School of Life Sciences | Ask A Biologist

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Why is Rudolp's nose red?

Identifying Bad Bacteria

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.

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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.

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

boiling water

You can remove almost all of the bacteria from water by boiling it.

book icon” title=“book icon Microbes

Story Behind the Scenes

 

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boiling water

You can remove almost all of the bacteria from water by boiling it.

book icon” title=“book icon Microbes

Story Behind the Scenes

 

For Teachers

Download comic as PDF packet
or
Get a MagCloud version for print or iPad

Share to Google Classroom

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 Volunteer page to get the process started.