Streptococci

Comprehensive Abstract

• Subject: Microbiology
• Instructor: Mazin Yashkur

Lecture Objectives

• Understand streptococci as normal flora and pathogens.
• Describe classification systems for streptococci.
• Explain laboratory diagnostic tests used for streptococci identification.
• Describe major species of streptococci along with their virulence factors.
• Explain the formation of dental plaque by Streptococcus mutans, with and without the presence of sucrose.

Definition of Streptococci

• Streptococci are:
  • Gram-positive cocci.
  • Arranged in chains or pairs.
  • Catalase negative (do not produce the enzyme catalase).
  • Facultative anaerobes (can grow in both the presence and absence of oxygen).
  • Many species are part of normal human flora.

Normal Flora Distribution

• Distribution of streptococci in the human body:
  • Oral cavity: Viridans streptococci, Streptococcus mutans.
  • Oropharynx: Streptococcus pneumoniae.
  • Skin & throat: Streptococcus pyogenes (present in small numbers).
  • Intestine: Enterococci.
  • Genital tract: Streptococcus agalactiae.

General Characteristics

• Streptococci exhibit the following characteristics:
  • Gram-positive cell wall.
  • Non-motile and non-spore forming organisms.
  • Fastidious organisms (require special nutrients to grow).
  • Grow well on blood agar (a rich growth medium).
  • The addition of CO₂ enhances their growth and hemolysis.

Hemolysis on Blood Agar

• Hemolysis patterns on blood agar can be classified into three types:
  • Alpha (α): Partial hemolysis, producing a green zone around colonies.
  • Examples: S. pneumoniae, Viridans streptococci.
  • Beta (β): Complete hemolysis, resulting in a clear zone around colonies.
  • Examples: S. pyogenes, S. agalactiae.
  • Gamma (γ): No hemolysis observed.
  • Examples: Enterococci.

Lancefield Classification

• Classification based on the C-carbohydrate antigen in the cell wall:
  • Group A: Streptococcus pyogenes.
  • Group B: Streptococcus agalactiae.
  • Group D: Enterococci.
  • Non-groupable: S. pneumoniae, Viridans streptococci.

Laboratory Diagnosis – Overview

• Diagnostic processes depend on the following factors:
  • Hemolysis pattern observed.
  • Specific biochemical tests utilized.
• Each hemolysis type requires different tests for accurate identification.

Bacitracin Test

• Principle: Group A streptococci are sensitive to bacitracin.
• Procedure:
  • Culture the organism on blood agar.
  • Apply a bacitracin disk to the agar.
  • Incubate for 24 hours.
• Result: A zone of inhibition indicates sensitivity, confirming S. pyogenes.

CAMP Test

• Principle: Group B streptococci produce CAMP factor, which enhances hemolysis in the presence of Staphylococcus aureus.
• Result: Arrow-shaped hemolysis indicates the presence of S. agalactiae.

Optochin Test

• Purpose: Used for alpha-hemolytic streptococci.
• Principle: S. pneumoniae is sensitive to optochin.
• Result: A zone of inhibition measuring ≥14 mm indicates S. pneumoniae; no inhibition suggests viridans streptococci.

Bile Solubility Test

• Principle: S. pneumoniae produces the enzyme autolysin, which is accelerated by bile salts.
• Result: A clear solution indicates S. pneumoniae; a turbid solution indicates viridans streptococci.

Summary of Laboratory Tests

Streptococcus pyogenes (Group A)

• Virulence Factors:
  • M protein (helps in evading the immune system).
  • Hyaluronic acid capsule (prevents phagocytosis).
  • Streptolysin O & S (leading to tissue damage).
  • Streptokinase (lyses blood clots).
• Diseases Caused:
  • Pharyngitis (strep throat).
  • Impetigo (skin infection).
  • Rheumatic fever (a non-suppurative complication).

Streptococcus agalactiae (Group B)

• Virulence Factors:
  • Polysaccharide capsule (important for evasion from host defenses).
  • CAMP factor (enhances hemolysis).
• Diseases Caused:
  • Neonatal sepsis (infection in newborns).
  • Neonatal meningitis (inflammation of the protective membranes covering the brain).

Streptococcus pneumoniae

• Virulence Factor:
  • Polysaccharide capsule (primary virulence factor).
• Identification:
  • Exhibits alpha hemolysis.
  • Sensitive to optochin.
  • Soluble in bile.
• Diseases Caused:
  • Pneumonia.
  • Meningitis.

Viridans Streptococci

• Characteristics:
  • Alpha-hemolytic.
  • Lacks a capsule.
  • Normal component of oral flora.
• Diseases Caused:
  • Dental caries (tooth decay).
  • Endocarditis (infection of the heart lining).

Streptococcus mutans

• Role: Normal oral flora.
• Significance: Major causative agent of dental caries.
• Characteristics:
  • Highly acidogenic (produces large amounts of acid).
  • Highly aciduric (tolerates acidic conditions).

Salivary Pellicle

• Definition: A thin protein layer derived from saliva.
• Function:
  • Covers tooth surfaces after cleaning.
  • Provides receptors for bacterial adhesion, promoting plaque formation.

Plaque Formation – WITHOUT Sucrose

• Sucrose-independent Mechanism:
  • Bacterial adhesion to the salivary pellicle.
  • Mediated by surface adhesins (Antigen I/II).
  • This attachment is weak and reversible.

Plaque Formation – WITH Sucrose

• Sucrose-dependent Mechanism:
  • Utilizes glucosyltransferase enzymes to convert sucrose into glucans.
  • This process leads to strong bacterial adhesion.
  • Formation of biofilm (dental plaque) occurs due to the accumulation of bacteria and dental materials.

Pathogenesis of Dental Caries

• Process details:
  • Biofilm traps acids produced during bacterial metabolism.
  • Lactic acid is produced, lowering the pH to below 5.5.
  • This low pH leads to the demineralization of enamel, resulting in cavities.
  • Progression of this process leads ultimately to dental caries.

Summary of Major Streptococci

Key Take Home Messages

• Many streptococci serve as normal flora in various body systems.
• The incidence of disease is dependent on virulence factors combined with environmental conditions.
• Correct laboratory tests are essential for accurate identification and management of streptococcal infections.
• S. mutans specifically contributes to caries development through biofilm formation.