Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Tuberculosis

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segment of DNA to be moved is physically cut from its origi-

nal location and then inserted into a new location. The DNA

from which the tranposon is removed is termed the donor

DNA, and the DNA to which the transposon is added is termed

the receptor DNA.

Transposons are not passive participants in transposi-

tion. Transposons carry the genes that code for the enzymes

needed for transposition. In essence, they carry the mecha-

nisms of transposition with them as they move or jump (hence

Barbara McClintok’s original designation of “jumping genes”)

throughout or across genomes. Transposons carry special

insertion sequences(IS elements) that carry the genetic infor-

mation to code for the enzyme transposase that is required to

accomplish transposition mutations. One of the most impor-

tant mechanisms of transposase is that they are the enzymes

responsible for cutting the receptor DNA to allow the insertion

of the transposon.

Transitions are a radical mutational mechanism. The

physical removal of both DNA and genes can severely damage

or impair the function of genes located in the transposons

(especially those near either terminus). Correspondingly, the

donor molecules suffer a deletion of material that may also

render the remaining genes inoperative or highly impaired

with regard to function.

McClintok’s discovery of transposons, also termed

“jumping genes” in the late 1940’s (before the formation of the

Waston-Crick model of DNA) resulted in her subsequent

award of a Nobel Prize for Medicine or Physiology.

Transposition segments termed retrotransposons may

also utilize an RNA intermediate complimentary copy to

accomplish their transposition.

Transposition can radically and seriously affect pheno-

typic characteristics including transfer of antibiotic resistance

in bacterium. Following insertion, transposed genetic ele-

ments usually generate multiple copies of the genes trans-

ferred, further increasing their disruption to both the genotype

and phenotypic expression.

See alsoAntibiotics; Microbial genetics

TREPONEMA•seeSYPHILIS

TRYPANOSOME•seeCHAGAS’DISEASE

TTuberculosisUBERCULOSIS

Tuberculosis (TB) is an infectious disease of the lungs caused

by the bacterium Mycobacterium tuberculosis.In the mid-

nineteenth century, about one-fourth of the mortality rate was

attributable to tuberculosis. It was particularly rampant in

early childhood and young adulthood. Its presence was felt

throughout the world, but by the 1940s, with the introduction

of antibiotics, there was a sharp decline of cases in developed

countries. For less-developed countries with poor public

healthstructures, tuberculosis is still a major problem. Since

1989, however, there has been an increase in reported cases in

economically advanced countries due mainly to immunosup-

pression associated with AIDS, and the emergence of antibi-

otic-resistant strains of TB.

The bacillus infects the lungs of those who inhale the

infected droplets formed during coughing by an individual

who has an active case of the disease. It can also be transmit-

ted by unpasteurized milk, as animals can be infected with the

bacteria. The disease is dormant in different parts of the body

until it becomes active and attacks the lungs, leading to a

chronic infection. Symptoms include fatigue, loss of weight,

night fevers and chills, and persistent coughing with sputum-

streaked blood. The virulent form of the infection can then

spread to other parts of the body. Without treatment, the con-

dition is eventually fatal.

Chest x rays and sputum examinations can show the

presence of tuberculosis. Tuberculin, a purified protein taken

from the tuberculosis bacilli, is placed under the skin of the

forearm during a tuberculosis skin test. In two or three days if

there is a red swelling at the site, the test is positive, and indi-

cates TB infection, but not necessarily active TB disease.

Early detection of the disease facilitates effective treatment to

avoid the possibility of it becoming active later on.

Populations at risk of contracting TB are people with

certain medical conditions or those using drugs for medical

conditions that weaken the immune system. Others at risk are

low-income groups, people from undeveloped countries with

high TB rates, people who work in or are residents of long-

term care facilities (nursing homes, prisons, hospitals), those

who are significantly underweight, alcoholics, and intra-

venous drug users.

Traces of lesions from tuberculosis have been found in

the lungs of ancient Egyptian mummies. The recent discovery

of a Pre-Columbian mummy has resolved the debate on

whether or not European explorers introduced the disease to

the New World. Lung samples from a Peruvian woman who

lived 500 years before Columbus discovered America show a

lump that was identified as tuberculosis by DNAtesting.

Hippocrates, a Greek physician who lived from 460 to 370

B.C., described the disease. The Greek name for the disease

was phthisis,derived from the verb phthinein,meaning to

waste away. Tuberculosis was also called consumption

because of the wasting away effects (notably, significant

losses of weight over a period of time) of the disease.

In 1839, Johann Schonlein is credited with first labeling

the disease tuberculosis. In 1882, the tubercle bacillus was

discovered by Robert Koch, the German physician who pio-

neered the science of bacteriology. This landmark discovery

was followed eight years later by his extraction of a protein

from dead bacilli called tuberculin. This protein is still used to

test for the presence of TB infection in a dormant or early

stage. Another important diagnostic breakthrough came in

1895 with the discovery of Wilhelm Conrad Roentgen’s x

rays. The presence of TB lesions was detected on x rays.

Two twentieth century French scientists, Albert

Calmette and Camille Guerin, developed a vaccineagainst

tuberculosis from a weakened strain of bovine bacillus. Called

BCG for Bacillus-Calmette-Guerin, this vaccine is the only

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