Polymerase Chain Reaction (PCR) WORLD OF MICROBIOLOGY AND IMMUNOLOGY446•
Poliomyelitis has been part of human history for mil-
lennia. An Egyptian stone engraving depicting the debilitating
effects of poliomyelitis dates from over 3,000 years ago. In
that time, the occurrence of polio was rare, as sanitation was
poor. The close proximity between people and raw sewage
bestowed protective immunityagainst the polioviruses, which
reside in the feces. As sewage treatment became better and
indoor plumbing became widespread in the twentieth century,
exposure to the virus became less and the protective immunity
was less likely to develop in children. By the time the disease
was first described in Britain in 1789 by Michael Underwood,
outbreaks in children were occurring. From the latter decades
of the nineteenth century through to the 1950s, polio epi-
demicsoccurred frequently. Children were most at risk and
could be crippled from polio, or suffer muscle damage severe
enough to require the assistance of iron lungs, early mechani-
cal ventilators, because their lungs had been damaged to the
point of incapacity.
The word poliomyelitis derives from the Greek word
polio(grey) and myelon(marrow, indicating the spinal cord).
It is the poliomyelitis effect on the spinal cord that is associ-
ated with the devastating paralysis of the severe form of the
disease.
Poliovirus is a member of the enterovirus group of the
family Picornaviridae. Poliovirus serotypes (an antigenic
means of categorizing viruses) P1, P2, and P3 are the agents of
poliomyelitis. The viruses are very susceptible to heat, ultravi-
olet light and chemicals such as formaldehyde and chlorine.
Poliovirus is spread most commonly via contact with
feces. Only humans are involved in the transmission, as only
humans harbor the virus. Typically, feces-soiled hands are not
washed properly and then are put into or round the mouth.
Spread of the virus by coughing or sneezing can also occur.
The virus multiplies in the pharynx or the gastrointestinal
tract. From there the virus invades adjacent lymph tissue,
enters the blood stream and can infect cells of the central nerv-
ous system. When neurons of the brain stem are infected, the
paralytic symptoms of poliomyelitis result.
About 95% of those who are infected by the poliovirus
may not exhibit symptoms. In those people who do exhibit
symptoms, about 4–8% exhibit a fever and flu-like malaise
and nausea. Recovery is complete within a short time. These
people can continue to excrete the virus in their feces for a
time after recovery, and so can infect others. About 2% of
those with symptoms develop a more sever form of nonpara-
lytic aseptic meningitis. The symptoms include muscular pain
and stiffness. In the severe paralytic poliomyelitis, which
occurs in less than 2% of all polio infections, breathing and
swallowing become difficult and paralysis of the bladder and
muscles occurs. Paralysis of the legs and the lung muscles is
common. This condition is known as flaccid paralysis.
The paralytic form of polio can be of three types. Spinal
polio is the most common, accounting for 79% of paralytic
polio in the Unites States from 1969 to 1979. Bulbar polio,
which accounts for 2% of cases, produces weakness in those
muscles that receive impulses from the cranial nerves. Finally,
bulbospinal polio, which is a combination of the two, accounts
for about 20% of all cases.At the time of the development of the vaccines to polio,
in the early 1950s, there were nearly 58,000 cases of polio
annually in the United States, with almost 20,000 of these peo-
ple being rendered paralyzed. Earlier, President Franklin
Roosevelt committed funds to a “war on polio.” Roosevelt
was himself a victim of polio.
Jonas Salkdeveloped a vaccineto the three infectious
forms of the poliovirus (out of the 125 known strains of the
virus) in the early 1950s. His vaccine used virus that had been
inactivated by the chemical formaldehyde. An immune
response was still mounted to the virus particles when they
were injected into humans. The vaccine was effective (except
for one early faulty batch) and quickly became popular.
Soon after the Salk vaccine appeared, Albert Sabin
developed a vaccine that was based on the use of still-live, but
weakened, polio virus. The vaccine was administered as an
oral solution. While effective as a vaccine, the weakened virus
can sometimes mutate to a disease-causing form, and the vac-
cine itself, rarely, can cause poliomyelitis (vaccine-associated
paralytic poliomyelitis). As of January 2000, the Centers for
Disease Controlhas recommended that only the Salk version
of the polio vaccine be used.
There is still no cure for poliomyelitis. In the post-vac-
cine era, however, poliomyelitis is virtually non-existent in
developed countries. For example, in the United States there
are now only approximately eight reported cases of polio each
year, mostly due to the vaccine-associated paralytic phenome-
non. The last cases of poliomyelitis in the Unites States caused
by wild virus occurred in 1979. Elsewhere, there are still bout
250,00 cases every year, mainly in the India subcontinent, the
Eastern Mediterranean, and Africa. However, an ongoing vac-
cinationcampaign by the World Health Organizationaims to
eradicate poliomyelitis by 2010.See alsoHistory of public healthPPolymerase Chain Reaction (PCR)OLYMERASECHAINREACTION (PCR)
PCR (polymerase chain reaction) is a technique in which
cycles of denaturation, annealing with primer, and extension
with DNApolymerase, are used to amplify the number of
copies of a target DNA sequence by more than 106 times in
a few hours. American molecular biologist Kary Mullis
developed the idea of PCR in the 1970s. For his ingenious
invention, he was awarded the 1993 Nobel Prize in physiol-
ogy or medicine.
The extraction of DNA polymerase from thermophilic
bacteriaallowed major advances in PCR technology.
PCR amplification of DNA is like any DNA replication
by DNA polymerase in vivo(in living cells). The difference is
that PCR produces DNA in a test tube. For a PCR to happen,
four components are necessary: template, primer, deoxyri-
bonecleotides (adenine, thymine, cytosine, guanine), and
DNA polymerase. In addition, part of the sequence of the tar-
geted DNA has to be known in order to design the according
primers. In the first step, the targeted double stranded DNA is
heated to over 194°F (90°C) for denaturation. During thiswomi_P 5/7/03 11:09 AM Page 446