Streptococci Overview and Characteristics

Gram-Positive Cocci: Streptococci Overview

• Gram-positive spherical cells arranged in chains or pairs.
• Colonies appear grayish, translucent to slightly opaque; pinpoint in size.
• Catalase negative and oxidase negative.
• Growth is enhanced by blood, serum, or glucose in agar plates.

Growth Characteristics

• Behave as facultative anaerobes; some species require increased CO2.
• Aerotolerant anaerobes; Peptostreptococci are obligate anaerobes.
• Young cultures exhibit a capsule, and they are common in normal human flora.

Serological Classification

• All streptococci except viridans group have C carbohydrate for serological classification.
• When they access sterile sites, they can cause serious infections, notably S. pyogenes and S. pneumoniae.

Colony Variation

• Variants include mucoid, matte or rough (high CHON), and smooth/glossy (low CHON).

Classification Based on Temperature

• Pyogenic Group: β-hemolytic, does not grow at 10°C or 45°C (e.g., S. pyogenes).
• Viridans Group: Grows at 45°C, not part of Lancefield group, alpha or non-hemolytic (e.g., S. salivarius).
• Enterococcus: Grows at both 10°C and 45°C; normal flora of the intestine (e.g., E. faecalis).
• Lactic Group: Grows at 10°C and is non-hemolytic; often found in dairy (e.g., S. lactis).

Classification Based on Hemolytic Patterns

• Alpha-Hemolytic: Partial hemolysis; green discoloration (e.g., S. pneumoniae).
• Beta-Hemolytic: Complete hemolysis; clear zone (e.g., S. pyogenes, S. agalactiae).
• Gamma-Hemolytic: No lysis; unchanged surrounding red cells (e.g., E. faecalis).

Lancefield Classification

• Based on antigenic nature of cell wall carbohydrates (C polysaccharides).
• Group classifications include:
  • α: None - S. pneumoniae
  • β: A - S. pyogenes, B - S. agalactiae
  • γ: D - Enterococcus spp.

Group A Streptococci

• Pathogenic; not part of normal flora; transmitted via respiratory droplets (e.g., S. pyogenes).
• M-protein serves as a principal virulence factor and is anti-phagocytic.
• Other factors include:
  • Protein F for cell attachment.
  • Lipoteichoic acid for adherence to respiratory epithelium.

Enzymes and Toxins of S. pyogenes

• Streptolysin O: Oxygen-labile, antigenic; responsible for subsurface hemolysis; lysis of WBCs.
• Streptolysin S: Oxygen-stable; surface hemolysis.
• Deoxyribonucleases: Hydrolyzes DNA.
• Streptokinase: Activates plasminogen for movement from clotted areas.
• Hyaluronidase: Spreads the organism by separating tissue.
• Pyrogenic toxins: Serotypes A, B, C; cause rashes and other symptoms.

Infections Associated with S. pyogenes

• Pharyngitis/Tonsillitis: Known as "strep throat."
• Scarlet Fever: Resulting from an S. pyogenes throat infection with erythrogenic toxin; features a characteristic rash and "strawberry tongue."
• Skin Infections: Includes cellulitis and erysipelas, with potential progression to necrotizing fasciitis and myositis.
• Rheumatic Fever and Acute Glomerulonephritis: Both are serious complications.

Diagnostic Tests

• Bacitracin Disk Test: Differentiates S. pyogenes, with positive results showing any zone of inhibition.
• Sulfamethoxazole-Trimethoprim Test (SXT): Positive with zones of inhibition.
• PYR Test: Specific for Group A; positive results yield a cherry-red color.

Group B Streptococci

• Normal flora of the female genital tract; can be opportunistic.
• Notable species include E. faecalis and S. agalactiae.

Viridans Streptococci

• Alpha-hemolytic, lack Lancefield antigen, opportunistic pathogens.
• Associated infections include subacute endocarditis and dental carries, particularly with S. mutans.

Abiotrophia

• Pyridoxal-dependent organisms; known as nutritionally variant streptococci.
• Associated with a low virulence but can cause endocarditis.

Streptococcus pneumoniae

• Diplococci; major cause of bacterial pneumonia and meningitis globally.
• Key virulence factor is the antiphagocytic capsular polysaccharide.
• Characteristic colonies range from mucoid young colonies to hard, dimpled older colonies.

Other Infections Caused by S. pneumoniae

• Includes pneumonia, meningitis, otitis media, bacteremia, endocarditis, and peritonitis.

Differential Tests for S. pneumoniae

• Optochin Susceptibility Test: Differentiates pneumococcus from viridans streptococci.
• Bile Solubility Test: Confirms pneumococcus by showing broth clearing after incubation.