mycobacteria 1 & 2

• The genus Mycobacterium is composed of over 130 recognized species.
• Mycobacterium tuberculosis


  Causative agent of tuberculosis (TB)
• Mycobacterium leprae

  Causative agent of Hansen disease (leprosy)
• Mycobacteria

  • Obligate pathogens
  • Associated with chronic illness and social stigma
  • Atypical mycobacteria inhabit the environment
• General characteristics of Mycobacteria

  • Slender, slightly curved or straight, rod-shaped organisms
  • Nonmotile
  • Do not form spores
  • High lipid content in cell wall
  • Resist staining with basic aniline dyes
  • Acid fastness
  • Strictly aerobic
• Mycobacteria resist decolorization with acid-ethanol.
• Pathogenic mycobacteria grow more slowly than most other bacteria pathogenic for humans.
• Incubation of pathogenic mycobacteria

  2 to 6 weeks on complex media at specific optimal temperatures
• M. leprae fails to grow in vitro.
• Mycobacterium tuberculosis complex

  • M. tuberculosis
  • M. bovis
  • M. africanum
  • M. caprae
  • M. pinnipedi
  • M. microti
  • M. canetti
• MTB

  Mycobacterium tuberculosis
• MTB was first described by Robert Kochin


  1882
• TB is one of the oldest documented communicable diseases.
• Immune deficiency state such as AIDS increases

  Risk for TB
• Closed environments with greater spread of TB

  • Nursing homes
  • Correctional facilities
  • Shelters for the homeless
• Development of TB after exposure

  1. Determined by cellular immune response

  2. Amount of exposure

  3. Virulence of the strain
• Tubercle bacilli are acquired from persons with active disease who are excreting viable bacilli by sneezing or talking.
• Infection process of M. tuberculosis

  1. Airborne droplets enter respiratory tract

  2. Reach lung alveoli

  3. Phagocytized by alveolar macrophages

  4. Intracellular multiplication
• The initial focus of infection together with the enlarged hilar lymph nodes forms the primary complex.
• Infection response in adequate cellular immunity

  1. Macrophage destroys intracellular mycobacteria

  2. Regression and healing of primary lesion
• In some exposed individuals, the immune system does not eliminate the bacteria.
• The pathologic features of TB are the result of a hypersensitivity reaction to mycobacterial antigen.
• Granuloma formation

  1. Hard tubercle forms

  2. Organization of lymphocytes

  3. Macrophages

  4. Fibroblasts

  5. Capillaries
• Ghon complex

  Parenchymal exudative lesion and draining lymph nodes
• Primary lesions usually occur in the lower lobes, whereas reactivation lesions usually occur in the apices.
• Healing process with granuloma formation

  1. Fibrosis

  2. Encapsulation

  3. Calcification

  4. Scar formation
• Lesions may heal without obvious pathology without granuloma or necrosis.
• Clinical diagnosis of primary TB is usually limited to signs and symptoms and a positive purified protein derivative (PPD) skin test.
• Main clinical feature of primary TB

  Cough, especially in children
• Reactivation of dormant foci of tubercle bacilli leads to post-primary tuberculosis.
• Post-primary tuberculosis

  • Tends to develop in the upper lobes of the lungs
  • Occurs with suppression of the cellular immune system
• Symptoms of post-primary tuberculosis

  • Fever
  • Shortness of breath
  • Night sweats
  • Chills
  • Fatigue
  • Anorexia
  • Weight loss
• About 20% of individuals may have no symptoms.
• Hemoptysis occurs in 25% of cases, indicating cavitation and necrosis.
• Complications of pulmonary TB

  • Empyema
  • Pleural fibrosis
  • Massive hemoptysis
  • Adrenal insufficiency
  • Hypercalcemia
• Extrapulmonary disease is a common presentation in individuals with HIV infection often in association with pulmonary disease.
• Miliary TB


  Seeding of many organs outside the pulmonary tree with AFB through hematogenous spread
• Common sites of spread of M. tuberculosis

  • Spleen
  • Liver
  • Lungs
  • Bone marrow
  • Kidney
  • Adrenal gland
  • Eyes
• Overall, children account for most cases of miliary TB.
• Definitive diagnosis of tuberculosis

  1. Detection of causative organism in clinical specimens

  2. Microscopy

  3. Cultural techniques