Mycobacteria and Their Diseases

Mycobacteria: Characteristics and Diseases

Learning Outcomes

  • Describe the three characteristics of mycobacteria.

  • Identify clinically relevant mycobacterial diseases and their transmission.

  • Explain the pathogenesis of various mycobacterial diseases in relation to pathogen-host interactions.

  • Describe the diagnosis, prevention, and control of mycobacterial diseases.

  • Discuss the importance of tuberculosis and joint diseases caused by mycobacteria.

Revision of Taxonomy

  • Taxonomy refers to the classification of bacteria into two names: genus and species.

    • Example: Staphylococcus aureus where Staphylococcus is the genus and aureus is the species.

  • Two types of classification:

    • Phenotypic Classification: Based on observable characteristics (morphology, staining properties).

    • Genotypic Classification: Based on genetic characteristics, such as DNA content.

Phenotypic Characteristics

  • Morphology, oxidative properties, staining attributes:

    • Gram Staining: Grams can be divided into:

    • Gram-Positive Bacteria: Thick peptidoglycan layer retains crystal violet stain, appear purple.

      • Example: Bacillus species.

    • Gram-Negative Bacteria: Thin peptidoglycan layer, decolorized by acetone, counterstained pink by safranin.

      • Example: E. coli, Salmonella.

Biochemical Tests

  • Biochemical reactions and enzymes are used in practical lab settings to characterize bacteria (e.g. catalase test, oxidase test).

    • Practical tests scheduled for March 4 to identify bacterial species.

Genotypic Taxonomy

  • Historically based on genomic content (e.g. G+C content).

  • Modern methods include:

    • Ribosomal RNA sequencing

    • Multi-locus sequence typing

    • Whole genome sequencing

    • Phylogenetic analysis

  • 16S rRNA gene is a highly conserved sequence used for classifying and differentiating bacteria.

Mycobacteria Overview

  • Mycobacterium species fall into two main groups:

    • Mycobacterium tuberculosis complex (MTBC): Causes tuberculosis in humans and animals.

    • Members include:

      • Mycobacterium tuberculosis: Affects humans and captive primates.

      • Mycobacterium bovis: Affects cattle.

      • Mycobacterium africanum: Affects individuals in Africa.

    • Mycobacterium avium complex (MAC): Causes diseases in birds and immunocompromised humans.

    • Members include:Mycobacterium avium, Mycobacterium intracellulare.

Mycobacterial Diseases

  • Tuberculosis (TB): An ancient disease causing severe weight loss and coughing of blood.

    • Treated with antibiotics but can be fatal if untreated.

    • Also affects various animals; zoonotic disease.

  • Leprosy: Caused by Mycobacterium leprae, affects skin and nerves in humans and other mammals.

  • Johne's Disease: Caused by Mycobacterium avium subspecies paratuberculosis, leads to chronic diarrhea and weight loss in ruminants.

Phenotypic Characteristics of Mycobacteria

  • Characteristics: Aerobic, non-spore forming, non-motile.

  • Staining: Acid fast due to the presence of mycolic acid in the cell wall, thus poorly staining with Gram's method.

    • Stained with Ziel-Nelson method.

  • Requires complex, enriched media for growth; slow-growing compared to other bacteria (21 days).

Pathogenesis

  • Entry and Survival: Mycobacteria enter via the respiratory system or intestines, survive inside macrophages by preventing phagosome-lysosome fusion.

    • They induce granuloma formation, characterized by:

    • Accumulation of macrophages, epithelioid cells, and multinucleated giant cells in response to infection.

    • Granulomas contain caseous necrosis, giving cheese-like appearance when opened.

Clinical Signs and Diagnosis

  • TB Clinical Signs: Chronic, progressive weight loss; cough; fever; lesions in lungs and other tissues.

  • Diagnosis methods include:

    • Tuberculin skin test: Evaluates delayed hypersensitivity to the specific tuberculin antigen.

    • Identification of symptoms and post-mortem analysis.

Control and Treatment

  • Culling of infected animals is preferred to treat powerfully, as antibiotic treatment can lead to resistance.

  • Vaccination: BCG vaccine mainly used in humans; few effective vaccines for animals, under research.

  • Focus on preventative measures in animal husbandry including maintaining biosecure herds.

Mycobacterium avium (Avian Tuberculosis)

  • Affects birds and can lead to significant losses in poultry production due to decreased egg production and high mortality rates.

Mycobacterium paratuberculosis (Johne's Disease)

  • Chronic, progressive disease affecting ruminants leading to persistent diarrhea and weight loss; long incubation (2-4 years).

  • Identification through fecal culture and post-mortem analysis of lesions in the intestines.

Mycobacterium leprae (Leprosy)

  • Affects skin and peripheral nerves; typically transmitted through contact rather than being zoonotic.

  • Treatment by surgical excision of lesions and possible supportive therapy.

Important Remarks

  • Mycobacteria are resilient and can persist in the environment for long periods, making control challenging.

  • Awareness of transmission pathways (air, feces) is critical for disease management in agricultural settings.