Streptococcus is a genus of Gram‑positive, catalase‑negative cocci that typically arrange in chains or pairs and includes many commensal and pathogenic species found in humans and animals.
Explanation
Members of the genus Streptococcus are facultative anaerobes that do not form spores and are non‑motile. They divide along a single axis, producing characteristic chains in clinical smears. Streptococci are classified based on their hemolytic properties on blood agar and their cell wall carbohydrate antigens, known as Lancefield groups. Beta‑hemolytic species completely lyse red blood cells and include group A Streptococcus pyogenes and group B Streptococcus agalactiae. Alpha‑hemolytic species cause partial hemolysis and include Streptococcus pneumoniae and the viridans group streptococci, while gamma‑hemolytic species are non‑hemolytic. S. pyogenes expresses virulence factors such as the M protein and streptolysins, enabling it to cause pharyngitis, impetigo, erysipelas, scarlet fever and invasive diseases like necrotizing fasciitis and toxic shock–like syndrome. S. agalactiae colonizes the gastrointestinal and genital tracts and can cause neonatal sepsis and meningitis. S. pneumoniae has a polysaccharide capsule that impedes phagocytosis and is a leading cause of pneumonia, meningitis and otitis media. Viridans streptococci are part of the normal oral flora and are implicated in dental caries (S. mutans) and subacute bacterial endocarditis. The genus also includes groups C and G streptococci and Enterococcus species, which were reclassified from Streptococcus.
Notable species and diseases
Streptococcus pyogenes causes “strep throat,” cellulitis and rheumatic fever; rapid antigen detection tests and throat cultures aid diagnosis. Group B S. agalactiae is screened for in pregnant women to prevent neonatal disease. Streptococcus pneumoniae is responsible for community‑acquired pneumonia and can be prevented by conjugate and polysaccharide vaccines. Viridans streptococci, such as S. mutans and S. sobrinus, contribute to dental caries by producing acid from sugars, while S. sanguinis and related species may enter the bloodstream during dental procedures and cause endocarditis. Streptococcus bovis (now Streptococcus gallolyticus) bacteremia is associated with colonic neoplasms. Identification of streptococcal species uses hemolysis patterns, Lancefield grouping, biochemical tests and molecular methods.
Collectively, streptococci range from benign commensals to major human pathogens. Understanding their classification, virulence factors and clinical presentations is essential for accurate diagnosis and appropriate therapy.
Related Terms: Streptococcus pyogenes, Streptococcus pneumoniae, Lancefield group, Beta-hemolysis, Gram-positive cocci