THE MYCOBACTERIA

OVERVIEW

• There are >70 species of mycobacteria
• Of these, two are major pathogens:
  1. Mycobacterium tuberculosis (Koch, 1882)
  2. Mycobacterium leprae (Hansen, 1874)
• The remaining mycobacteria are environmental organisms-collectively known as MOTTS (Mycobacteria Other Than Tuberculosis)
• MOTT organisms are responsible for opportunistic infections, especially in  people with AIDS

CLASSIFICATION

Mycobacteria belong to the

• Order: ACTINOMYCETALES
• Family: MYCOBACTERIACEAE
• Genus: MYCOBACTERIUM

All mycobacteria are:

1. ACID FAST- i.e. they do not destain with acid and alcohol once stained with arylmethane dyes
2. AEROBIC
3. CONTAIN MYCOLIC ACIDS
4. THEIR GENOMES HAVE A 59-66% GC CONTENT

INCIDENCE OF TUBERCULOSIS

• TB is responsible for 25% of adult deaths in the developing world- more than those caused by diarrhea, malaria and AIDS combined
• South Africa has one of the highest incidences: >200/100 000
  • In Ravensmead near Cape Town there are more than 400 cases per year. The total population of that suburb is 40 000 people.
• 35 people die of TB every day in South Africa
• The incidence in the 'first world' is less than 10/100 000

THE GLOBAL EMERGENCY

• 8-12 million new infections with M. tuberculosis per year
• 2-3 million people die from TB per year
• If present trends continue, deaths will exceed 4 million by 2004
• There is also the emergence of multi-drug resistant M. tuberculosis
• These trends, and the association of TB with HIV infection, has led to TB being declared

INCIDENCE OF NON-TUBERCULOUS MYCOBACTERIAL INFECTIONS

• There were more than 800 000 cases of leprosy world-wide in 1998; of these more than 690 000 were new cases
• In the USA, environmental mycobacteria are isolated more frequently than M. tuberculosis is from clinical specimens; this is due to their association with AIDS and the low incidence of TB in that country
• Less than 3% of mycobacterial isolates in South Africa are of MOTT organisms
• Buruli Ulcer is the 3rd most common mycobacterial infection and is endemic in West Africa

THE MYCOBACTERIAL CELL WALL

• Unique cell wall: waxy, hydrophobic and high lipid content
• Up to 60% of the dry weight of the organisms may be mycolic acids- long chain, branched fatty acids
• The type of mycolic acid can be used to distinguish different mycobacteria
• The mycolic acids and short chain fatty acids form a pseudo outer membrane and are responsible for the unusual staining characteristics of the cells
• The wall is also responsible for the hydrophobicity of these organisms
• The wall has adjuvant properties and may be responsible for the development of delayed type hypersensitivity (DTH)
• All mycobacterial pathogens are intracellular pathogens- the wall helps the organism to survive within the macrophage by resisting oxydative damage

TUBERCULOSIS

Tuberculosis is a chronic, gramulomatous disease caused by the microorganism Mycobacterium tuberculosis

• Tuberculosis is an ancient disease
  • prehistoric human bones show tuberculous damage
  • mummies have been shown to contain AFBs
  • drawings from ancient Egyptian times show people with characteristic tuberculous deformities
  • M. tuberculosis may have entered the human population at the time cattle were domesticated