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Mycobacterium bovis is the etiologic agent of TB in cows and rarely in humans. Both cows and humans can serve as reservoirs. Humans can also be infected by the consumption of unpasteurized milk. This route of transmission can lead to the development of extrapulmonary TB, exemplified in history by bone infections that led to hunched backs.

Other human pathogens belonging to the Mycobacterium genus include Mycobacterium avium which causes a TB-like disease especially prevalent in AIDS patients, and Mycobacterium leprae, the causative agent of leprosy.

History and Present Day Importance

Mycobacterium tuberculosis (MTB) was the cause of the "White Plague" of the 17th and 18th centuries in Europe. During this period nearly 100 percent  of the European population was infected with MTB, and 25 percent of all adult deaths were caused by MTB (Note: The White Plague is not to be confused with the "Black Plague", which was caused by Yersinia pestis and occurred about 3 centuries earlier).

General Characteristics

Mycobacterium tuberculosis is a fairly large nonmotile rod-shaped bacterium distantly related to the Actinomycetes. Many non pathogenic mycobacteria are components of the normal flora of humans, found most often in dry and oily locales. The rods are 2-4 micrometers in length and 0.2-0.5 um in width.

Mycobacterium tuberculosis is an obligate aerobe. For this reason, in the classic case of tuberculosis, MTB complexes are always found in the well-aerated upper lobes of the lungs. The bacterium is a facultative intracellular parasite, usually of macrophages, and has a slow generation time, 15-20 hours, a physiological characteristic that may contribute to its virulence.

Two media are used to grow MTB Middlebrook's medium which is an agar based medium and Lowenstein-Jensen medium which is an egg based medium. MTB colonies are small and buff colored when grown on either medium. Both types of media contain inhibitors to keep contaminants from out-growing MT. It takes 4-6 weeks to get visual colonies on either type of media.

Colonies of Mycobacterium tuberculosis on Lowenstein-Jensen medium. CDC.

Chains of cells in smears made from in vitro-grown colonies often form distinctive serpentine cords. This observation was first made by Robert Koch who associated cord factor with virulent strains of the bacterium.

MTB is not classified as either Gram-positive  or Gram-negative because it does not have the chemical characteristics of either, although the bacteria do contain peptidoglycan (murein) in their cell wall. If a Gram stain is performed on MTB, it stains very weakly Gram-positive or not at all (cells referred to as "ghosts").

Mycobacterium species, along with members of a related genus Nocardia, are classified as acid-fast bacteria due to their impermeability by certain dyes and stains. Despite this, once stained, acid-fast bacteria will retain dyes when heated and treated with acidified organic compounds. One acid-fast staining method for Mycobacterium tuberculosis is the Ziehl-Neelsen stain. When this method is used, the MTB. smear is fixed, stained with carbol-fuchsin (a pink dye), and decolorized with acid-alcohol. The smear is counterstained with methylene-blue or certain other dyes. Acid-fast bacilli appear pink in a contrasting background.

In order to detect Mycobacterium tuberculosis in a sputum sample, an excess of 10,000 organisms per ml of sputum are needed to visualize the bacilli with a 100X microscope objective (1000X mag). One acid-fast bacillus/slide is regarded as "suspicious" of an MTB infection.

Mycobacterium tuberculosis. Acid-fast stain. CDC.

Cell Wall Structure

The cell wall structure of Mycobacterium tuberculosis deserves special attention because it is unique among procaryotes, and it is a major determinant of virulence for the bacterium. The cell wall complex contains peptidoglycan, but otherwise it is composed of complex lipids. Over 60% of the mycobacterial cell wall is lipid. The lipid fraction of MTB's cell wall consists of three major components, mycolic acids, cord factor, and wax-D.

Mycolic acids are unique alpha-branched lipids found in cell walls of Mycobacterium and Corynebacterium. They make up 50% of the dry weight of the mycobacterial cell envelope.  Mycolic acids are strong hydrophobic molecules that form a lipid shell around the organism and affect permeability properties at the cell surface.  Mycolic Acids are thought to be a significant determinant of virulence in MTB. Probably, they prevent attack of the mycobacteria by cationic proteins, lysozyme, and oxygen radicals in the phagocytic granule. They also protect extracellular mycobacteria from complement deposition in serum.

Cord Factor is responsible for the serpentine cording mentioned above. Cord factor is toxic to mammalian cells and is also an inhibitor of PMN migration. Cord factor is most abundantly produced in virulent strains of MTB.

Wax-D in the cell envelope is the major component of Freund's complete adjuvant (CFA).

The high concentration of lipids in the cell wall of Mycobacterium tuberculosis have been associated with these properties of the bacterium:
Impermeability to stains and dyes
Resistance to many antibiotics
Resistance to killing by acidic and alkaline compounds
Resistance to osmotic lysis via complement deposition
Resistance to lethal oxidations and survival inside of macrophages