Koch’s postulates WORLD OF MICROBIOLOGY AND IMMUNOLOGY328•
discovery that tuberculin was a valuable diagnostic tool (caus-
ing a reaction in those infected but none in those not infected),
rather than a cure, helped restore Koch’s reputation. In 1892,
there was a cholera outbreak in Hamburg. Thousands of peo-
ple died. Koch advocated strict sanitary conditions and isola-
tion of those found to be infected with the bacillus. Germany’s
senior hygienist, Max von Pettenkofer, was unconvinced that
the bacillus alone could cause cholera. He doubted Koch’s
ideas, going so far as to drink a freshly isolated culture.
Several of his colleagues joined him in this demonstration.
Two developed symptoms of cholera, Pettenkofer suffered
from diarrhea, but no one died; Pettenkofer felt vindicated in
his opposition to Koch. Nevertheless, Koch focused much of
his energy on testing the water supply of Hamburg and Berlin
and perfecting techniques for filtering drinking water to pre-
vent the spread of the bacillus.
In the following years, he gave the directorship of the
Institute over to one of his students so he could travel again.
He went to India, New Guinea, Africa, and Italy, where he
studied diseases such as the plague, malaria, rabies, and vari-
ous unexplained fevers. In 1905, after returning to Berlin from
Africa, he was awarded the Nobel Prize for physiology and
medicine for his work on tuberculosis. Subsequently, many
other honors were awarded him recognizing not only his work
on tuberculosis, but his more recent research on tropical dis-
eases, including the Prussian Order Pour le Merits in 1906 and
the Robert Koch medal in 1908. The Robert Koch Medal was
established to honor the greatest living physicians, and the
Robert Koch Foundation, established with generous grants
from the German government and from the American philan-
thropist, Andrew Carnegie, was founded to work toward the
eradication of tuberculosis.
Meanwhile, Koch settled back into the Institute where
he supervised clinical trials and production of new tuberculins.
He attempted to answer, once and for all, the question of
whether tuberculosis in cattle was the same disease as it was
in humans. Between 1882 and 1901 he had changed his mind
on this question, coming to accept that bovine tuberculosis
was not a danger to humans, as he had previously thought. He
presented his arguments at conferences in the United States
and Britain during a time when many governments were
attempting large-scale efforts to minimize the transmission of
tuberculosis through limiting meat and milk.
Koch did not live to see this question answered. On April
9, 1910, three days after lecturing on tuberculosis at the Berlin
Academy of Sciences, he suffered a heart attack from which he
never fully recovered. He died at Baden Baden the next month
at the age of 67. He was honored after death by the naming of
the Institute after him. In the first paper he wrote on tuberculo-
sis, he stated his lifelong goal, which he clearly achieved: “I
have undertaken my investigations in the interests of public
health and I hope the greatest benefits will accrue therefrom.”See alsoBacteria and bacterial infection; History of microbi-
ology; History of public health; Koch’s postulates; Laboratory
techniques in microbiologyKKoch’s postulatesOCH’S POSTULATES
Koch’s postulates are a series of conditions that must be met
for a microorganism to be considered the cause of a disease.
German microbiologist Robert Koch(1843–1910) proposed
the postulates in 1890.
Koch originally proposed the postulates in reference to
bacterial diseases. However, with some qualifications, the
postulates can be applied to diseases caused by virusesand
other infectious agents as well.
According to the original postulates, there are four con-
ditions that must be met for an organism to be the cause of a
disease. Firstly, the organism must be present in every case of
the disease. If not, the organism is a secondary cause of the
infection, or is coincidentally present while having no active
role in the infection. Secondly, the organism must be able to
be isolated from the host and grown in the artificial and con-
trolled conditions of the laboratory. Being able to obtain the
microbe in a pure form is necessary for the third postulate that
stipulates that the disease must be reproduced when the iso-
lated organism is introduced into another, healthy host. The
fourth postulate stipulates that the same organism must be able
to be recovered and purified from the host that was experi-
mentally infected.
Since the proposal and general acceptance of the postu-
lates, they have proven to have a number of limitations. For
example, infections organisms such as some the bacterium
Mycobacterium leprae, some viruses, and prionscannot be
grown in artificial laboratory media. Additionally, the postu-
lates are fulfilled for a human disease-causing microorganism
by using test animals. While a microorganism can be isolated
from a human, the subsequent use of the organism to infect a
healthy person is unethical. Fulfillment of Koch’s postulates
requires the use of an animal that mimics the human infection
as closely as is possible.
Another limitation of Koch’s postulates concerns
instances where a microorganism that is normally part of the
normal flora of a host becomes capable of causing disease
when introduced into a different environment in the host (e.g.,
Staphylococcus aureus), or when the host’s immune systemis
malfunctioning (e.g., Serratia marcescens.
Despite these limitations, Koch’s postulates have been
very useful in clarifying the relationship between microorgan-
ismsand disease.See alsoAnimal models of infection; Bacteria and bacterial
infection; Germ theory of disease; History of immunology;
History of microbiology; History of public health;
Laboratory techniques in immunology; Laboratory tech-
niques in microbiologyKKöhler, Georges ÖHLER, GEORGES(1946-1995)
German immunologistFor decades, antibodies, substances manufactured by the
plasma cells to help fight disease, were produced artificially
by injecting animals with foreign macromolecules, thenwomi_K 5/6/03 3:29 PM Page 328