people who simply inhaled the air around an infected animal were likely to
inhale the anthrax bacterium spores and come down with the disease. Koch’s
investigations into anthrax led him to discover how microorganisms work.
Anthrax is caused by Bacillus anthracis(Fig. 1-7), which is a bacterial type
of microorganism consisting of one cell. Bacillus anthracis,whether in a dor-
mantor an active state,is called a spore. A spore is not infectious. However,
under the right conditions, the Bacillus anthracisspores germinate and enter the
active state and rapidly multiply and become infectious.
The question that Koch raised is: Would taking active Bacillus anthracisfrom
one animal and injecting it into a healthy animal cause the healthy animal to
come down with anthrax? If so, then he could prove that a microorganism was
actually the cause of disease.
Bacillus anthraciswas present in the blood of infected animals, so Koch
removed a small amount of blood and injected it into a healthy animal. The ani-
mal came down with anthrax. He repeated the experiment by removing a small
amount of blood from the newly infected animal and gave it to another healthy
animal. It, too, came down with anthrax.
Koch expanded his experiment by cultivating Bacillus anthracison a slice of
potato. He then exposed the potato to the right blend of air, nutrients, and tem-
perature. Koch took a small sample of his homegrown Bacillus anthracisand
injected it into a healthy animal. The animal came down with anthrax.
Based on his findings, Koch developed the Germ Theory. The Germ Theory
states that a disease-causing microorganism should be present in animals
infected by the disease and not in healthy animals. The microorganism can be
cultivated away from the animal and used to inoculate a healthy animal. The
CHAPTER 1 The World of the Microorganism^13
Fig. 1-7. Bacillus anthracisrapidly multiplies in
the active state and becomes infectious.