Laboratory Investigations in Microbiology

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Chapter 1: Contamination

Microbiology encompasses the study of various microorganisms using similar laboratory techniques. These techniques include culturing, microscopy, and aseptic technique - the handling of microbial cultures without introducing contamination. Of all the difficulties you may encounter in performing the lab exercises this semester, the most common one (and most easily avoidable) is the problem of contamination. Contamination has even been found in rock samples from the moon!

Contamination is unwanted microbial growth. This applies to sterile culture media as well as desired cultures. A culture medium that is supposedly sterile can easily be evaluated for contamination: agar plates should have no visible growth on them, and liquid (broth) cultures should not be cloudy. Contamination in an experimental culture is more difficult to discern. We usually look for growth of colonies that differ from our desired culture in terms of color, size, shape etc. Contamination of a broth culture is even more difficult to detect - often you will not know it is contaminated unless you grow a sample on an agar plate first. 

In this exercise, you will be able to observe some sources of microbial contamination and compare the extent of contamination with that from other sources. You will also get a first look at some of the diversity of microbial growth, including bacterial and fungal colonies.

Materials & Methods

First period
  1. Obtain an agar plate and label the bottom of the plate with a permanent marker.
  2. Choose a source of contamination that you wish to sample. For example:
  3. Write your source on the board. Choose a sample location that nobody else has chosen yet.
  4. Contaminate your agar surface as follows:
  5. Bring all completed/labeled plates to the front lab desk to be incubated.
Next Lab Period
  1. Observe your agar plate for bacterial and fungal growth. 
  2. Complete the data sheet.
  3. Save your agar plate! Observe your plate again after 1 week.
Data Sheet & Review Questions (printable)

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