Streptococcus

Streptococcus is a genus of Gram-positive, non-spore-forming cocci that occur in pairs or chains. They are facultative anaerobes or microaerophiles and are catalase-negative, distinctions that help separate them from staphylococci.

Streptococci are classified into Lancefield groups (A through W) based on cell-wall polysaccharide antigens. The most medically important groups are A (S. pyogenes) and B (S. agalactiae); Streptococcus pneumoniae belongs to a separate pneumococcus lineage and is not assigned to a Lancefield group. Many viridans streptococci lack a group antigen.

They inhabit human mucosal surfaces and the skin and are transmitted mainly by respiratory droplets or close contact. They include species such as S. pyogenes, S. agalactiae, S. pneumoniae, and various commensal viridans streptococci (e.g., S. mutans, S. anginosus).

Key virulence factors include the M protein (group A), capsule (pneumococcus), streptolysins, and hyaluronidase, enabling immune evasion and tissue invasion. These factors underlie diverse diseases from throat infections to severe invasive disease.

Clinical syndromes include pharyngitis, impetigo and cellulitis, scarlet fever, rheumatic fever, post-streptococcal glomerulonephritis, pneumonia, meningitis, otitis media, and dental caries. Neonatal sepsis and meningitis are classically caused by group B streptococci.

Diagnosis relies on culture on blood agar showing beta-hemolysis for many groups and Lancefield grouping; S. pneumoniae is alpha-hemolytic and optochin sensitive. Treatment is usually penicillin or its derivatives, with alternatives for resistant strains. Vaccines are available for S. pneumoniae; no universal vaccine covers all Streptococcus species.