Bacterial Infections of Skin, Soft Tissues & Musculoskeletal System-II

Overview

• Pyogenic bacteria:
  • Key organisms: Staphylococcus aureus and Streptococcus pyogenes.
  • Date: 15/10/2025
  • Presenters: Dr. Mona Elfaki & Dr. Mohamed Almagrabi, Department of Clinical Parasitology and Microbiology, KKU, Abha, KSA.

Learning Objectives

• Describe microbiological characteristics of S. aureus and S. pyogenes.
• Explain virulence factors responsible for skin and soft tissue infections.
• Recognize main clinical syndromes caused by these bacteria.
• Understand laboratory diagnosis and differentiation between the two organisms.
• Discuss antimicrobial therapy and prevention of infections caused by these pathogens.

Introduction to Pyogenic Infections

• Definition: Pyogenic infections produce pus, primarily composed of dead neutrophils, bacteria, and tissue debris.
• Significant pathogens:
  • Staphylococcus aureus
  • Streptococcus pyogenes (Group A β-hemolytic Streptococcus)
• Microbiological characteristics:
  • Both are Gram-positive cocci.
  • Part of normal flora but can cause mild to severe disease, including:
  • Superficial lesions
  • Life-threatening necrotizing infections
  • Toxin-mediated syndromes.

Staphylococcus aureus

Morphology and Laboratory Characteristics

• Appearance: Gram-positive cocci in clusters (grape-like formation).
• Tests:
  • Catalase: Positive
  • Coagulase: Positive
  • Growth:
  • Grows on nutrient and blood agar, forms golden-yellow colonies with β-hemolysis.
  • Ferments mannitol on Mannitol Salt Agar (MSA), which is selective and differential.

Reservoir and Transmission

• Colonization: Normal flora in 30% of healthy individuals (anterior nares, skin, perineum).
• Transmission:
  • Direct contact
  • Contaminated surfaces
  • Fomites.

Virulence Factors

• Categories and Examples:
  • Structural components:
  • Capsule: Inhibits phagocytosis, promotes adhesion.
  • Protein A: Binds IgG, inhibits opsonization.
  • Teichoic acid: Promotes adhesion.
  • Enzymes:
  • Coagulase: Facilitates blood clot formation to evade immune response.
  • Hyaluronidase: Breaks down connective tissue to facilitate spread.
  • Lipase: Destroys lipids in host tissues.
  • DNase: Degrades DNA in pus.
  • Toxins:
  • α-toxin: Cytolytic activity leading to tissue necrosis.
  • Exfoliative toxins (A & B): Cause tissue damage.
  • Enterotoxins: Associated with food poisoning.
  • TSST-1: Causes Toxic Shock Syndrome.
  • Biofilm formation:
  • Important in prosthetic infections, promotes antibiotic resistance and persistence.

Clinical Syndromes

• Localized skin infections:
  • Folliculitis: Inflammation of hair follicles.
  • Furuncle (boil): Same as folliculitis but larger and more painful.
  • Carbuncle: Cluster of furuncles.
  • Impetigo: Bullous lesions, especially in children due to exfoliative toxins.
• Wound and surgical infections: Characterized by purulent discharge, often hospital-acquired.
• Toxin-mediated syndromes:
  • Scalded Skin Syndrome (Ritter’s disease): Exfoliative toxins A & B cause epidermal splitting.
  • Toxic Shock Syndrome: Symptoms include fever, rash, hypotension, multi-organ failure.
• Systemic Spread:
  • Conditions such as osteomyelitis, endocarditis, bacteremia, and pneumonia (hematogenous spread).

Laboratory Diagnosis

• Methicillin-Resistant Staphylococcus aureus (MRSA):
  • Resistant to all β-lactams.
  • Identified by detection of mecA gene (PBP2a).
  • Treatment: vancomycin, linezolid, daptomycin.
• Stepwise Diagnosis:
  1. Microscopy: Gram stain - shows Gram-positive cocci in clusters.
  2. Culture: On blood agar/MSA (Golden-yellow β-hemolytic colonies). Mannitol fermentation used for differentiation.
  3. Biochemical Tests: Catalase positive, coagulase positive, differentiates from streptococci and coagulase-negative staphylococci.
  4. Antibiotic Susceptibility: Cefoxitin test specifically for MRSA to determine methicillin resistance.

Treatment and Prevention

• Localized infections: Incision and drainage recommended.
• Systemic infections: Use of β-lactamase resistant penicillins (e.g., oxacillin) or vancomycin for MRSA.
• Prevention:
  • Hand hygiene
  • Decolonization strategies (e.g., mupirocin)
  • Aseptic surgical techniques.

Streptococcus pyogenes (Group A β-hemolytic Streptococcus)

Morphology and Laboratory Characteristics

• Appearance: Gram-positive cocci in chains.
• Tests:
  • Catalase: Negative
  • Hemolysis: β-hemolytic on blood agar (complete hemolysis).
  • Sensitivity: Bacitracin-sensitive.
  • Classification: Lancefield classification identifies Group A antigen.

Reservoir and Transmission

• Colonization: Common in the oropharynx and skin.
• Transmission:
  • Respiratory droplets
  • Direct skin contact.

Virulence Factors

• Categories and Examples:
  • Surface proteins:
  • M protein: Antiphagocytic; major virulence factor.
  • Capsule: Hyaluronic acid mimics host connective tissue, aiding immune evasion.
  • Enzymes:
  • Streptokinase: Converts plasminogen to plasmin, aiding tissue invasion.
  • Hyaluronidase: Deteriorates connective tissue for spread.
  • DNases: Degrade DNA.
  • C5a peptidase: Inhibits the chemotaxis of neutrophils.
  • Toxins:
  • Streptolysins O and S: Cytolytic effect (hemolysins causing cell lysis).
  • Pyrogenic exotoxins (SpeA, SpeB): Cause scarlet fever and toxic shock-like syndromes, induce systemic effects.

Clinical Syndromes

• Superficial infections:
  • Impetigo (non-bullous): Characterized by honey-colored crusted lesions usually in children; often co-infected with S. aureus.
• Spreading infections:
  • Erysipelas: Presenting as bright red, sharply demarcated rash involving dermis and lymphatics, primarily on face and legs.
  • Cellulitis: Diffuse, accounts for rapid spreading infection of dermis and subcutaneous tissue, symptoms include pain, swelling, and fever.
• Deep infections:
  • Necrotizing fasciitis: Known as “flesh-eating bacteria”; rapid tissue necrosis with severe systemic toxicity, can occur with S. pyogenes as the primary agent (Type II).
• Toxin-mediated syndromes:
  • Scarlet fever: Erythrogenic toxin-mediated rash, results in multi-organ involvement.
  • Streptococcal Toxic Shock: Associated systemic effects.

Laboratory Diagnosis

• Stepwise Diagnosis:
  1. Microscopy: Gram-positive cocci in chains observed.
  2. Culture: Confirm presence of β-hemolytic colonies on blood agar.
  3. Bacitracin Sensitivity Test: Confirmation via inhibition zone.
  4. Rapid antigen detection: Detection of Group A carbohydrate.
  5. Serology: ASO titer useful for post-streptococcal complications (not for acute skin infection).

Treatment and Prevention

• Drug of choice: Penicillin G, no resistance reported.
• Alternatives:
  • Erythromycin
  • Clindamycin (for penicillin-allergic patients).
• Necrotizing fasciitis management: Urgent surgical debridement alongside high-dose penicillin and clindamycin to suppress toxin production.
• Prevention strategies:
  • Good hygiene, prompt treatment of wounds, eradication of carriers.

Comparison Between S. aureus and S. pyogenes

Laboratory Diagnosis of Pyogenic Skin Infections

General Workflow

• Specimen Collection: Pus aspirate or deep wound swab.
• Microscopy: Perform Gram stain to define cocci type and arrangement.
• Culture: Inoculate on blood agar and MacConkey agar (for mixed infections).
• Identification: Conduct biochemical tests (catalase, coagulase, hemolysis pattern).
• Antimicrobial Susceptibility Testing: Essential for guiding treatment.
• Special Tests: PCR or MALDI-TOF for rapid species confirmation.

General Treatment Principles

• Abscess management: Drainage recommended when present.
• Empirical antibiotic therapy: Should cover both S. aureus and S. pyogenes (e.g., co-amoxiclav, cefazolin).
• Adjust therapy: Modify based on culture and sensitivity results.
• Supportive care: Manage fever, provide analgesics, ensure hydration.

Key Microbiological Takeaways

• S. aureus and S. pyogenes are major bacterial causes of pyogenic skin and soft tissue infections.
• Key differentiating features include arrangement of cocci and catalase tests.
• S. aureus causes localized abscess-forming infections, while S. pyogenes is associated with spreading infections (cellulitis, erysipelas).
• Toxin-mediated syndromes from both organisms can be severe, necessitating urgent management.
• Laboratory confirmation is critical to guide therapy and control outbreaks.

Thank You

• Presenters: Dr. Mona Elfaki & Dr. Mohamed Almagrabi
• Date: 15/10/2025
• Department: Clinical Parasitology & Microbiology, KKU, Abha, KSA.