Microbiology and Immunology

Thermal death WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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cines were produced, at a cost of approximately two cents per

unit, and given away to people in tropical countries and the

United States. The vaccine was so effective that the

Rockefeller Foundation ended its yellow-fever program in

1949, safe in the knowledge that the disease had been effec-

tively eradicated worldwide and that any subsequent out-

breaks could be controlled with the new vaccine.

Unfortunately, almost all yellow-fever research ended around

this time and few people studied how to cure the disease. For

people in tropical climates who live outside of the major urban

centers, yellow fever is still a problem. A major outbreak in

Ethiopia in 1960–1962 caused 30,000 deaths. The World

Health Organizationstill uses Theiler’s 17D vaccine and had

mounted efforts to inoculate people in remote areas.

The success of the vaccine brought Theiler recognition

both in the U.S. Over the next ten years, he received the

Chalmer’s Medal of the Royal Society of Tropical Medicine

and Hygiene (1939), the Lasker Award of the American Public

Health Association, and the Flattery Medal of Harvard

University (1945).

In 1951, Theiler received the Nobel Prize in medicine or

physiology “for his discoveries concerning yellow fever and

how to combat it.”

After developing the yellow-fever vaccine, Theiler

turned his attention to other viruses, including some unusual

and rare diseases, such as Bwamba fever and Rift Valley fever.

His other, less exotic research focused on polio and led to his

discovery of a polio-like infection in mice known as

encephalomyelitis or Theiler’s disease. In 1964, he retired

from the Rockefeller Foundation, having achieved the rank of

associate director for medical and natural sciences and direc-

tor of the Virus Laboratories. In that same year, he accepted a

position as professor of epidemiologyand microbiology at

Yale University in New Haven, Connecticut. He retired from

Yale in 1967.

Theiler married in 1938 and had one daughter. Theiler

died on August 11, 1972, at the age of 73.

See alsoEpidemics, viral; Epidemiology, tracking diseases

with technology; History of immunology; History of public

health; Immune stimulation, as a vaccine; Viruses and

responses to viral infection; Zoonoses

TThermal deathHERMAL DEATH

Thermal death is the death of a population of microorganisms

due to exposure to an elevated temperature.

The nature of the thermal death varies depending on the

source of the heat. The heat of an open flame incinerates the

microorganisms. The dry heat of an oven causes the complete

removal of water, which is lethal for biological structures. In

contrast, the moist heat delivered by a sterilizer such as an

autoclave causes the proteins in the sample to coagulate in a

way that is analogous to the coagulation of the proteins of an

egg to form the familiar cooked egg white.

The coagulation of proteins by heat is a drastic alter-

ation in the three-dimensional shape of these protein mole-

cules. Typically, the alteration is irreversible and renders a

protein incapable of proper function.

Thermal death also involves the destruction of the mem-

branes surrounding microorganisms such as bacteria. The high

temperatures can cause the phospholipid constituents of the

membrane to dissolve and thus destroy the membrane struc-

ture. Finally, the high heat will also cause the destruction of

the nucleic acid of the target microorganism. In the case of

double-stranded DNA, the heat will result in the disassociation

of the two DNA strands.

Thermal death can be related to time. A term known as

the thermal death time is defined as the time required to kill a

population of the target microorganism in a water-based solu-

tion at a given temperature. The thermal death time of

microorganisms can vary, depending on the thermal tolerance

of the microbes. For example, thermophilic bacteria such as

Thermophilus aquaticusthat can tolerate high temperatures

will have a thermal death time that is longer than the more

heat-sensitive bacterium Escherichia coli.

Another aspect or measure of thermal death is termed

the thermal death point. This is defined as the lowest temper-

ature that will completely kill a population of a target microor-

ganism within 10 minutes. This aspect of thermal death is

useful in purifying water via boiling. Whereas Escherichia

colipopulations will be readily killed within 10 minutes at

212°F (100°C), spores of bacteria such as Bacillus subtilus

and Clostridium perfringenswill have a higher thermal death

point, because a higher temperature is required to kill spores

within 10 minutes.

Exact temperatures and times are usually used in calcu-

lating thermal death variables because terms such as “boiling”

are not precise. For example, the boiling point of water (i.e.,

the temperature of boiling water) depends upon pressure. As

altitude above sea level increases, the boiling temperature of

water (H 2 O) lowers.

See alsoLaboratory techniques in microbiology; Sterilization

THERMOTOLERANT BACTERIA•see

EXTREMOPHILES

THIN SECTIONS AND THIN SECTIONING•

seeELECTRON MICROSCOPIC EXAMINATION OF MICROORGAN-

ISMS

TThrushHRUSH

Thrush, or oropharyngeal candidiasis, is an infection of the

mouth and throat caused by the fungus Candida, a genus of

yeast. This microorganism is naturally present on the skin and

mucous membranes, but overgrowth can cause disease.

Candidiasis is not considered communicable because the

microorganism is ubiquitous (common and widespread).

Symptoms of thrush include cottage cheese-like white

patches in the mouth and throat, with raw areas underneath.

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