Omnipresence of Microorganisms in the Environment
This procedure was adapted from A Laboratory Manual for Microbiology, third edition, by John M. Larkin.
Microorganisms are ubiquitous in our environment. Practically any area of the earth is inhabited by a micro ecosystem of life forms that may be uniquely adapted to that environment.
All living cells share certain requirements for growth. C, H, O, N, P and S are the elements considered basic for cell growth. Water is essential for growth, since it comprises approximately 70% of the cell. Molecular oxygen is essential for the growth of many but not all cells. All cells have a need for energy, which may be obtained by trapping sunlight and converting light energy to chemical energy, or by releasing the chemical energy contained in bonds within compounds.
This experiment will demonstrate the numbers and diversity of microorganisms found in the environment. It will utilize the concepts of growth requirements and demonstrate the use of a selective medium. Two media which support the growth of organisms commonly found in the environment are utilized.
Nutrient agar contains
- beef extract(concentrate of water soluble components from lean beef; supplies carbohydrates, organic nitrogen, salts, vitamins)
- peptone(enzymatic digest of certain proteins; provides readily available nitrogen)
- agar(extract of marine algae which is used solely as a solidifying agent at 1.5%)
- water pH=6.8
Sabouraud agar contains
- water pH=5.
Nutrient agar is one of the most widely used media in microbiology. The Sabouraud agar is a selective medium, which is a culture medium designed to suppress the growth of unwanted microorganisms and encourage the growth of desired ones. The lower pH of the Sabouraud agar inhibits the growth of most bacteria, making it selective for yeasts and fungi. Inoculation from the same environmental site onto the two media should readily demonstrate the bacterial versus fungal flora of the site.
Materials per pair of students:
- 1 petri dish of nutrient agar (NA)
- 1 petri dish of Sabouraud agar (SDA)
- 2 cotton tipped swabs
- Obtain one petri dish of each medium.
- Inoculate each dish with the same sample from any source which interests you by placing the sample onto the surface of the media. Dirt, dust, food, etc. will all yield good results. (Students may be more interested if they are allowed to take their dishes home in order to investigate some environment about which they are curious.)
- Label the bottom of the dish with your name and the inoculum. Put tape around the sides of the Sabouraud dishes.
- Incubate the inverted dishes in a warm place for several days. (Ideally, this could be done on a Friday and plates could incubate until Monday.)
- Examine plates for growth. Typically, fungal growth is fuzzy, "hairy" and may be dark, while bacterial or yeast growth occurs in smooth, flat circular areas called colonies. Record your macroscopic observations in the circles provided below.
- You may wish to save these plates for later use in microscopic viewing