BACT211_Week_7_Mycobacteria

Mycobacteria Overview

  • Mycobacteria are non-motile, non-spore forming, slender, slightly curved or straight, rod-shaped organisms that tend to clump.

  • Characterized by a highly lipid-rich cell wall, granting them exceptional resilience against basic aniline dyes, notably with Gram staining techniques.

  • They require prolonged staining time or the application of heat for effective dye absorption and resist decolorization with acid-ethanol, a property termed acid fastness (AFB).

  • Strictly aerobic; growth is enhanced with elevated carbon dioxide levels.

Objectives

  • Describe general characteristics of Mycobacterium spp.

  • Explain the staining characteristics using the Gram stain and acid-fast staining.

  • Identify common pathogenic Mycobacterium species, their transmission mode, natural habitat, and reservoirs.

  • Differentiate clinical infections of Mycobacterium tuberculosis (M. tuberculosis) including primary, latent, disseminated, and reactivation.

  • Compare culture media for mycobacterial isolation.

  • Discuss identification tests for mycobacteria.

Pathogenic Mycobacterium Species

Common Pathogens

  • Mycobacterium tuberculosis: Causative agent of tuberculosis (TB).

  • Mycobacterium bovis: Related to bovine TB, affects cattle and humans.

  • Mycobacterium ulcerans: Causes skin infections.

  • Mycobacterium leprae: Causes leprosy.

  • Mycobacterium avium complex: Potential pathogen in immunocompromised individuals.

Tuberculosis and Its Complex

  • The Mycobacterium tuberculosis complex comprises

    • Mycobacterium tuberculosis

    • Mycobacterium bovis (including Bacillus Calmette-Guérin vaccine strain).

  • TB primarily affects the respiratory system and is transmitted via aerosolized droplets from infected individuals.

  • Granulomas (tubercle formations) develop, resulting in fibrosis and possible reactivation of TB.

  • Diagnosis includes a positive PPD skin test coupled with sputum smear/culture to confirm infection.

Extrapulmonary Tuberculosis

  • Miliary TB: Dissemination of bacteria to multiple organs via blood.

  • Affects various organs leading to diverse manifestations such as pleurisy, lymphadenitis, genitourinary, skeletal TB (Pott disease), and meningitis.

  • Dagnosis of meningitis reveals elevated protein levels and decreased glucose in cerebrospinal fluid (CSF).

Clinical Presentation of Tuberculosis

Symptoms of Primary TB

  • Symptoms: fever, fatigue, night sweats, weight loss, dyspnea, cough, chest pain, hemoptysis.

    • Presentations include exudative and granulomatous lesions.

Secondary TB (Reactivation)

  • Reactivation or reinfection in previously infected individuals.

Colonization and Identification

  • Mycobacterium colonies on media are raised, dry, and buff-colored with characteristic cord formations.

  • Growth optimal temperature: 35°C to 37°C.

  • Identifying tests include niacin accumulation, nitrate reduction, catalase production, and growth on specific selective media (e.g., thiophene-2-carboxylic acid hydrazide [T2H]).

Mycobacterium bovis

  • Primarily affects cattle but can also infect humans, mirroring M. tuberculosis pathology.

  • Notable characteristics: niacin-negative and does not reduce nitrate.

Acid-Fast Smear Reporting

  • Classification of acid-fast bacilli (AFB) based on the number seen in various magnifications, and interpretation guidelines for results (e.g., no AFB, indeterminate results).

Treatment Strategies for TB

Quadruple Therapy (DOTS)

  • Drug regimen includes:

    • Isoniazid (INH), Rifampicin, Pyrazinamide, Ethambutol, Streptomycin.

  • DOTS ensures patient compliance for effective treatment.

Commonly Tested Antitubercular Agents

  • Treatment typically lasts 9 months, focusing on Isoniazid and Rifampin with alternatives for resistant strains.

  • Second-line drugs: Kanamycin, Ciprofloxacin, etc.

Drug Resistance

  • MDR-TB: Resistant to INH and RMP (two major first-line drugs).

  • XDR-TB: Resistant to additional quinolones and injectables.

Non-Tuberculous Mycobacteria (NTM)

  • Encountered in the environment; colonize healthy host skin and respiratory tracts but are not transmissible.

  • Types include: Mycobacterium avium complex, Mycobacterium kansasii, Mycobacterium marinum, etc.

Runyon Classification of NTM

  • Photochromogens: Produce pigment upon light exposure (e.g., M. kansasii).

  • Scotochromogens: Produce pigment in the dark (e.g., M. scrofulaceum).

  • Nonphotochromogens: Do not produce pigment (e.g., M. avium complex).

  • Rapid Growers: Fast growth (<7 days) (e.g., M. fortuitum).

Specific NTM Characteristics

  • Mycobacterium avium: Environmental saprophyte associated with diseases in animals, notably zoonosis.

  • Mycobacterium haemophilum: Requires heme for growth, causes lymphadenitis.

  • Mycobacterium kansasii: Resistant to several antibiotics, forms distinct colony characteristics (wavy edges).

  • Mycobacterium marinum: Associated with aquatic environments, can cause cutaneous infections in humans after exposure.

  • Mycobacterium ulcerans: Causes Buruli ulcer; characterized by non-cross-banded long bacilli.

Conclusion

  • Understanding the characteristics and pathogenicity of Mycobacteria, including their transmission, cultures, and treatments, is critical for effective diagnosis and therapy of mycobacterial infections.