Mycobacteria Lecture Notes

Lecture Overview

  • Introduction to Mycobacteria

  • Learning Outcomes:

    • Describe the three characteristics of mycobacteria.

    • Describe clinically relevant mycobacterial diseases and their transmission, the affected hosts, and pathogenesis.

    • Discuss how various mycobacteria can be diagnosed, controlled, and prevented.

    • Explain the significance of tuberculosis and Johne's disease.

Taxonomy of Bacteria

  • Classification in bacteriology is done using a two-name system: genus and species.

    • Example: Staphylococcus aureus (genus: Staphylococcus, species: aureus).

  • Two types of classification/taxonomy:

    • Phenotypic: Based on observable traits, such as morphology (shape), coloration (Gram staining), and biochemical characteristics.

    • Genotypic: Based on genetic characteristics.

    • Involves genomic content and DNA hybridization techniques.

    • Commonly uses 16S rRNA gene for taxonomy because of its highly conserved nucleotide regions.

Phenotypic Characteristics of Bacteria

  • Morphology: Observing shape and arrangement.

    • Examples of Gram-positive and Gram-negative bacteria:

    • Gram-positive: Bacillus species, Streptococcus species (thick peptidoglycan layer).

    • Gram-negative: E. coli, Salmonella, Campylobacter (thin peptidoglycan layer, two membranes).

  • Staining methods:

    • Gram Staining: Differentiates bacteria based on cell wall composition.

    • Acid-fast Staining: Used for mycobacteria due to mycolic acid in the cell wall which makes them resistant to regular staining methods.

Oxygen Requirements

  • Aerobic: Require oxygen for survival.

  • Anaerobic: Cannot survive in the presence of oxygen.

  • Microaerophilic: Require low levels of oxygen.

Mycobacteria Overview

  • Classification of Mycobacteria:

    • Divided into two main groups:

    • Mycobacterium tuberculosis complex (MTBC):

      • Mycobacterium tuberculosis (human TB)

      • Mycobacterium bovis (bovine TB)

      • Mycobacterium africanum (TB in Africa)

    • Mycobacterium avium complex (MAC): Causes Johne's disease and tuberculosis in avian species.

Mycobacterial Diseases

  • Tuberculosis (TB):

    • Caused by Mycobacterium tuberculosis, affecting humans and some primates.

    • Symptoms include chronic cough, weight loss, and hemoptysis (coughing up blood).

  • Johne’s disease:

    • Caused by Mycobacterium avium spp. paratuberculosis in ruminants.

    • Characterized by chronic diarrhea and weight loss.

  • Leprosy:

    • Caused by Mycobacterium leprae, affects skin and peripheral nerves.

Pathogenesis of Mycobacterial Diseases

  • Mycobacterium enters host through inhalation or ingestion, reaching alveoli in lungs or intestinal mucosa.

  • Phagocytosis: Macrophages attempt to engulf mycobacteria, forming a phagosome.

    • Mycobacteria prevent fusion with lysosomes, allowing survival and replication within macrophages.

  • Granuloma Formation:

    • Characteristic immune response characterized by the accumulation of immune cells (macrophages, epithelioid cells, giant cells) results in granuloma formation.

    • Granulomas may undergo caseous necrosis (cheese-like material).

  • Survival in the host can become latent or active, depending on the immune status of the host.

Clinical Signs and Diagnosis

  • Tuberculosis: Clinical signs develop slowly over years. Common signs include:

    • Emaciation, chronic cough, intermittent fever, and in advanced cases, mastitis.

  • Diagnosis:

    • Tuberculin Skin Test: Intradermal injection of tuberculin protein to observe for a hypersensitivity reaction indicating infection.

Control and Treatment

  • Culling of infected animals is often necessary to control the spread.

  • Antibiotic treatment exists but poses risks for antimicrobial resistance.

  • Vaccination strategies, such as BCG vaccine, are being explored but vary in effectiveness.

Mycobacterium avium and Related Diseases

  • Mycobacterium avium: Causes enteric diseases in birds, characterized by chronic wasting and gastrointestinal lesions.

  • No available vaccines; management through sanitation and removal of infected animals.

Mycobacterium liberum

  • Affects felines and rodents, causing leprosy-like skin lesions.

  • Transmission through bites and scratches, primarily affecting the skin.

Conclusion and Summary

  • Mycobacterium species can cause chronic and persistent diseases in various hosts.

  • Effective control strategies include diagnosis, management, culling, and ongoing research for vaccines and treatments.

Review Questions

  • Any questions about today's session or class?