tb

• Mycobacterium

  • Aerobic
  • Non–spore forming
  • Non-motile
  • Very thin
  • Slightly curved or straight rods
• Mycobacterium cell wall

  Contains N-glycolylmuramic acid instead of N-acetylmuramic acid, and has a very high lipid content, which creates a hydrophobic permeability barrier
• Mycobacteria are difficult to stain
• Mycobacteria growth rates

  • Rapid growers (Visible colonies in <7 days)
  • Slow growers (Visible colonies in > 7 days)
• Major groups of mycobacteria


  • M. tuberculosis complex
  • NTM group
• Mycobacterial species in M. tuberculosis complex


  • M. tuberculosis
  • Mycobacterium bovis
  • Mycobacterium africanum
  • Mycobacterium caprae
  • Mycobacterium microti
• All species in M. tuberculosis complex are capable of causing tuberculosis
• M. tuberculosis is the cause of most cases of human tuberculosis, particularly in developed countries
• Inhalation of a single viable M. tuberculosis organism


  Leads to infection, although close contact is usually necessary
• Ingestion of milk from infected cows


  M. bovis may penetrate the gastrointestinal mucosa or invade the lymphatic tissue of the oropharynx
• An attenuated strain of M. bovis, bacillus Calmette-Guérin (BCG), has been used extensively in many parts of the world to immunize susceptible individuals against tuberculosis
• Tuberculosis may mimic other diseases, such as pneumonia, neoplasm, or fungal infections
• Clinical manifestations of M. tuberculosis complex infection


  • Asymptomatic
  • Acutely symptomatic with systemic symptoms, pulmonary signs and symptoms, signs and symptoms related to other organ involvement, or a combination of these features
• Specimens received by the laboratory for mycobacterial smear and culture must be handled in a safe manner
• Acid-fast bacilli can infect almost any tissue or organ of the body
• Processing to recover acid-fast bacilli from clinical specimens


  1. Decontamination

  2. Concentration
• Acid-fastness


  Characteristic of mycobacteria due to the long-chain, multiple cross-linked fatty acids (mycolic acids) in their cell walls
• Visualization of acid-fast bacilli in sputum or other clinical material should be considered presumptive evidence of tuberculosis

  
• Ziehl-Neelsen staining


  Classic carbolfuchsin stain that requires heating of the slide for better penetration of the stain into the mycobacterial cell wall

  
• With Ziehl-Neelsen staining, Mycobacterium spp. appear red or have a red-blue, beaded appearance, whereas nonmycobacteria appear blue

  
• Drug-resistant tuberculosis is a major health threat
• More than 500,000 cases of multidrug-resistant (MDR) tuberculosis occur each year
• MDR tuberculosis is resistant to rifampin and isoniazid, the two drugs most often used as effective treatment against tuberculosis
• Strains of extensively drug-resistant tuberculosis (XDR TB) are emerging that are resistant not only to rifampin and isoniazid, but also to quinolones and other drugs, such as aminoglycosides
• Drugs used to treat tuberculosis


  • Isoniazid
  • Rifampin
  • Ethambutol
  • Pyrazinamide
• To prevent the selection of resistant mutants, treatment of tuberculosis requires four drugs