show/hide words to know

Anthrax: a disease caused by bacteria that can infect animals, including humans. Anthrax can be life-threatening......more

Bacteria: one-celled, microscopic organisms that grow and multiply everywhere on Earth. They can be either useful or harmful to animals... more

Culture: a growing “soup” of microscopic organisms and the nutrients they need to survive.

Microbe: a living thing so tiny that you would need a microscope to see it... more

Postulate: a set of rules used to argue that something is true.

Spontaneous: to appear suddenly without an obvious cause.

Sterilize: to remove or kill all bacteria.

arrow left back to comic

Figuring out the "bad guys"

Many important discoveries in microbiology happened during the 1800s. Two major questions needed answers. First, did microbes just suddenly (spontaneously) appear in rotting material? Many scientists back then thought that microbes could grow in dead material without being produced by other microbes. If the microbes didn’t appear spontaneously, they would have to come from some other process.

The great French scientist Louis Pasteur found the answer. He showed that boiled water or any type of material in which all microbes were killed or removed (called sterilized)  would remain clear of bacterial growth. This meant microbes did not spontaneously grow. However, sterilized fluid would only stay free of bacteria as long as bacteria carried in the air did not “infect” that fluid.

good and bad bacteriaThe second question seems silly today. Scientists wanted to know whether microbes cause specific diseases. If they did, how could one be sure which microbe was responsible? Which bacteria were good and which were bad? Robert Koch was one of many who took on the task of answering this question. The German scientist developed a set of rules, or postulates. Koch’s rules help us tell the difference between actual disease-causing microbes and those that are harmless.

Today, scientists still use Koch’s postulates. For a microbe to be considered a disease-causing agent, Koch’s rules state the following:

  • postulates

    The microbe must be regularly found in the diseased tissue (like in the wounds caused by certain diseases).
  • The microbe must be able to be grown alone (called cultured), without other bacteria. (Keeping unwanted microbes out is important to this process.)
  • If you introduce the microbe culture into an organism that could carry the disease, it should cause the disease.
  • The same microbe should be found once again in the diseased tissue of the new host.

Koch used his own postulates to learn about anthrax, a life-threatening disease. He showed that anthrax in cattle was caused by a bacterium. Scientists began classifying all kinds of bacteria and the diseases they caused by using Koch’s ideas.

arrow left back to comic

Additional images from Wikimedia via Markus Schweiss (boiling water).

View Citation

You may need to edit author's name to meet the style formats, which are in most cases "Last name, First name."

Bibliographic details:

  • Article: Identifying Bad Bacteria
  • Author(s): Dr. Biology
  • Publisher: Arizona State University School of Life Sciences Ask A Biologist
  • Site name: ASU - Ask A Biologist
  • Date published: July 9, 2014
  • Date accessed: March 18, 2018
  • Link:

APA Style

Dr. Biology. (2014, July 09). Identifying Bad Bacteria. ASU - Ask A Biologist. Retrieved March 18, 2018 from

American Psychological Association. For more info, see

Chicago Manual of Style

Dr. Biology. "Identifying Bad Bacteria". ASU - Ask A Biologist. 09 July, 2014.

MLA 2017 Style

Dr. Biology. "Identifying Bad Bacteria". ASU - Ask A Biologist. 09 Jul 2014. ASU - Ask A Biologist, Web. 18 Mar 2018.

Modern Language Association, 7th Ed. For more info, see
You can remove almost all of the bacteria from water by boiling it.

Be Part of
Ask A Biologist

By volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program. If you are interested in helping with the website we have a Volunteers page to get the process started.

Donate icon  Contribute

Share this page:

Share to Google Classroom