Immunology_chapter_14

Chapter 14: Streptococcal Infections - Part One

Overview of Streptococcus pyogenes

• Common Cause: Most common cause of bacterial pharyngitis; causes strep throat.

• Clinical Manifestations:

  • Pyrogenic Exotoxin: Can lead to bacteriopharyngitis and scarlet fever (characterized by rash).

  • Impetigo: Most common cause of this skin infection.

  • Necrotizing Fasciitis: Severe infection that destroys tissue; may require surgery and can be fatal.

Post-Streptococcal Sequelae

• Definition: Diseases that occur after an untreated infection by S. pyogenes; results from antibodies targeting the heart or kidneys rather than direct bacterial presence.

• Conditions Include:

  • Post-streptococcal glomerulonephritis.

  • Toxic shock syndrome: Immune response to exotoxins causing a cytokine storm.

• Other Infections: Otitis media, sinusitis, osteomyelitis, septic arthritis, neonatal septicemia, pneumonia (rare).

Morphological Characterization of S. pyogenes

• Lancefield Group: Identified as Lancefield group A; based on C carbohydrate detection through agglutination tests.

• Hemolytic Activity: Typically beta hemolytic; identified by blood agar growth showing phenotypes.

• Gram Staining: Appear as gram-positive cocci in chains.

• Virulence Factors:

  • Fimbrae: Hair-like structures aiding adherence.

  • M Protein: Major virulence factor; inhibits phagocytosis. Essential for causing infections; more than 60 serotypes identified.

Extracellular Products of S. pyogenes

• Lyse Cells:

  • Streptolysin O: Oxygen-labile hemolysin, indicator of recent infection; binds red cell membranes leading to lysis.

  • Streptolysin S: Oxygen-stable hemolysin disrupting red cell membrane permeability.

  • Hyaluronidase: Breaks down connective tissue, facilitating bacterial spread.

  • Deoxyribonucleases: Breaks down DNA, aiding invasion.

  • Streptokinase: Converts plasminogen to plasmin; used as a therapeutic clot buster.

  • Erythrogenic Exotoxin: Associated with the rash of scarlet fever.

Pathogenicity and Carrier State

• Obligate Pathogen: Not normal flora, but can form a carrier state in some, including pets.

• Transmission: Carrier individuals can transmit the bacteria without showing symptoms.

• Rheumatic Fever Risk: Higher in school-age children, particularly in developing countries; complications include rheumatic heart disease and endocarditis.

Clinical Symptoms of Infection

• Upper Respiratory Infections: Symptoms include rhinorrhea, cough, fever, anorexia, pharyngeal erythema, and cervical adenopathy.

• Skin Infections:

  • Impetigo: Starts as red, itchy pustule, crusting to leave honey-colored lesions.

  • Cellulitis: Subcutaneous infection; area becomes warm, tender, and swollen.

• Scarlet Fever: Followed by pharyngitis, characterized by bright red rash due to pyrogenic exotoxin.

Autoimmune Complications

• Post-Streptococcal Conditions: Caused by autoimmune responses to streptococcal antigens; affects organs not directly infected.

  • Rheumatic Fever: Related to untreated S. pyogenes pharyngitis.

  • Glomerulonephritis: Associated with select M serotypes following skin/respiratory infections.

Chapter 14: Streptococcal Infections - Part Two

Diagnostic Evaluation for S. pyogenes

• Common Tests:

  • Thermocultures: Traditional throat swab on blood agar; observe for beta hemolysis.

  • Rapid Antigen Tests: Direct detection of S. pyogenes antigens.

  • DNA Probes: Nucleic acid hybridization for qualitative detection of RNA.

• Serologic Tests:

  • Anti-Streptolysin O (ASO) Tests: Indicates previous infection; useful for identifying post-streptococcal complications.

    • Antibody levels peak 4-6 weeks post-infection.

    • Rising titer indicates recent infection severity.

  • Anti-DNase B Tests: More reliable for recent skin infections; useful in diagnosing glomerulonephritis.

Streptococcal Toxic Shock Syndrome (TSS)

• Characteristics: Associated with S. pyogenes exotoxin, bacteria may spread from minor injuries or surgical procedures.

• Symptoms: Fever, shock, blotchy rash, and painful skin areas; potentially fatal.

• Mortality and Population Risk: Approximately 70% mortality; higher incidence in children and elderly. Rarely linked to strep throat.

Treatment

• Antimicrobials: Primarily penicillin and beta-lactam antibiotics; IV fluids for support.

Chapter 14: Streptococcal Infections - Part Three

Group B Streptococcus (Streptococcus agalactiae)

• Carrier State: At least 30% of women carry the bacteria in the genitourinary tract asymptomatically.

• Risk in Neonates: Leading cause of early-onset neonatal sepsis; universal screening during pregnancy (35-37 weeks) recommended.

• Characteristic Features:

  • Gram Positive Cocci: Typically beta hemolytic; identified through blood agar tests.

  • Diseases in Newborns: Potential infections include pneumonia, meningitis, and sepsis.

Screening and Diagnosis

• Pregnancy Screening: Vaginal-rectal swabbing; traditional cultures recommended for accuracy.

• Identification: Utilization of biochemical tests post-culture for precise strain identification.

• Transmittance to Newborns: Bacteria may infect amniotic fluid or newborn during passage through the birth canal, leading to potential serious infections.