http://www.life.umd.edu/classroom/bsci424/BSCI223WebSiteFiles/KochsPostulates.htm
Koch's Postulates
Four criteria that were established by Robert Koch to identify the causative agent of a particular disease, these include:
1.the microorganism or other pathogen must be present in all cases of the disease
2.the pathogen can be isolated from the diseased host and grown in pure culture
3.the pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal
4.the pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen
http://dobzhanskycenter.bio.spbu.ru/pdf/sjop/MS341_O'Brien_HIV Newsline 2ndary.pdf
The HIV-AIDS Debate Is Over
By Stephen J. O'Brien, Ph.D.
February 1997
During the early years of the AIDS epidemic, both defenders and critics of the theory
that HIV causes AIDS agreed that HIV failed to completely fulfill Koch's postulates (1, 7,
13, 14, 25). As defenders of the theory were quick to point out, a number of other
diseases, notably typhoid fever, diphtheria, and leprosy, also fail to meet these
stringent tests of causality -- yet there is no controversy about what causes these
illnesses. We know the pathogens that produce these diseases; what we cannot do with
consistency is culture those pathogens in vitro.
This was the problem with HIV as well, until recently. There was little question, even
among the counter-theorists, that HIV clearly satisfied the first and second of Koch's
postulates, but it proved considerably harder to show that HIV also fulfilled the third.
Today, however, overwhelming epidemiological and experimental data have been
assembled to fulfill all three of Koch's postulates, establishing to a virtual certainty that
HIV causes AIDS (26).
[postulate 1]
Demonstrating the epidemiological concordance of HIV exposure and AIDS was
relatively straightforward, once the etiologic agent had been identified. Numerous
studies have shown, for example, that prompt and progressive depletion of CD4
lymphocytes -- and a subsequent diagnosis of AIDS -- follows HIV seroconversion in the
vast majority of HIV-infected hemophiliacs (27, 28), and HIV antibodies have been
detected in more than 90% of transfusion recipients who received blood from donors
who were HIV-positive. In the latter group seroconversion has likewise led to
progressive depletion of CD4 cells and the onset of AIDS (27-29).
Two recent cohort studies of HIV-positive hemophiliacs have provided an even more
direct link between HIV infection and mortality: They show a ten-fold increase in deaths
among antibody-positive patients compared to uninfected individuals, irrespective of
the severity of the subjects' hemophilia (30, 31). Significantly, since the screening of
donated blood for the presence of HIV was instituted, new infections have dropped
almost to zero among hemophiliacs and transfusion recipients -- further proof that HIV
is the cause of AIDS.
The fact that HIV itself (or antibodies to the virus) can be detected in more than 95% --
but fewer than 100% -- of AIDS patients worldwide is explained by the relative
insensitivity of the early tests for the presence of HIV in patients' peripheral blood. By
the more sensitive HIV RNA assays now used to detect the virus, it is possible to
confirm the presence of HIV in individuals who have as few as 20 viral particles per mL
of blood (see "The HIV RNA Assay: A Valuable New Diagnostic Tool," Vol. 2, No. 2).
Sensitive as these new diagnostic tests are, they will not detect HIV in all profoundly
immunocompromised patients -- not because the virus fails Koch's test for
pathogenicity but because other disorders cause the body's immune system to collapse
(32, 33). Certain drugs also produce immune suppression, as do chemical carcinogens,
irradiation, and cigarette smoke.
[postulate 2]
The isolation component of Koch's postulates has been repeatedly demonstrated since
the discovery of HIV. Scores of isolates have been cultured from AIDS patients; the
virus has been cultivated in fresh human T lymphocytes; and cultured-cell lines have
been developed for in vitro propagation (10, 34). ...
[postulate 3]
This leaves only Koch's third postulate
-- transmission pathogenesis -- as a matter of contention. Ethical considerations
preclude the experimental inoculation of uninfected individuals with HIV, and this
makes empirical verification of Koch's last postulate exceedingly difficult.
Difficult, but not impossible. For while we cannot deliberately infect anyone with HIV
merely to satisfy Koch's postulates and Duesberg's curiosity, we can examine the
evidence that has been gathered on healthcare workers who were accidentally infected
with HIV in the course of their professional work. Take, for example, the cases of three
laboratory technicians who were inadvertently exposed to the HTLV-IIIb strain of HIV-1
while working with that strain in their laboratories (35). All three of these technicians
developed antibodies to HIV, and within five years all three showed marked CD4
lymphocyte depletion. Two had their CD4 counts fall to less than 200 cells/mm3, and
one of those developed PCP.
In all three of these cases it was possible to establish the precise phylogenetic type of
the virus that had infected the laboratory workers. When genetic sequencing tests were
performed on the laboratory virus and on viral samples taken from the three workers,
the sequence divergence was less than 3% (36). This low level of divergence is
equivalent to the variation observed in cases of HIV transmission from mothers to their
infants -- and it is less than one third as great as the extent of variation seen when
viral samples from unconnected patients are compared (37, 38). Thus, these three
unfortunate individuals provide prima facie evidence of transmission pathogenesis,
Koch's third postulate.
This same high level of genetic concordance was also seen when the C.D.C. compared
viral samples taken from a Florida dentist who died of AIDS with samples taken from
five of his patients who tested positive for HIV and who had no HIV risk factors other
than multiple visits to the dentist for invasive procedures (39, 40). Two independent
research groups reached the same conclusion after examining the HIV gene sequences
of these six individuals: the dentist had almost certainly infected his patients in the
course of those invasive procedures, although the experts could not say exactly how
those infections had occurred (41-44).
It is unlikely that we will ever learn how transmission occurred in this unique cluster of
infections, but the genetic data gathered from the victims of this tragedy teach us an
important lesson: They establish, as conclusively as science can establish such things,
that when HIV is inadvertently transferred from a person with AIDS to an uninfected
host, it does indeed produce AIDS in that host (45). And thus it satisfies the last, and
most rigorous, of Koch's postulates.