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Germ Invasion

Time Required Long (2-4 weeks)
Material Availability Agar plates must be specially ordered
Cost Low ($20 - $50)
Safety Must follow general safety precautions for handling microorganisms


Microbes are everywhere in our environment, but for the most part they escape our notice. This project shows you how to safely culture and study common bacteria from your everyday surroundings.


The goal of the proposed project is to determine the microbial diversity present in one's immediate environment. Of the 100 million or so bacteria that are proposed to exist, how many distinct species can one identify? How can these different species be identified: By size, shape color, growth rate?

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Laurie Usinger, Bio-Rad Laboratories
Images courtesy of Bio-Rad Laboratories

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Science Buddies Staff. "Germ Invasion." Science Buddies, 28 July 2017, Accessed 12 Nov. 2018.

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Last edit date: 2017-07-28


Microorganisms are the most fundamental, diverse, and prevalent biological organisms that inhabit the earth today. Prokaryotic microorganisms, organisms without a nucleus, can be generally divided into three classes: bacteria, protozoa, and fungi. By far the most predominant of these three classes of organisms would include bacteria, single celled organisms which inhabit every type of environment on earth, and which have been in existence for greater than 3.5 billion years. Bacteria are ubiquitous, and are found in almost any environment. They thrive in the hot environments of deep sea sulfur vents, the frozen tundra of the Antarctic, the saline environments of the Dead Sea, and extremely acidic environments such as the stomachs of organisms. In this science fair project, you will investigate what kind of bacteria, and how much bacteria, grows in different locations around your home.

Bacteria can be both pathogenic, responsible for a variety of diseases, and non-pathogenic, or harmless. Pathogenic bacteria are responsible for the outbreaks of cholera, tuberculosis, and gonorrhea, whereas non-pathogenic bacteria have many roles, which include the symbiotic residence in the stomachs and intestines of humans, the break down indigestible foods, and in return, producing necessary nutrients, such as Vitamin K. Moreover, a variety of naturally occurring antibiotics are synthesized from bacteria, such as streptomycin.

The non-pathogenic and ubiquitous E. coli bacterial strain, isolated from the guts of humans, is the most commonly grown laboratory organism. E. coli is used to produce drugs, such as insulin and human growth hormone, and is a commonly used organism in almost all biotechnology laboratories, producing a variety of reagents used by the typical scientific researcher.

Terms and Concepts

Any basic biology text will have a chapter on prokaryotic organisms. Begin by reading a text on basic microbiology, such as Chapter 16 in Biology, Exploring Life, by Campbell, Williamson, and Heyden. Topics and terms which should be researched include:
  • Prokaryote vs. eukaryote
  • Gram-negative vs. gram positive bacteria
  • Pathogenic vs. non-pathogenic bacteria
  • Bacterial motility
  • Phototrophs vs. autotrophs vs. chemotrophs vs. heterotrophs
  • Anaerobic vs. aerobic bacteria
  • Symbiotic bacteria and their importance to higher organisms
  • Bacteria, and E. coli, as scientific research tools


Visit these websites for an introduction to bacteria:

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Materials and Equipment

  • Nutrient agar plates (20), available from online supplies such as Carolina Biological, catalog # 821862. You will need to buy a quantity of two to have 20 plates.
  • Permanent marker
  • Heavy-duty tape or saran wrap
  • Bleach
  • Lab notebook

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Germ Invasion

Experimental Procedure

Working with Biological Agents

For health and safety reasons, science fairs regulate what kinds of biological materials can be used in science fair projects. You should check with your science fair's Scientific Review Committee before starting this experiment to make sure your science fair project complies with all local rules. Many science fairs follow Intel® International Science and Engineering Fair (ISEF) regulations. For more information, visit these Science Buddies pages: Projects Involving Potentially Hazardous Biological Agents and Scientific Review Committee. You can also visit the webpage ISEF Rules & Guidelines directly.

  1. Identify 9 different locations where you would like to assess biodiversity. Suggested sites could include the bathroom, kitchen, locations near heating vents, bedrooms, the refrigerator, the backyard, the garage, etc.
  2. At each site, place 2 nutrient agar plates per site. Use the permanent marker to label the bottom of each plate with its location. Leave the plates open and exposed for a period of 3 hours.
  3. An additional two plates should be unopened and used as negative controls. In other words, you'll study what grows on these plates even though you never expose them to the air. (Hopefully, very little if anything will grow!) Use the permanent marker to write "negative control" on the bottom of each of these plates.
  4. At the end of 3 hours, seal plates with heavy-duty tape or saran wrap.
  5. Allow the plates to incubate by placing all of them in one single location that has a fairly constant room temperature (about 22 degrees Celsius) for 1-3 weeks, until distinct bacterial colonies can be observed. (Don't forget to put the two, unopened control plates in this same location.)
  6. Collect data over the course of the three weeks. Every other day, write down the number of colonies, the color, and the size in your lab notebook.
  7. After the end of the three week period, make various graphs of the data. Suggestions include, but are not limited to:
    1. Colony count on each plate.
    2. Colony count at each location (take an average of two plates).
    3. Different types of microorganisms, based upon:
      1. Size
      2. Color
      3. Shape
  8. Keep the microbial plates during the duration of your project, and while you are writing up your paper. You will want to make many observations.

Discussion Points to Consider When Writing up the Results

  • Which environmental areas resulted in the most microbial growth?
    • What environmental features unique to those locations might facilitate microbial growth?
      • Moisture content?
      • Air circulation?
      • Cleanliness?
  • Based upon your background reading, can you possibly identify any of the bacterial/microbial colonies based upon their morphological features? Tip: The Science Buddies guide to Interpreting Plates should be helpful.
    • Color?
    • Shape?
    • Size?
  • If you had access to reagents in a common laboratory, how could you use more sophisticated methods to type, classify, and characterize the individual colonies?
  • Based upon your background reading, what percentage of the microbial organisms that are present in your environment did you isolate and identify?

Bacterial Safety

Bacteria are all around us in our daily lives and the vast majority of them are not harmful. However, for maximum safety, all bacterial cultures should always be treated as potential hazards. This means that proper handling, cleanup, and disposal are necessary. Below are a few important safety reminders. You can also see the Microorganisms Safety Guide for more details. Additionally, many science fairs follow ISEF Rules & Guidelines, which have specific guidelines on how bacteria and other microorganisms should be handled and disposed of.

  • Keep your nose and mouth away from tubes, pipettes, or other tools that come in contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.
  • Make sure to wash your hands thoroughly after handling bacteria.
  • Proper Disposal of Bacterial Cultures
    • Bacterial cultures, plates, and disposables that are used to manipulate the bacteria should be soaked in a 10% bleach solution (1 part bleach to 9 parts water) for 1–2 hours.
    • Use caution when handling the bleach, as it can ruin your clothes if spilled, and any disinfectant can be harmful if splashed in your eyes.
    • After bleach treatment is completed, these items can be placed in your normal household garbage.
  • Cleaning Your Work Area
    • At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly clean any surfaces you have used.
    • Be aware of the possible hazards of disinfectants and use them carefully.

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