Genus Mycobacterium. M.tuberculosis

Question 1

Classification

Answer 1

Family: mycobacteriaceae
Genus: mycobacterium

80 species

2 major pathogens:

• M.tuberculosis
• M.leprae

> the remaining are atypical mycobacterium - known as MOTT (mycobacterium other than Tuberculosis)

Question 2

General characteristics

Answer 2

Mycobacterium cell well:
Unique: waxy, hydrophobic and high lipid content
Up to 60% mycolic acids (Long chain, branched fatty acids)

The lipid wall is responsible for:

• difficulty in staining (they cannot stain by Gram)
• development of delayed type hypersensitivity (DTH)
• relative resistance to host phagocytic defense

Question 3

Mycobacterium tuberculosis

Answer 3

M tuberculosis complex:

• M.tuberculosis
• M.africanum
• M.bovis
• M.microti

BAT (Man)

The last three are now considered to be sub-species of M.tuberculosis

Present in human lungs
Transmitted by:
- respiratory droplets (coughing) 95%
- by alimentary route 5%

Question 4

Morphology

Answer 4

Acid fast rods
Slender
Straight or slightly curved
Arranged singly or in groups

Zeihl-Neelsen: pink in a blue background

Question 5

Growth characteristics

Answer 5

• Obligate aerobic
• slow growth (duplication time ~ 24hrs)
• catalase positive (required to activate isoniazid)

Question 6

Antigen structure

Answer 6

Cell wall antigens:

• peptidoglycan layer
• arabinogalactan layer
• mycolic acid layer
• mycosides

Question 7

Virulence factors

Answer 7

• cord factor = glycolysis derivative -> inhibits migration of PMNLs and elicits granuloma formation
• sulfatides -> inhibit phagolysosome formation
• antibacterial resistance

Question 8

Disease

Answer 8

Pulmonary TB

After primary infection, three outcome can result:

1) healed latent TB
2) systemic TB (Military TB - lethal)
3) reactivated latent TB

Extrapulmonary TB:

TB meningitis 
TB peritonitis
TB pericarditis 
TB osteomyelitis 
Renal TB
TB of lymph nodes

Question 9

Microbiological diagnosis

Answer 9

• nonsterile materials taken are decontaminated with NaOH and concentrated by centrifugation

DME: Ziehl-Neelsen stain:

• bright red, slender or slightly curved rods;
• blue stained tissue cells and other organisms

Culture:
(Routinely observed for 6 to 10 weeks)
Lowenstein-JENSEN medium - rough, dysgenic colonies, resembling “bread crumbs”

Middlebrook medium with antibiotics

Tuberculin skin tests (Mantoux method):
Intracutaneous injection of purified protein derivative (PPD) to detect hypersensitivity

positive if induration measuring 10 mm or more appears 48 hours after inoculation

Show person has been infected with TB, but doesn’t necessarily show that the person has the disease

Question 10

Treatment

Answer 10

Multiple drug therapy for 6 to 8 months

Izoniazid, rifampin and ethambutol are frequently combined

Prevention:

Izoniazid prophylaxis

BCG vaccine - contains live, attenuated strains of M. Bovis

Question 11

Prevention

Answer 11

• BCG vaccine containing live, attenuated Mycobacterium bovis organisms may prevent or limit extent of disease but does not prevent infection with M.tuberculosis
• chemoprophylaxis:
  > treatment of latent (asymptomatic) infections consists of isoniazid taken for 6 to 9 months or isoniazid + rifampin for 3 months