Entamoeba histolytica WORLD OF MICROBIOLOGY AND IMMUNOLOGY186•
fatally, when injected into another body, and there was always
the danger some tissue might remain in the vaccine serum after
the virus had been harvested from the culture. The discovery
that the polio virus could grow outside the nervous system pro-
vided a revolutionary breakthrough in the search for a vaccine.
As many as 20 specimens could be taken from a single mon-
key, enabling the virus to be cultivated in far larger quantities.
Because no nervous system tissue had to be used, there was no
danger of an allergic reaction through inadvertent transmission
of the tissue. In addition, the technique of cultivating the virus
and studying its effects also represented a new development in
viral research. Enders and his assistants placed parts of the tis-
sues around the inside walls of the test tubes, then closed the
tubes and placed the cultures in a horizontal position within a
revolving drum. Because this method made it easier to observe
reaction within the culture, Enders was able to discover a
means of distinguishing between the different viruses in human
cells. In the case of polio, the virus killed the cell, whereas the
measles virus made the cells fuse together and grow larger.
Because his breakthrough made it possible to develop a
vaccine against polio, Enders, Robbins, and Weller were
awarded the Nobel Prize for medicine or physiology in 1954.
Interestingly enough, Enders originally opposed Salk’s pro-
posal to vaccinate against polio by injecting killed viruses into
an uninfected person to produce immunity. He feared that this
would actually weaken the immunity of the general population
by interfering with the way the disease developed. In spite of
their disagreements, Salk expressed gratitude to Enders by
stating that he could not have developed his vaccine without
the help of Enders’ discoveries.
Enders’ work in the field of immunologydid not stop
with his polio research. Even before he won the Nobel Prize,
he was working on a vaccine against measles, again winning
the acclaim of the medical world when he announced the cre-
ation of a successful vaccine against this disease in 1957.
Utilizing the same techniques he had developed researching
polio, he created a weakened measles virus that produced the
necessary antibodies to prevent infection. Other researchers
used Enders’ methodology to develop vaccines against
German measles and chicken pox.
In spite of his accomplishments and hard work, Enders’
progress in academia was slow for many years. Still an assis-
tant professor when he won the Nobel Prize, he did not
become a full professor until two years later. This may have
resulted in his dislike for university life—he once said that he
preferred practical research to the “arid scholarship” of acade-
mia. Yet, by the mid-fifties, Enders began receiving his due
recognition. He was given the Kyle Award from the United
States Public Health Service in 1955 and, in 1962, became a
university professor at Harvard, the highest honor the school
could grant. Enders received the Presidential Medal of
Freedom in 1963, the same year he was awarded the American
Medical Association’s Science Achievement Award, making
him one of the few non-physicians to receive this honor.
Enders married his first wife in 1927, and in 1943, she
passed away. The couple had two children. He married again
in 1951. Affectionately known as “The Chief” to students and
colleagues, Enders took a special interest in those he taught,keeping on the walls of his lab portraits of those who became
scientists. When speaking to visitors, he was able to identify
each student’s philosophy and personality. Enders wrote some
190 published papers between 1929 and 1970. Towards the
end of his life, he sought to apply his knowledge of immunol-
ogy to the fight against AIDS, especially in trying to halt the
progress of the disease during its incubation period in the
human body. Enders died September 8, 1985, of heart failure,
while at his summer home in Waterford, Connecticut.See alsoAntibody and antigen; Antibody formation and kinet-
ics; Immunity, active, passive and delayed; Immunity, cell
mediated; Immunity, humoral regulation; Immunization;
Immunochemistry; Poliomyelitis and polioEEntamoeba histolyticaNTAMOEBA HISTOLYTICA
Entamoeba histolyticais a eukaryotic microorganism; that is,
the nuclear genetic material is enclosed within a specialized
membrane. Furthermore, the microbe is a protozoan parasite.
It requires a host for the completion of its life cycle, and its
survival comes at the expense of the host organism.
Entamoeba histolyticacauses disease in humans. Indeed, after
malariaand schistosomiasis, the dysenterycaused by the
amoeba is the third leading cause of death in the world. One-
tenth of the world’s population, some 500 million people, are
infected by Entamoeba histolytica, with between 50,000 and
100,000 people dying of the infection each year.
The bulk of these deaths occurs in underdeveloped areas
of the world, where sanitation and personal hygieneis lacking.
In developed regions, where sanitation is established and
where water treatment systems are in routine use, the dysen-
tery caused by Entamoeba histolyticais almost nonexistent.
A characteristic feature of Entamoeba histolyticais the
invasion of host tissue. Another species, Entamoeba dispar
does not invade tissue and so does not cause disease. This non-
pathogenic species does appear similar to the disease-causing
species, however, which can complicate the diagnosis of the
dysentery caused by Entamoeba histolytica.
Both microorganismshave been known for a long time,
having been originally described in 1903. Even at that time the
existence of two forms of the microorganisms were known.
The two forms are called the cyst and the trophozoite. A cyst is
an environmentally hardy form, designed to protect the genetic
material when conditions are harsh and unfavorable for the
growth of the organism. For example, cysts are found in food
and water, and are the means whereby the organism is trans-
mitted to humans. Often, the cysts are ingested in water or food
that has been contaminated with the fecal material of an
infected human. Within the small intestine, the cyst undergoes
division of the nuclear material and then resuscitation and divi-
sion of the remaining material to form eight trophozoites.
Some of the trophozoites go on to adhere to the intes-
tinal wall and reproduce, so as to colonize the intestinal sur-
face. The adherent trophozoites can feed on bacteriaand cell
debris that are present in the area. Some of the trophozoites are
able to break down the membrane barrier of the intestinal cellswomi_E 5/6/03 2:12 PM Page 186