Chapter 26 MTB

Created by

Angela Marie

Cards (38)

Mycobacterium tuberculosis 
Slender, slightly curved or straight rods
Nonmotile and nonspore-forming
Cell wall has high lipid content (AFB)
Strictly aerobic
Grow more slowly (2 to 6 weeks)
M. leprae fails to grow in vitro
M. tuberculosis Complex 
M. tuberculosis
M. bovis (BCG)
M. africanum (TB in tropical Africa)
M. canettii
M. microti (TB in both immunocompetent and immunocompromised persons)
Robert Koch 
Discovered M. tuberculosis
TB is one of the oldest communicable diseases
Types of TB 
Primary TB
Reactivation TB
Extrapulmonary TB
Primary TB 
Disease of the respiratory tract
Airborne transmission
Hypersensitivity reaction leads to the formation of a hard tubercle or granuloma
Bacilli remain viable in granulomas for months or years (reactivation)
Positive PPD skin test
Reactivation TB 
Risk factor: Malnutrition
Alteration or suppression of the cellular immune system
Chronic cases: Fibrosis of the lungs and calcifications; negative PPD test; confirmed by smear and culture of sputum
Fiberoptic bronchoscopy
Extrapulmonary TB 
Less common
Miliary TB – seeding of many organs outside the pulmonary tree with AFB
Occurs after primary pulmonary TB
Most cases occur in children and HIV (+) persons
Examples: Pleurisy (unexplained pleural effusion), Lymphadenitis, Skeletal TB (Pott disease), Meningitis
Identification of M. tuberculosis
Colonies: Raised, dry, rough, nonpigmented
Cord formation (cord factor)
Grows at 35° to 37°C
Niacin (+)
Reduces nitrate to nitrite
Catalase (+); heat stable catalase negative
Inhibited by NAP
Differs from M. bovis: T2H susceptible, pyrazinamidase (+)
Cord formation 
Characteristic of M. tuberculosis
Treatment of M. tuberculosis
9-month course with isoniazid and rifampin
Pyrazinamide may be added
M. bovis 
TB in cattle and other animals
In humans, it closely resembles M. tuberculosis and is treated similarly
Colonies: Small, granular, rounded, white, nonpigmented
In Middlebrook 7H10 medium, colonies similar to M. tuberculosis but grows slowly
Niacin (-)
Do not reduce nitrate
T2H susceptible
Slow Growers (Nontuberculous Mycobacteria)
M. avium Complex
M. avium subsp. paratuberculosis
M. kansasii
M. asiaticum
M. celatum
M. genavense
M. gordonae
M. haemophilum
M. malmoense
M. marinum
M. scrofulaceum
M. simiae
M. szulgai
M. terrae Complex
M. ulcerans
M. xenopi
M. avium Complex (MAC)
1. M. avium
2. M. intracellulare
Disease in poultry and swine
Most common NTM causing TB in the US
M. avium subsp. paratuberculosis
Causative agent of Johne disease (intestinal infection; chronic diarrhea in animals)
Very slow growth rate (3 to 4 months)
Requires mycobactin-supplemented medium
M. kansasii 
Second to MAC as the cause of NTM lung disease
Chronic pulmonary disease involving the upper lobes
Photochromogenic
M. gordonae 
"tap-water bacillus"
M. marinum 
Isolated from aquariums
Causes "swimming pool granuloma"
M. scrofulaceum 
Cervical lymphadenitis in children
M. ulcerans 
Buruli ulcer
Third most common species next to M. tuberculosis and M. leprae
M. xenopi 
Cornmeal agar: "Bird's nest" appearance
Rapid Growers (Nontuberculous Mycobacteria) 
M. chelonae-M. abscessus Group
M. fortuitum Group
M. smegmatis Group
M. leprae 
Causative agent of Hansen disease (leprosy)
Two major forms: Tuberculoid leprosy (skin lesions; nerve involvement; effective CMI response) and Lepromatous leprosy (uneffective CMI response; progressive, malignant, life-threatening)
Tuberculoid leprosy has skin lesions, nerve involvement, and an effective cell-mediated immune response
Lepromatous leprosy has an ineffective cell-mediated immune response, is progressive and malignant, and can be life-threatening
Mycobacteriology lab should be separate, have nonrecirculating ventilation, use a biological safety cabinet, and use disinfectants like sodium hypochlorite, phenol, and formaldehyde
Specimen Collection 
Sputum and other Respiratory Specimens (sputum, bronchial aspirates, bronchoscopy)
Gastric Aspirates and Washings
Urine
Stool
Blood
Tissue and Other Body Fluids (CSF, exudates, fluids)
Digestion and Decontamination of Specimens
Purposes: liquefy the sample and kill nonmycobacterial organisms
Agents used: sodium hydroxide, N-Acetyl-L-Cysteine, benzalkonium chloride, oxalic acid
Centrifugation at 3000 x g
Specimens that require both digestion and decontamination: sputum, gastric washing, BAL, bronchial washing, transtracheal aspirate
Specimens that require decontamination only: voided urine, autopsy tissue, abdominal fluid, any contaminated fluid
Specimens that do not require decontamination: blood, CSF, synovial fluid, biopsy tissue
Staining for AFB 
Ziehl-Neelsen
Kinyoun
Auramine-Rhodamine Fluorochrome Stains
Auramine-Rhodamine Fluorochrome Stain 
Stains mycobacteria yellow-orange fluorescence
Culture Media and Isolation Methods
Strictly aerobic
5% to 10% CO2
pH 6.5 to 6.8
Types: Egg-based media (Lowenstein-Jensen, Petragnani, ATS), Serum albumin agar media (Middlebrook 7H10, 7H11), Liquid media (Middlebrook 7H9, 7H12, 7H13)
Preliminary Identification of Mycobacteria
Acid-Fast Stain
Colony Morphology (smooth/soft or rough/friable, cording)
Growth Rate (rapid vs slow)
Temperature (30-32°C, 42°C)
Photoreactivity (photochromogens, scotochromogens, nonchromogens)
Biochemical Identification of Mycobacteria
Niacin Accumulation (positive for 95% of MTB)
Nitrate Reduction (differentiates MTB from scotochromogens and MAC)
Catalase (heat-stable catalase negative for MTB)
Hydrolysis of Tween 80 (distinguishes scotochromogen from nonphotochromogen)
Iron Uptake (distinguishes M. chelonae from other rapid growers)
Arylsulfatase (positive result is pink color)
Pyrazinamidase (positive result is red pigment, differentiates M. bovis from MTB)
Tellurite Reduction (characteristic of MAC and rapid growers)
Urease (differentiates M. scrofulaceum from M. gordonae)
Inhibitory Tests 
NAP – MTB complex is susceptible
T2H – M. bovis is susceptible
Sodium Chloride Tolerance (5% NaCl) – Most rapid growers
Growth on MacConkey Agar (w/o crystal violet) – M. fortuitum-chelonae complex
Tuberculin Skin Test