17

Lecture 17: Anaerobes

Learning Objectives

• Upon completing this lecture, students should be able to:

  • LO1: List the clinical features of anaerobic bacterial infections of the skin and soft tissue.

  • LO2: Describe the general approach for management of anaerobic bacterial infections of the skin and soft tissues.

  • LO3: Describe the general characteristics of the etiologic agents covered and identify them in images as appropriate.

  • LO4: Discuss virulence factors and pathogenesis of specific agents and conditions.

  • LO5: Describe the clinical presentation of specific infections.

  • LO6: Identify lesions in images and discuss associated limitations.

  • LO7: Discuss the diagnosis and management of specific skin and soft tissue infections.

Anaerobic Bacterial Skin & Soft Tissue Infections

General Characteristics of Anaerobic Bacterial Infections (Applicable to any organ)

• Putrid drainage: Infections typically present with foul-smelling discharge, which is diagnostic.

• Polymicrobial infections: Many anaerobic infections involve multiple bacterial species.

• Involving endogenous flora: These bacteria usually originate from the patient's own body flora.

• Abscess formation: Anaerobes frequently lead to the formation of pus-filled cavities.

General Approach to Anaerobic Bacterial Infections

• Empirical Treatment: Treatment is initiated based on Gram stain results and the anatomical site of infection.

• Culture and Susceptibility Challenges:

  • Often impractical due to the complexity of infections.

  • Culturing multiple organisms can be time-consuming and labor-intensive.

  • Routine cultures may fail to yield results due to inadequate standardization of methods.

• Common Treatments:

  • Most infections are treated using:

    • Metronidazole

    • Carbapenems

    • β-lactam-β-lactamase inhibitor combinations

Clostridium perfringens

General Description

• Morphology: Large, rectangular, Gram-positive anaerobic rods.

• Spores: Rarely observed. Granular pus cells are typically absent in Gram-stained smears.

• Gas Production: Growth is marked by the production of hydrogen and carbon dioxide, which can lead to gas gangrene.

• Infection Mechanism: Infection occurs through traumatic inoculation of microorganisms from the environment or intestinal flora.

Pathogenesis

• Exotoxins Production:

  • α-Toxin:

  • Phospholipase C, lecithinase, and sphingomyelinase activities.

  • Disrupts cell membranes of erythrocytes, leukocytes, and myocytes through hydrolysis of lecithin and sphingomyelin.

  • Triggers platelet aggregation, leading to capillary occlusion, ischemia, and subsequent tissue necrosis.

  • Can be absorbed systemically, potentially triggering shock.

  • Perfringolysin O: A pore-forming toxin that compromises capillary endothelium resulting in edema and can attack immune cells.

Manifestations of Gas Gangrene (Clostridial Myonecrosis)

• Clinical Indicators:

  • History of delayed surgical intervention suggests presence of gas gangrene.

  • Symptoms include:

  • Severe pain.

  • Sensation of heaviness or pressure.

  • Edema, tenderness, and pallor.

  • Discoloration and hemorrhagic bullae.

  • Tissue crepitus.

  • Potential outcomes include shock, renal failure, coma, and death.

Manifestations of Anaerobic Cellulitis

• Characteristics:

  • A milder wound infection compared to myonecrosis.

  • Increased gas production.

  • Absence of severe pain, swelling, or systemic toxicity.

Diagnosis and Treatment

• Diagnosis:

  • Predominantly based on clinical presentation.

  • Cultures may readily isolate the organism through routine methods.

• Treatment:

  • Surgical excision of all devitalized tissues is critical.

  • Penicillin for clostridia in combination with cephalosporins for contaminants.

  • Antimicrobial therapy is often sufficient for cellulitis management.

Bacteroides fragilis

General Description

• Morphology: Anaerobic, Gram-negative rods.

• Prevalence: Most common anaerobic pathogen isolated from soft tissues and blood, part of endogenous intestinal flora.

Pathogenesis

• Virulence Factors:

  • Polysaccharide Capsule: Provides protection against phagocytosis, promotes adhesion, and aids in abscess formation.

  • Oxygen Tolerance: Displays relative tolerance to oxygen, allowing survival and proliferation in varying environments.

Manifestations of Necrotizing Fasciitis Type I

• Common Pathogens: Typically caused by facultative anaerobes (e.g., Enterobacteriaceae) in conjunction with anaerobes (e.g., Bacteroides, Clostridium, Propionibacterium).

• Symptoms:

  • Rapid progression of symptoms.

  • Erythema with diffuse margins and severe pain.

  • Edema that extends beyond visible erythema and fever.

  • Possible crepitus, bullae, ecchymosis, and diminished sensation that may precede other symptoms.

  • Skin discoloration, frank gangrene, shock in later stages of infection.

Treatment

• Interventions: Surgical intervention is required.

• Antimicrobial Therapy: Typical anaerobic antimicrobial therapy as outlined previously.

Case Example: Bilateral Thigh Necrotizing Soft Tissue Infection

• Clinical Presentation:

  • A 72-year-old diabetic patient presented with increasing confusion, tenderness, and skin changes in the bilateral thighs over a few days.

  • During surgical exploration and debridement, characteristic appearance of necrotic skin, underlying fascia, and dish-water fluid drainage was observed.

Relevant Information for Final Exam

• Focus Areas:

  • Microbiology, including images of Clostridium perfringens and Bacteroides fragilis.

  • Infections caused by the two organisms.

  • Virulence factors for all organisms discussed.

  • Pathogenesis is not required for any of the organisms covered in this lecture.