Microbiology and Immunology

Toxoplasmosis WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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immune system. The immune cells release a compound called

cytokine. Normally, only a small proportion of the immune cells

are releasing cytokine. But the massive cytokine release that

occurs in response to the staphylococcal toxin produces the

myriad of physiological changes in the body.

Treatment of toxic shock syndrome depends on the

prompt recognition of the symptoms and their potential sever-

ity. Immediate administration of antibioticsis essential.

The number of cases of toxic shock syndrome has been

reduced since the suspect superabsorbant tampons were with-

drawn from the marketplace.

See alsoBacteria and bacterial infection; Enterotoxin and exo-

toxin; Immune system

TToxoplasmosisOXOPLASMOSIS

Toxoplasmosis is an infectious disease caused by the proto-

zoan Toxoplasma gondii. The infection results from a parasitic

association with a human host.

Cats are the primary carrier of the protozoan

Toxoplasma gondii. In the United States, approximately 30%

of cats are at some time infected by Toxoplasma gondii. Cattle,

sheep, or other livestock can also excrete a form of the proto-

zoan known as an oocyst. Although oocysts are not capable of

producing an infection, they are important because they act to

preserve the infectious capability of the protozoan during

exposure to inhospitable environments. In this capacity they

are analogous to the bacterial spore. Oocysts are often capable

of resuscitation into the infectious form after prolonged peri-

ods of exposure to adverse environments.

Humans can also become infected by eating fruits and

vegetables that have themselves become contaminated when

irrigated with untreated water contaminated with oocyte-con-

taining feces.

Humans typically contract toxoplasmosis by eating

cyst-contaminated raw or undercooked meat, vegetables, or

milk products. The protozoan can also be spread from litter

boxes or a sandbox soiled with cat feces. In all cases, the agent

that is ingested can be the inactive oocyst or the actively grow-

ing and infectious egg form of the parasite.

In the human host, the parasite is able to grow and

divide. This causes the symptoms of the infection.

Symptoms of toxoplasmosis include a sporadic and

reoccurring fever, muscle pain, and a general feeling of

malaise. Upon recovery, a life-long immunityis conferred. In

some people, the disease can become chronic and cause an

inflammationof the eyes, called retinochoroiditis, that can lead

to blindness, severe yellowing of the skin and whites of the

eyes (jaundice), easy bruising, and convulsions. As well

inflammation of the brain (encephalitis), one-sided weakness

or numbness, mood and personality changes, vision distur-

bances, muscle spasms, and severe headaches can result.

Person to person transmission is not frequent. Such

transmission occurs only during pregnancy. Some six out of

1,000 women contract toxoplasmosis during pregnancy.

Nearly half of these infections are passed on to the fetus.

Congenital toxoplasmosis afflicts approximately 3,300 new-

borns in the United States each year. In such children, symp-

toms may be severe and quickly fatal, or may not appear until

several months, or even years, after birth.

As for many other microbial diseases, the observance of

good hygiene(including appropriate hand washing protocols)

is a key means of preventing toxoplasmosis.

See alsoImmunodeficiency diseases; Protozoa; Zoonoses

TRACKING DISEASES WITH TECHNOLOGY

• seeEPIDEMIOLOGY, TRACKING DISEASES WITH TECHNOLOGY

TTranscriptionRANSCRIPTION

Transcription is defined as the transfer of genetic information

from deoxyribonucleic acid(DNA) to ribonucleic acid(RNA).

The process of transcription in prokaryotic cells (e.g., bacte-

ria) differs from the process in eukaryotic cells (cells with a

true nucleus) but the underlying result of both transcription

processes is the same, which is to provide a template for the

formation of proteins.

The use of DNA as a blueprint to manufacture RNA

begins with an enzyme called RNA polymerase. The enzyme

is guided to a certain region on the DNA, called the promoter,

by association with molecules known as sigma factors. There

are many promoters on DNA, located just before a region of

DNA that codes for a protein. The promoter serves to position

the RNA polymerase so that transcription of the full coding

region is accomplished.

Once the polymerase has bound to a promoter, the

sigma factors detach and can serve another polymerase. The

attached polymerase then begins to move along the DNA,

unwinding the two strands of DNA that are linked together and

using the sequence on one of the strands as the blueprint for

RNA manufacture. The strand from which RNA is made is

known as the template or the antisense strand, while the other

strand to which it is complimentary is called the sense or the

coding strand.

As the polymerase moves along the DNA, the strands

link back together behind the polymerase. The effect is some-

what similar to a zipper with a bulge, where the two links of

the zipper have come apart. The bulging region can move

along the zipper, with separation and reannealing of the

strands occurring continuously with time. The promoter can

accommodate the binding of another polymerase as soon as

the region is free. Thus, the same stretch of DNA can be

undergoing several rounds of transcription at the same end,

with polymerase molecules positioned all along the DNA.

The RNA that is produced is known as messenger RNA

(or mRNA). The species derives its name from its function. It

is the tangible form of the message that is encoded in the

DNA. The mRNA in turn functions as a template for the next

step in the genetic process, that of translation. In translation

the mRNA information is used to manufacture protein.

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