitis and otitis media (middle ear infections). These infections involve inflammation and fluid buildup in the sinuses or behind the eardrum.
Group B Streptococcus (GBS) is significant due to its impact on infants. In newborns, GBS causes early-onset infections, usually within the first week of life, presenting as sepsis or meningitis. This occurs when the baby is exposed during birth. GBS also causes invasive infections in adults, particularly those over 65 or with underlying health issues, leading to bacteremia, endocarditis, or bone infections.
Transmission, Diagnosis, and Treatment
Streptococcal bacteria spread through several routes depending on the species and infection site. Group A Streptococcus is highly contagious, primarily transmitted through respiratory droplets from coughing or sneezing. Direct contact with infected skin lesions or nasal secretions is another common transmission route.
Diagnosis begins with a clinical assessment, but confirmation requires laboratory testing. For suspected strep throat, a rapid strep test (RADT) quickly detects the GAS antigen from a throat swab. If the rapid test is negative, a throat culture may be performed, especially in children, to ensure the infection is not missed. More severe or invasive infections necessitate culturing the bacteria from sterile sites like blood or cerebrospinal fluid.
The standard treatment for most streptococcal infections is a course of antibiotics, with penicillin or amoxicillin being the first-line choices. These medications are highly effective against the bacteria and help reduce the duration of symptoms. Patients must complete the entire course of antibiotics as prescribed to fully eradicate the bacteria and prevent potential complications.
Long-Term Health Impacts and Prevention
A significant concern with Group A Streptococcus infections is the risk of developing non-suppurative, immune-mediated complications after the acute infection has cleared. The immune system’s response can mistakenly attack the body’s own tissues through molecular mimicry. This can lead to Acute Rheumatic Fever (ARF), a serious condition that damages the joints, skin, and heart valves, potentially resulting in Rheumatic Heart Disease (RHD).
Another delayed complication is Post-Streptococcal Glomerulonephritis (APSGN), causing inflammation in the kidneys. In children, a link has been observed between GAS infection and the sudden onset of neurological symptoms, known as Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections (PANDAS). These long-term sequelae underscore the importance of prompt diagnosis and treatment of acute strep infections.
Prevention strategies include consistent hygiene practices, such as frequent hand washing, which limits the spread of respiratory droplets and skin bacteria. For Streptococcus pneumoniae, vaccination is the most effective preventative measure, with pneumococcal vaccines (PCV13 and PPSV23) available to protect against common disease-causing strains. While an effective vaccine against Group A Streptococcus is still under development, timely antibiotic treatment remains the best defense against severe streptococcal disease.