MOD 7

• Tuberculosis (TB) is one of the top 10 causes of death worldwide and the leading cause of death from a single infectious agent (ranking above HIV/AIDS)
• Most people who develop TB are adults, and there are more cases among men than women
• TB is a disease of poverty, and economic distress, vulnerability, marginalization, stigma and discrimination are often faced by people affected by TB
• Acid-fast


  A term used to describe bacteria that resist decolorization with acidified alcohol once they have been stained, so they retain the pink to red color with carbol fuchsin
• Mycobacteria


  • Their acid-fastness property depends on the integrity of their unique cell walls that contain large amounts of lipids (long-chain fatty acids C78–C90) called mycolic acids or hydroxymethoxy acids
  • They are generally considered gram-positive, but because of their thick waxy cell wall, the bacterial cells of mycobacteria do not stain well with crystal violet, the primary stain used in the Gram's stain
• Classification of mycobacteria


  • Mycobacterium tuberculosis complex (MTBC)
  • Nontuberculous Mycobacteria (NTM)
  • Mycobacterium leprae
• Mycobacterium tuberculosis (MTB)


  Also known as Koch's bacillus or human tubercle bacillus
• Mycobacterium tuberculosis


  • Thin, slightly curved bacilli that measure 0.3 to 0.6 × 1 to 4 µm
  • Strongly acid-fast (pink to red staining), with a distinct beaded appearance due to volutin granules known as Much's granules
  • Either grow as discrete rods in Chinese letter (X, Y, V & L) configuration, or as aggregates of numerous bacilli that are arranged in long, parallel strands called serpentine cords
  • Nonmotile
  • Non-spore-forming
  • Strict aerobe
  • Slow grower, with a generation time of 15-20 hours
  • Produces niacin
  • Produces heat-sensitive catalase
• Immune response to TB infection

  1. Infected macrophages present TB antigens on MHC Class II to T-cells

  2. T-cell activation and release of cytokines like IFN-gamma

  3. Activated macrophages form granulomas around infected macrophages
• Tubercle


  • Grayish white tissue nodule, 1-2 cm in diameter
  • Central area of giant cells containing TB bacilli
  • Mid zone of pale epithelioid cells
  • Peripheral zone of fibroblasts, lymphocytes, and monocytes
• Caseation necrosis


  Center of tubercle breaks down into necrotic lesion with semi-solid or "cheesy" consistency
• Ghon complex


  Healed lesion consisting of Ghon focus (calcified primary complex) and hilar lymphadenopathy
• MTB cannot multiply within tubercles due to low pH and anoxic environment, but remain dormant
• Secondary (Reactivation) TB


  • Caseous centers of tubercles liquefy, rupture, and form air-filled cavities
  • Allows rapid extracellular multiplication and spread of MTB, usually in lung apices
• Extrapulmonary TB (EPTB)


  • Seeding of many organs outside lungs via bloodstream
  • Common sites are spleen, liver, bone marrow, kidney, adrenal gland, genital tract
• Miliary TB


  Metastasizing tubercles about the size of a millet seed
• Renal TB


  • Necrosis and scarring of renal medulla, pelvis, ureters, bladder
  • Causes painful urination, fever, blood and TB bacilli in urine
• Genital TB


  • Damages reproductive organs in both males and females, affecting reproductive function
• Bone and joint TB

  • Frequent site is spine, can cause vertebral collapse and abnormal curvature (Pott's disease)
  • Can cause paralysis and sensory loss from nerve compression
• Tuberculous meningitis


  • Active brain lesion seeds bacilli into meninges, causing mental deterioration, retardation, blindness, deafness
• Cord factor


  Inhibits migration of WBCs and causes chronic granulomas
• Sulfatides


  Prevents fusion of phagosome and lysosome, allowing MTB to survive and multiply within macrophages
• Mycolic acid


  • High lipid concentration in cell wall accounts for impermeability, resistance to antimicrobials, acidic/alkaline compounds, and complement
• Slow generation time of MTB

  • Immune system may not readily recognize or be triggered to eliminate them
• Intracellular growth and granuloma formation


  • Allows MTB to evade immune system, antibodies and complement are ineffective, caseous material blocks drug penetration
• Other causative agents of TB in humans

  • Mycobacterium bovis (Bovine TB)
  • Mycobacterium africanum (intermediate form between MTB and M. bovis)
  • Other MTBC species (M. caprae, M. microti, M. canettii, M. mungi, M. orygis, M. pinnipedii)
• BCG vaccine


  • Live attenuated strain of M. bovis, given at birth
  • 60-80% effective in children, does not prevent infection only disease
  • Can cause false positives in tuberculin skin tests
• TB prevention and control

  1. Prompt treatment of active TB

  2. TB preventive treatment for high-risk individuals

  3. Respiratory separation and isolation of infectious TB cases

  4. Eradication of TB in cattle and pasteurization of milk

  
• Runyon classification of nontuberculous mycobacteria based on growth rate and pigmentation
• Assessing photoreactivity of mycobacteria

  Inoculate media, incubate wrapped and exposed, expose to light, observe for pigment production

  
• Plate incubation and inspection


  1. Plate is returned to the incubator

  2. Plate is inspected after 24–48 hours for the appearance of yellow pigment
• Action of light on mycobacteria (Lowenstein-Jensen medium, 6 weeks at 37oC)
• Two slopes sown with the same culture

  • One exposed to light during growth
  • One grown in the same incubator but shielded from light
• Photochromogen


  NTM colonies that develop yellow pigment on exposure to light after being grown in the dark (nonpigmented in dark) and take longer than 7 days to appear on solid media
• Scotochromogen


  NTM colonies that develop pigment in the dark or light and take longer than 7 days to appear on solid media
• Runyon Group Number and Group Name

  • Runyon Group I PHOTOCHROMOGENS
  • Runyon Group II SCOTOCHROMOGENS
  • Runyon Group III NONPHOTOCHROMOGENS
  • Runyon Group IV RAPID GROWERS
• Runyon Group I PHOTOCHROMOGENS


  • NTM colonies that develop yellow pigment on exposure to light after being grown in the dark (nonpigmented in dark) and take longer than 7 days to appear on solid media
• Runyon Group II SCOTOCHROMOGENS


  • NTM colonies that develop pigment in the dark or light and take longer than 7 days to appear on solid media
• Runyon Group II SCOTOCHROMOGENS

  • M. scrofulaceum
  • M. szulgai (35-37 oC)
  • M. gordonae
  • M. flavescens
• Runyon Group III NONPHOTOCHROMOGENS

  • NTM colonies that are nonpigmented regardless of whether they are grown in the dark or light and take longer than 7 days to appear on solid media