Streptococcus
Introduction
Gram-positive cocci in chains or pairs.Facultative anaerobes, non-motile, non-spore forming, catalase-negative.Normal flora of upper respiratory tract, skin, intestine, genital tract.Causes pyogenic, toxigenic, and immune-mediated diseases.
Streptococci Species
1.β-hemolytic Streptococci
Group A: S. pyogenes
Group B: S. agalactiae
2.α-hemolytic Streptococci
S. pneumoniae (pneumococcus)
Viridans streptococci (e.g., S. mutans, S. mitis)
3.γ-hemolytic (non-hemolytic)
Mostly Enterococcus (previously Group D).
Morphology
Gram-positive cocci, 0.5–1 µm, arranged in chains.
Non-capsulated (except S. pneumoniae).
Some strains show capsule (antiphagocytic).
Culture Media
Blood agar: Hemolysis patterns (α = green, β = clear zone, γ = none).
Chocolate agar: Growth enhanced.
MacConkey agar: No growth.
Enrichment broth: Todd-Hewitt broth for group B.
Biochemical Reactions
Catalase test: Negative (differentiates from Staphylococcus).
Bacitracin sensitivity: Group A (S. pyogenes) sensitive.
CAMP test: Group B (S. agalactiae) positive.
Optochin sensitivity: S. pneumoniae sensitive, Viridans resistant.
Bile solubility: Positive for S. pneumoniae.
Resistance
Increasing resistance to macrolides (erythromycin), tetracyclines.
Penicillin resistance emerging (especially in S. pneumoniae due to altered PBPs).
Antigen Structure
Group-specific carbohydrate (C-substance) – Lancefield classification.
M protein (Group A S. pyogenes) – antiphagocytic, major virulence factor.
Capsular polysaccharide (S. pneumoniae) – antiphagocytic.
Lipoteichoic acid – adherence.
Toxins & Enzymes
Hemolysins: Streptolysin O (oxygen-labile, antigenic), Streptolysin S (oxygen-stable, non-antigenic).
Streptokinase: Fibrinolysis.
Hyaluronidase: Tissue spread.
DNase.
Pyrogenic exotoxins: Superantigens → scarlet fever, streptococcal toxic shock.
Pathogenesis
Pyogenic infections: Pharyngitis, cellulitis, impetigo, erysipelas.
Toxin-mediated: Scarlet fever, streptococcal toxic shock syndrome.
Immune-mediated (post-infectious):
Acute rheumatic fever (molecular mimicry with M protein).
Acute glomerulonephritis.
Antibiotic Sensitivity
Penicillin = drug of choice (Group A, B, Viridans).
Alternatives: Macrolides, clindamycin.
S. pneumoniae – penicillin or ceftriaxone; resistant strains → vancomycin, levofloxacin.
Epidemiology
Reservoir: Human throat, skin, genital tract.
Transmission: Droplets, direct contact.
Risk groups: Children (pharyngitis), neonates (S. agalactiae sepsis), immunocompromised.
Laboratory Diagnosis
Specimens: Throat swab, pus, CSF, urine, blood.
Microscopy: Gram-positive cocci in chains.
Culture: Blood agar → hemolysis.
Rapid antigen detection: For S. pyogenes.
Serology: ASO titer (antistreptolysin O) – post-streptococcal diseases.
Optochin, bile solubility for S. pneumoniae.
Treatment
Penicillin G/V = DOC.
S. pneumoniae – ceftriaxone, vancomycin for resistant strains.
Supportive: Drain abscesses, manage complications.
Related Posts
Growth Media in Microbiology – Types, Examples & Uses
Growth Media in Microbiology – Types, Uses, and Examples Introduction…
Staphylococcus
Staphylococcus Introduction Staphylococcus is a genus of Gram-positive cocci that…
