In the United States TB is on the decline. In 2007 a total of 13,293 cases were reported. The TB rate declined to 4.4 cases per 100,000 population, the lowest recorded rate since national reporting began in 1953. Despite this overall improvement, progress toward TB elimination has slowed in recent years; the average annual percentage decline in the TB rate slowed from 7.3% per year during 1993--2000 to 3.8% during 2000--2007. Also, since 1993 there has been a gradual decline in the number of TB patients with coinfection with HIV, and the number of cases of multiple drug-resistant TB has gradually dropped.
On the other hand, the proportion of TB cases contributed by foreign-born persons has increased each year since 1993. In 2007 the TB rate in foreign-born persons in the United States was 9.7 times higher than in U.S.-born persons. In many states, especially in the West, the upper Midwest, and the Northeast, most new cases of TB now occur in individuals who are foreign born.
Mycobacterium tuberculosis is the etiologic agent of tuberculosis in humans. Humans are the only reservoir for the bacterium. 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. 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...