Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Germ theory of disease

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observed that, “There are similarities between the diseases of

animals or man and the diseases of beer and wine.” The notion

of spontaneous generation received another blow in 1858,

when the German scientist Rudolf Virchow introduced the

concept of biogenesis. This concept holds that living cells can

arise only from preexisting living cells. This was followed in

1861 by Pasteur’s demonstration that microorganisms present

in the air can contaminate solutions that seemed sterile. For

example, boiled nutrient media left uncovered will become

contaminated with microorganisms, thus disproving the notion

that air itself can create microbes.

In his classic experiments, Pasteur first filled short-

necked flasks with beef broth and boiled them. He left some

opened to the air to cool and sealed others. The sealed flasks

remained free of microorganisms, while the open flasks were

contaminated within a few days. Pasteur next placed broth in

flasks that had open-ended, long necks. After bending the

necks of the flasks into S-shaped curves that bent downward,

then swept sharply upward, he boiled the contents. Even

months after cooling, the uncapped flasks remained uncontam-

inated. Pasteur explained that the S-shaped curve allowed air to

pass into the flask; however, the curved neck trapped airborne

microorganisms before they could contaminate the broth.

Pasteur’s work followed earlier demonstrations by both

himself and Agostino Bassi, an amateur microscopist, show-

ing that silkworm diseases can be caused by microorganisms.

While these observations in the 1830s linked the activity of

microorganisms to disease, it was not until 1876 that German

physician Robert Kochproved that bacteria could cause dis-

eases. Koch showed that the bacterium Bacillus anthraciswas

the cause of anthraxin cattle and sheep, and he discovered the

organism that causes tuberculosis.

Koch’s systematic methodology in proving the cause of

anthrax was generalized into a specific set of guidelines for

determining the cause of infectious diseases, now known as

Koch’s postulates. Thus, the following steps are generally

used to obtain proof that a particular organism causes a partic-

ular disease:

1. The organism must be present in every case of the dis-
   ease.


2. The organism must be isolated from a host with the cor-
   responding disease and grown in pure culture.


3. Samples of the organism removed from the pure culture
   must cause the corresponding disease when inoculated
   into a healthy, susceptible laboratory animal.


4. The organism must be isolated from the inoculated ani-
   mal and identified as being identical to the original
   organisms isolated from the initial, diseased host.
   By showing how specific organisms can be identified as
   the cause of specific diseases, Koch helped to destroy the
   notion of spontaneous generation, and laid the foundation for
   modern medical microbiology. Koch’s postulates introduced
   what has been called the “Golden Era” of medical bacteriol-
   ogy. Between 1879 and 1889, German microbiologists iso-
   lated the organisms that cause cholera, typhoid fever,
   diphtheria, pneumonia, tetanus, meningitis, gonorrhea, as
   well the Staphylococcusand Streptococcusorganisms.

Even as Koch’s work was influencing the development

of germ theory, the influence of the English physician Joseph

Listerwas being felt in operating rooms. Building on the work

of both Semmelweis and Pasteur, Lister began soaking surgi-

cal dressings in carbolic acid (phenol) to prevent postoperative

infection. Other surgeons adopted this practice, which was one

of the earliest attempts to control infectious microorganisms.

Thus, following the invention of microscopes, early sci-

entists struggled to show that microbes can cause disease in

humans, and that public healthmeasures, such as closing

down sources of contaminationand giving healthy people vac-

cines, can prevent the spread of disease. This led to reduction

of disease transmission in hospitals and the community, and

the development of techniques to identify the organisms that

for many years were considered to exist only in the imagina-

tions of those researchers and physicians struggling to estab-

lish the germ theory.

See alsoBacteria and bacterial infection; Bacterial growth and

division; Centers for Disease Control (CDC); Colony and

colony formation; Contamination, bacterial and viral;

Epidemics and pandemics; Epidemics, bacterial; Epidemics,

viral; Epidemiology, tracking diseases with technology;

Louis Pasteur, one of the “fathers” of microbiology. Among his

accomplishments was the demonstration of the involvement of

microorganisms in disease.

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