Microbiology and Immunology

Enzyme-linked immunosorbant assay (ELISA) WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Enterovirus infection is spread easily, as the virus is

found in saliva, sputum or nasal secretions, and also in the

feces of those who are infected. Humans are the only known

reservoir of enteroviruses. Following spread to water via

feces, enteroviruses can persist in the environment. Thus, sur-

face and ground water can be a source of enterovirus.

Spread of an enterovirus occurs by direct contact with

the fluids from an infected person, by use of utensils that have

been handled by an infected person, or by the ingestion of con-

taminated food or water. For example, coughing into some-

one’s face is an easy way to spread enterovirus, just as the

cold-causing rhinoviruses are spread from person to person.

Fecal contact is most common in day care facilities or in

households where there is a newborn, where diapers are

changed and soiled babies cleaned up.

The spread of enterovirus infections is made even eas-

ier because some of those who are infected do not display any

symptoms of illness. Yet such people are still able to transfer

the infectious virus to someone else.

The common respiratory infection can strike anyone,

from infants to the elderly. The young are infected more fre-

quently, however, and may indeed be the most important

transmitters of the virus. Common symptoms of infection

include a runny nose, fever with chills, muscle aches and

sometimes a rash. In addition, but more rarely, an infection of

the heart (endocarditis), meninges (meningitis) or the brain

(encephalitis) can develop. In newborns, enterovirus infection

may be related to the development of juvenile-onset diabetes,

and, in rare instances, can lead to an overwhelming infection

of the body that proves to be lethal.

Although enterovirus-induced meningitis is relatively

rare, it afflicts between 30,000 and 50,000 people each year in

the United States alone.

Evidence is accumulating that suggests that enterovirus

infections may not only be short in duration (also referred as

acute) but may also become chronic. Diseases such as chronic

heart disease and chronic fatigue syndrome may well have an

enterovirus origin. Moreover, juvenile diabetes may involve

an autoimmune response.

The climate affects the prevalence of the infections. In

tropical climates, where warm temperatures are experienced

throughout the year, enterovirus infections occur with similar

frequency year-round. But, in more temperate climates, where

a shift in seasons is pronounced, enterovirus infections peak in

the late summer and fall.

Another factor in the spread of enterovirus infections is

the socio-economic conditions. Poor sanitation that is often

coincident with lower economic standing is often associated

with the spread of enterovirus infections.

Following inhalation or ingestion of enterovirus, viral

replication is thought to occur mainly in lymphoid tissues of

the respiratory and gastrointestinal tract that are in the imme-

diate vicinity of the virus. Examples of tissues include the ton-

sils and the cells lining the respiratory and intestinal tracts.

The virus may continue to replicate in these tissues, or can

spread to secondary sites including the spinal cord and brain,

heart or the skin.

As with other viruses, enteroviruses recognize a receptor

molecule on the surface of host cells and attach to the receptor

via a surface molecule on the virus particle. Several viral mol-

ecules have been shown to function in this way. The virus then

enters the host cell and the genetic material is released into the

cytoplasm(the interior gel-like region) of the host cell. The

various steps in viral replication cause, initially, the host cell

nucleusto shrink, followed by shrinkage of the entire. Other

changes cause the host cell to lose its ability to function and

finally to explode, which releases newly made virus.

Currently, no vaccineexists for the maladies other than

polio. One key course of action to minimize the chances of

infection is the observance of proper hygiene. Handwashing is

a key factor in reducing the spread of many microbial infec-

tions, including those caused by enteroviruses. Spread of

enteroviruses is also minimized by covering the mouth when

coughing and the nose when sneezing.

See alsoCold, common; Viruses and responses to viral in-

fection

ENZYME-LINKED IMMUNOSORBANT ASSAY

(ELISA)Enzyme-linked immunosorbant assay (ELISA)

The enzyme-linked immunosorbant assay, which is commonly

abbreviated to ELISA, is a technique that promotes the bind-

ing of the target antigen or antibodyto a substrate, followed by

the binding of an enzyme-linked molecule to the bound anti-

gen or antibody. The presence of the antigen or antibody is

revealed by color development in a reaction that is catalyzed

by the enzyme which is bound to the antigen or antibody.

Typically, an ELISA is performed using a plastic plate

which contains an 8 x 12 arrangement of 96 wells. Each well

permits a sample to be tested against a whole battery of

antigens.

There are several different variations on the ELISA

theme. In the so-called direct ELISA, the antigen that is fixed

to the surface of the test surface is the target for the binding of

a complimentary antibody to which has been linked an

enzyme such as horseradish peroxidase. When the substrate of

the enzyme is added, the conversion of the substrate to a col-

ored product produces a darkening in whatever well an anti-

gen-antibody reaction occurred.

Another ELISA variation is known as the indirect tech-

nique. In this technique a specific antibody recognizes the

antigen that is bound to the bottom of the wells on the plastic

plate. Binding between the antigen and the antibody occurs.

The bound antibody can then be recognized by a second anti-

body, to which is fixed the enzyme that produces the color

change. For example, in this scheme the first, or primary, anti-

body could be a rabbit antibody to the particular antigen. The

so-called secondary antibody could be a goat-antirabbit anti-

body. That is, the primary antibody has acted as an antigen to

produce an antibody in a second animal. Once again, the dark-

ening of a well indicates the formation of a complex between

the antigen and the antibodies.

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