Makindo Medical Notes

Related Subjects: |Streptococcus Pneumoniae (Pneumococcus) |Streptococcus pyogenes Group A |Streptococcus viridans |Streptococcus milleri |Streptococcus agalactiae Group B |Microbiology and Assessment of Streptococcus

Streptococcus is a genus of Gram-positive bacteria characterized by their spherical shape (cocci) and tendency to form chains or pairs. They are facultative anaerobes and catalase-negative, distinguishing them from Staphylococcus species.

Classification

1. Haemolytic Properties on Blood Agar:
   • Alpha-haemolytic Streptococci: Partial haemolysis causing a greenish discoloration on blood agar. Examples include Streptococcus pneumoniae and the viridans group streptococci.
   • Beta-haemolytic Streptococci: Complete haemolysis resulting in a clear zone around colonies. Further classified into Lancefield groups based on carbohydrate antigens in their cell walls.
   • Gamma-haemolytic Streptococci: Non-haemolytic; no change in the blood agar around colonies.
2. Lancefield Grouping (Based on Cell Wall Antigens):
   • Group A: Streptococcus pyogenes
   • Group B: Streptococcus agalactiae
   • Groups C and G: Various species causing zoonotic infections and human diseases
   • Group D: Includes Enterococcus species (formerly classified under Streptococcus)

Streptococcus pyogenes (Group A Streptococcus, GAS)

• Morphology: Gram-positive cocci in chains.
• Haemolysis: Beta-haemolytic.
• Virulence Factors:
  • M Protein: Anti-phagocytic, major virulence factor.
  • Streptolysins O and S: Exotoxins causing haemolysis.
  • Pyrogenic Exotoxins: Superantigens leading to scarlet fever and toxic shock syndrome.
• Diseases Caused:
  • Suppurative Infections: Pharyngitis (strep throat), impetigo, cellulitis, erysipelas.
  • Invasive Diseases: Necrotizing fasciitis, streptococcal toxic shock syndrome.
  • Post-Infectious Sequelae: Rheumatic fever, acute glomerulonephritis.

Streptococcus agalactiae (Group B Streptococcus, GBS)

• Morphology: Gram-positive cocci in chains.
• Haemolysis: Beta-hemolytic (narrow zone).
• Virulence Factors:
  • Capsule: Inhibits phagocytosis.
  • Beta-Haemolysin/Cytolysin: Contributes to tissue injury.
• Diseases Caused:
  • Neonatal Infections: Meningitis, pneumonia, sepsis.
  • Infections in Adults: Urinary tract infections, skin and soft tissue infections.

Streptococcus pneumoniae

• Morphology: Gram-positive diplococci, lancet-shaped.
• Haemolysis: Alpha-hemolytic.
• Virulence Factors:
  • Polysaccharide Capsule: Major virulence factor; over 90 serotypes.
  • Pneumolysin: Damages host cells, activates complement.
• Diseases Caused:
  • Respiratory Tract Infections: Pneumonia, otitis media, sinusitis.
  • Invasive Diseases: Meningitis, bacteremia.

Viridans Group Streptococci

• Characteristics: Heterogeneous group; alpha or non-hemolytic.
• Habitat: Normal flora of the oral cavity, gastrointestinal tract.
• Diseases Caused:
  • Dental Caries: S. mutans produces acid from sugar fermentation.
  • Subacute Bacterial Endocarditis: Especially in patients with damaged heart valves.

Enterococcus Species (Group D Streptococcus)

• Morphology: Gram-positive cocci in pairs or short chains.
• Haemolysis: Variable (alpha, beta, or non-hemolytic).
• Notable Species: Enterococcus faecalis, Enterococcus faecium.
• Diseases Caused:
  • Nosocomial Infections: Urinary tract infections, bacteremia, endocarditis.
  • Antibiotic Resistance: Notably to vancomycin (VRE).

Laboratory Identification

1. Gram Stain: Reveals Gram-positive cocci in chains or pairs.
2. Culture on Blood Agar: Observes haemolysis patterns.
3. Catalase Test: Negative (differentiates from Staphylococcus).
4. Bacitracin Sensitivity: Group A Streptococcus is sensitive.
5. CAMP Test: Positive for S. agalactiae.
6. Optochin Sensitivity and Bile Solubility: S. pneumoniae is optochin-sensitive and bile soluble.
7. PYR Test: Positive for S. pyogenes.

Assessment of Streptococcal Infections: Clinical Assessment: History and Symptoms

• Pharyngitis:
  • Sudden onset sore throat, fever, headache.
  • Absence of cough and nasal congestion suggests bacterial etiology.
• Skin and Soft Tissue Infections:
  • Redness, warmth, swelling, pain at the affected site.
  • Rapid progression in cases like necrotizing fasciitis.
• Systemic Symptoms:
  • High fever, chills, malaise.
  • Signs of septic shock in severe cases.

Physical Examination

• Pharyngeal Inspection:
  • Red, swollen tonsils with or without exudate.
  • Palatal petechiae.
• Lymphadenopathy:
  • Tender cervical lymph nodes.
• Skin Examination:
  • Erythema, edema, possible bullae or necrosis.
• Vital Signs:
  • Fever, tachycardia, hypotension in severe cases.

Laboratory Assessment

Specimen Collection

• Throat Swab: For patients with pharyngitis.
• Skin Swab or Biopsy: From infected wounds or lesions.
• Blood Cultures: If bacteraemia or sepsis is suspected.
• Cerebrospinal Fluid: In suspected meningitis.

Diagnostic Tests

• Rapid Antigen Detection Tests (RADTs):
  • Detect Group A Streptococcal antigens from throat swabs.
  • High specificity; negative tests in children may require confirmation with culture.
• Culture:
  • Blood Agar Plates: Incubated aerobically and anaerobically.
  • Observation of Haemolysis: Aids in preliminary identification.
• Serological Tests:
  • Anti-Streptolysin O (ASO) Titer: Elevated levels indicate recent S. pyogenes infection.
  • Anti-DNase B Test: Supports ASO findings, especially in skin infections.
• Molecular Methods:
  • PCR Assays: For rapid identification and detection of virulence genes.
  • 16S rRNA Sequencing: In complex cases or for epidemiological studies.

Antibiotic Susceptibility Testing

• Disk Diffusion or MIC Determination: Guides antibiotic therapy, especially important for S. pneumoniae due to resistance patterns.

Antimicrobial Therapy

• Group A Streptococcus:
  • First-line: Penicillin V or amoxicillin.
  • Penicillin Allergy: Cephalosporins (if not anaphylactic), macrolides, or clindamycin.
• Group B Streptococcus:
  • First-line: Penicillin G.
  • Intrapartum Prophylaxis: To prevent neonatal transmission.
• Streptococcus pneumoniae:
  • Empiric Therapy: High-dose amoxicillin.
  • Resistance Considerations: Use of cephalosporins or fluoroquinolones.
• Enterococcus Species:
  • Ampicillin or Vancomycin: Often combined with gentamicin for synergy.
  • VRE Infections: Linezolid or daptomycin.

Supportive Care

• Symptom Relief: Analgesics and antipyretics for pain and fever.
• Hydration and Rest: Essential for recovery in systemic infections.

3. Infection Control and Prevention

• Hand Hygiene: Reduces transmission in community and healthcare settings.
• Vaccination:
  • Pneumococcal Vaccines: PCV13 and PPSV23 recommended for children, the elderly, and high-risk individuals.
• Screening:
  • Pregnant Women: For GBS colonization at 35–37 weeks gestation.

Complications to Monitor

• Rheumatic Fever:
  • Autoimmune response following untreated GAS pharyngitis.
  • Affects heart valves, joints, skin, and brain.
• Acute Glomerulonephritis:
  • Immune-mediated kidney inflammation.
  • Presents with haematuria, proteinuria, hypertension.
• Invasive Streptococcal Disease:
  • Necrotizing fasciitis, streptococcal toxic shock syndrome.
  • Requires prompt surgical intervention and intensive care.

Prognosis

• Prompt Treatment: Generally leads to full recovery in uncomplicated cases.
• Delayed or Inadequate Therapy: Increases risk of complications and mortality in severe infections.

Disclaimer: This information is intended for educational purposes and should not replace professional medical advice. For specific patient care decisions, consult a qualified healthcare provider.

Note: Antibiotic sensitivities can vary based on local resistance patterns. Always consult current clinical guidelines and perform susceptibility testing for appropriate antibiotic selection.