Strep A vs. Strep B

What's the Difference?

Strep A and Strep B are both strains of the Streptococcus bacteria, but they differ in several key aspects. Strep A, also known as Streptococcus pyogenes, is responsible for a wide range of infections, including strep throat, scarlet fever, and skin infections. It can also cause more severe conditions like rheumatic fever and necrotizing fasciitis. On the other hand, Strep B, or Streptococcus agalactiae, primarily affects newborns and pregnant women. It can cause infections in the urinary tract, bloodstream, and soft tissues. Strep B is also a leading cause of meningitis in newborns. While both strains can be harmful, Strep A is generally more prevalent and associated with a broader range of infections, while Strep B is more specific to certain populations.


AttributeStrep AStrep B
Scientific NameStreptococcus pyogenesStreptococcus agalactiae
Gram StainPositivePositive
Virulence FactorsM Protein, Streptolysin O, StreptokinaseCapsule, CAMP Factor, Sialidase
DiseasesStrep throat, Scarlet fever, ImpetigoUrinary tract infections, Neonatal sepsis, Pneumonia
Antibiotic SensitivityPenicillin, ErythromycinPenicillin, Ampicillin

Further Detail


Streptococcus, commonly known as strep, is a group of bacteria that can cause various infections in humans. Among the different types of strep, Strep A (Streptococcus pyogenes) and Strep B (Streptococcus agalactiae) are two significant species that have distinct attributes. In this article, we will explore and compare the characteristics of Strep A and Strep B, including their prevalence, clinical manifestations, associated diseases, diagnostic methods, and treatment options.


Strep A is more prevalent in the general population compared to Strep B. It is estimated that Strep A colonizes the throat and skin of approximately 5-15% of healthy individuals, making it a common commensal bacterium. On the other hand, Strep B is found in the gastrointestinal and genitourinary tracts of about 10-30% of healthy adults, making it less prevalent than Strep A.

Clinical Manifestations

Strep A is notorious for causing a wide range of clinical manifestations, including pharyngitis (strep throat), skin infections (such as impetigo and cellulitis), scarlet fever, and invasive diseases like necrotizing fasciitis and streptococcal toxic shock syndrome. Strep B, on the other hand, primarily affects pregnant women and newborns. It can cause urinary tract infections, chorioamnionitis (infection of the fetal membranes), and neonatal sepsis, pneumonia, and meningitis.

Associated Diseases

Strep A is associated with several post-infectious complications, such as acute rheumatic fever (ARF) and post-streptococcal glomerulonephritis (PSGN). ARF can lead to inflammation of the heart, joints, skin, and nervous system, while PSGN affects the kidneys. Strep B, on the other hand, is primarily associated with infections in pregnant women and newborns, as mentioned earlier.

Diagnostic Methods

Both Strep A and Strep B can be diagnosed using various laboratory methods. Strep A is commonly identified through a rapid antigen detection test (RADT) or a throat culture. RADT provides quick results within minutes, while a throat culture takes longer but is more accurate. Strep B, on the other hand, is typically detected through a vaginal-rectal screening culture during late pregnancy to identify carriers. Additionally, Strep B can also be identified from clinical samples, such as urine or blood cultures, in cases of suspected infections in newborns.

Treatment Options

Strep A infections are usually treated with antibiotics, primarily penicillin or amoxicillin. In cases of penicillin allergy, alternative antibiotics like erythromycin or clindamycin may be prescribed. It is important to complete the full course of antibiotics to prevent complications and reduce the risk of antibiotic resistance. Strep B infections, on the other hand, are typically managed with intravenous antibiotics during labor for pregnant women to prevent transmission to the newborn. In newborns with suspected or confirmed Strep B infections, intravenous antibiotics are administered promptly.


In conclusion, Strep A and Strep B are two distinct species of streptococcal bacteria with different prevalence, clinical manifestations, associated diseases, diagnostic methods, and treatment options. While Strep A is more prevalent and causes a wide range of infections in the general population, Strep B primarily affects pregnant women and newborns. Understanding the attributes of these bacteria is crucial for accurate diagnosis, appropriate treatment, and prevention of complications associated with streptococcal infections.

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