Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Epidemics and pandemics

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specificity. The active site of an enzyme consists of two key

regions, the catalytic site, which interacts with the substrate

during the reaction, and the binding site, the chemical groups

of the enzyme that bind the substrate, allowing the interactions

at the catalytic site to occur. The crevice of the active site cre-

ates a microenvironment whose properties are critical for

catalysis. Environmental factors influencing enzyme activity

include pH, polarity and hydrophobicity of amino acids in the

active site, and a precise arrangement of the chemical groups

of the enzyme and its substrate.

Enzymes have high catalytic power, high substrate

specificity, and are generally most active in aqueous solvents

at mild temperature and physiological pH. Most enzymes cat-

alyze the transfer of electrons, atoms, or groups of atoms.

There are thousands of known enzymes, but most can be cat-

egorized according to their biological activities into six major

classes: oxidoreductases, transferases, hydrolases, lyases,

isomerases, and ligases.

Enzymes generally have an optimum pH range in which

they are most active. The pH of the environment will affect the

ionization state of catalytic groups at the active site and the

ionization of the substrate. Electrostatic interactions are there-

fore controlled by pH. The pH of a reaction may also control

the conformation of the enzyme by influencing amino acids

critical for the three-dimensional shape of the macromolecule.

Inhibitors can diminish the activity of an enzyme by

altering the binding of substrates. Inhibitors may resemble the

structure of the substrate, thereby binding the enzyme and

competing for the correct substrate. Inhibitors may be large

organic molecules, small molecules, or ions. They can be used

for chemotherapeutic treatment of diseases.

Regulatory enzymes are characterized by increased or

decreased activity in response to chemical signals. Metabolic

pathways are regulated by controlling the activity of one or

more enzymatic steps along that path. Regulatory control

allows cells to meet changing demands for energy and

metabolites.

See alsoBiochemical analysis techniques; Biotechnology;

Bioremediation; Cloning, application of cloning to biological

problems; Enzyme induction and repression; Enzyme-linked

immunosorbant assay (ELISA); Food preservation; Food

safety; Immunologic therapies; Immunological analysis

techniques

EEpidemics and pandemicsPIDEMICS AND PANDEMICS

Epidemics are outbreaks of disease of bacterial or viral origin

that involve many people in a localized area at the same time.

An example of an epidemic is the hemorrhagic fever outbreak

caused by the Ebola virusin Zaire in 1976. When Ebola fever

occurs, it tends to be confined to a localized area, and can

involve many people. If an outbreak is worldwide in scope, it

is referred to as a pandemic. The periodic outbreaks of

influenzacan be pandemic.

Some maladies can be both epidemic and pandemic.

This can be a function of time. An example is Acquired

ImmunodeficiencySyndrome (AIDS). Initially, the acknowl-

edged viral agent of AIDS, the Human Immunodeficiency

Virus(HIV), was prevalent in a few geographic regions, such

as Haiti, and among certain groups, such as homosexual men

in the United States. In these regions and populations, the

infection was epidemic in scope. Since these early days, AIDS

has expanded to become a worldwide disease that cuts across

all racial, cultural, economic and geographic categories. AIDS

is now a pandemic.

Influenza can also be epidemic or pandemic. In this

case, the antigenic composition of the viral agent of the dis-

ease determines whether the virus becomes global in its distri-

bution or not. Antigenic variants of the virus that are quite

different from varieties that have preceded it, and so require an

adaptive response by the immune system before the

infection can be successfully coped with, tend to become

pandemic.

Pandemics of influenza can be devastating. The huge

number of people who become ill can tax the capability of a

regions’ or countries’ health infrastructure. The preparation to

attempt to thwart an influenza pandemic is immense. For

example, the preparation and distribution of the required vac-

cine, and the subsequent inoculation of those who might be at

risk, is a huge undertaking. In human terms, influenza pan-

demics exact a huge toll in loss of life. Even thought the death

rate from influenza is typically less than one percent of those

who are infected, a pandemic involving hundreds of millions

of people will result in a great many deaths.

Epidemics and pandemics have been a part of human

history for millennia. An example of this long-standing pres-

ence is cholera. Cholera is an infection that is caused by a

bacterium called Vibrio cholerae. The bacterium is present in

the feces, and can be spread directly to drinking water, and to

food via handling of the food in an unhygienic manner. The

resulting watery diarrhea and dehydration, which can lead to

collapse of body functions and death if treatment is not

prompt, has devastated populations all over the world since

the beginning of recorded history. The first reports that can be

identified as cholera date back to 1563 in India. This and

other epidemics in that part of the world lead to the spread of

the infection. By 1817 cholera had become pandemic. The

latest cholera pandemic began in 1961 in Indonesia. The out-

break spread through Europe, Asia, Africa, and finally

reached South America in the early 1990s. In Latin America,

cholera still causes 400,000 cases of illness and over 4000

deaths each year.

Influenza is another example of am illness that has been

present since antiquity. Indeed, the philosopher Hippocrates

first described an influenza outbreak in 412 B.C. There were

three major outbreaks of influenza in the sixteenth century

(the one occurring in 1580 being a pandemic), and at least

three pandemics in the eighteenth century. In the twentieth

century there were pandemics in 1918, 1957, and 1968. These

were caused by different antigenic types of the influenza virus.

The 1918 pandemic is thought to have killed some 30 million

people, more than were killed in World War I.

A common theme of epidemics and pandemics through-

out history has been the association of outbreaks and sanitary

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