Unit-6.2-Streptococcaceae-Clinical-Bacteriology-Sham

Beta Hemolysis, Alpha Hemolysis, Gamma Hemolysis

  • Beta Hemolysis: Complete lysis of red blood cells (RBCs), indicated by a clear zone surrounding bacterial colonies on blood agar.

  • Alpha Hemolysis: Partial lysis of RBCs, causing a greenish discoloration of the agar around the colonies.

  • Gamma Hemolysis: No lysis of RBCs, no color change around the colonies.

Classification of Streptococci and Related Organisms

Family: Streptococcaceae

  • Streptococcus and Enterococcus belong to this family.

  • Catalase-negative, gram-positive cocci, usually found in pairs or chains.

    • Weak false-positive catalase reactions can occur from blood media due to hemoglobin peroxidase activity.

General Characteristics

  • Cells tend to form chains in broth cultures.

  • Most members are facultative anaerobes; some species are capnophilic, needing increased carbon dioxide (CO2) concentrations for optimal growth.

Bergey’s Academic Classification

Physiological Divisions of Streptococcus

  • Pyogenic: Grows at 11-44°C; example: Streptococcus pyogenes.

  • Viridans: Grows at 45°C but not 100°C; examples: S. salivarius, S. mutans, S. sanguis, S. anginosus.

  • Enterococci: Grows at both 45°C and 100°C; example: Streptococcus faecalis.

  • Lactic: Grows at 100°C but not 45°C; examples: S. cremoris, S. lactis.

Lancefield’s Classification

  • Based on the presence of serologically active C-carbohydrate polysaccharides.

  • Groups identified through Rebecca Lancefield’s work (1933).

Smith and Brown Classification

  • Based on hemolytic reactions on Blood Agar Plates.

Hemolytic Patterns

  • Alpha: Partial hemolysis, green discoloration; examples: S. pneumoniae, S. viridans.

  • Beta: Complete hemolysis, clear zones; examples: S. pyogenes, S. agalactiae.

  • Gamma: Non-hemolytic; examples: S. faecalis.

Classification Table 15-2

Species

Lancefield Group

Hemolysis

Common Terms

Disease Associations

S. pyogenes

A

B

Group A streptococci

Rheumatic fever, scarlet fever, pharyngitis, glomerulonephritis, pyogenic infections

S. agalactiae

B

B

Group B streptococci

Neonatal sepsis, meningitis, puerperal fever, pyogenic infections

S. dysgalactiae, S. equi

C

B

S. bovis group

Notable infections include pyogenic infections

E. faecalis, E. faecium

D

n/a

Nonenterococcus

UTIs, pyogenic infections

S. pneumoniae

n/a

α

Pneumococcus

Pneumonia, meningitis, pyogenic infections

Viridans streptococci

no group

α

Various groups

Dental caries, bacteremia

Group A Streptococci: Streptococcus pyogenes

Antigenic Structure

  • Similar to other gram-positive bacteria; unique group antigen (Lancefield group A).

Virulence Factors

  1. M Protein: Encoded by emm genes; resists phagocytosis and aids in adherence.

    • Over 80 serotypes exist, with M1 being common in pharyngitis.

  2. Adhesion Molecules: Lipoteichoic acid and fibronectin binding protein secure adherence to epithelial cells.

  3. Hyaluronic Acid Capsule: Prevents opsonized phagocytosis and camouflages bacterial antigens.

  4. Streptolysin O (SLO): Causes hemolysis, immunogenic and measured by antistreptolysin O (ASO) test.

  5. Streptolysin S (SLS): Oxygen-stable hemolysin, non-immunogenic.

  6. Streptococcal Pyrogenic Exotoxins: Cause symptoms of scarlet fever and act as superantigens (SpeA, SpeB, SpeC, SpeF).

Clinical Infections

Bacterial Pharyngitis

  • Most common cause of bacterial pharyngitis seen predominantly in children (5-15 years).

Pyodermal Infections

  • Include impetigo, cellulitis, erysipelas, and more.

Necrotizing Fasciitis

  • Rapidly progressing invasive infection, often termed "flesh-eating disease."

Streptococcal Toxic Shock Syndrome

  • Systemic condition potentially leading to death linked to M-protein toxins.

Poststreptococcal Sequelae

  1. Rheumatic Fever: Follows streptococcal pharyngitis, leads to inflammatory responses.

  2. Acute Glomerulonephritis: Immunologically mediated, usually post-infection.

Specimen Collection

  • Oropharyngeal Swab: Remains viable for 48-72 hours on dry swabs but should be cultured within 4 hours.

Cultural Characteristics

  • Colonies appear transparent to translucent, convex, circular, and exhibit a wide zone of beta-hemolysis.

Laboratory Diagnosis

Tests for Identification

  1. Bacitracin Test: Differentiates Group A from other beta-hemolytic streptococci.

  2. PYR Test: Detects hydrolysis of L-pyrrolidonyl-beta-napthylamide.

  3. Dick’s Test: Evaluates susceptibility to scarlet fever based on toxins.

  4. Schultz-Charlton Reaction: Determines the presence of erythrogenic toxin effects.

  5. Treatment: Penicillin or alternatives for penicillin-allergic patients.

Group B Streptococci: Streptococcus agalactiae

Virulence Factors

  1. Sialic Acid Capsule: Prevents phagocytosis; loss of virulence without it.

  2. CAMP Factor: Pore-forming protein aiding in pathogenicity.

  3. Neuraminidase: Invasiveness facilitator.

Disease Association

  • Main cause of neonatal sepsis and death in infants; colonization can occur in a significant percentage of pregnant women.

Laboratory Diagnosis for Group B Streptococci

  1. Culture Media: Todd-Hewitt broth, StrepB Carrot Broth to promote growth.

  2. CAMP Test: Typical arrowhead hemolysis indicates positive identification.

Alpha-Hemolytic: Streptococcus pneumoniae

Clinical Significance

  • Common cause of pneumonia and bacterial meningitis; presents with a multitude of virulence factors such as neuraminidase and pneumolysin O.

Pathogenesis

  • Key features include proteases for colonization and unique autolytic properties of colonies.

Laboratory Diagnosis

  1. Optochin Test: Distinguishes S. pneumoniae from viridans streptococci based on inhibition zones.

  2. Bile Solubility Test: Determines strain viability in bile.

  3. Inulin Fermentation Test: Assesses presence of fermentative ability in differentiating species.

  4. Neufeld Quellung Reaction: Provides capsular swelling evidence when exposed to anti-sera.

Gamma-Hemolytic: Enterococci

Characteristics

  • Enterococci possess group D antigen and are significant in various infections, primarily in immunocompromised individuals.

Treatment Protocols

  • Generally resistant to many penicillin variants, treatment often involves combinations of penicillin and aminoglycosides.