To introduce methods for observing and measuring microbial growth on a macroscopic scale.

Students observed bacteria/yeast grow from "nothing" into "something." The lesson began with the introduction of the concept of a microbial colony. Although it is not possible to see one individual bacteria/yeast, they can be seen by eye when many are clustered together.

Each student had an opportunity "to streak" from a bacterial or yeast colony onto a fresh petri plate. Students were not immediately able to see what they put on the plate. When the plates were left at room temperature for a couple of days, colonies formed. In addition, a small amount of bacteria/yeast was put into a liquid medium. Over time the liquid got cloudier and cloudier as the bacteria/yeast reproduced. Because it is important to have control comparisons, students also observed plates and liquid media that were not inoculated with bacteria/yeast. There was no growth in these controls. The comparison demonstrated that bacteria/yeast do not magically appear.

This lesson reemphasized the idea that microorganisms are very small. A small sample of one colony, which contains many bacteria or yeast, can be seen under the microscope.

All students observed growth both on plates and in liquid culture. (video clip- how did this change- Deb) On plates, students interpreted growth as colonies appearing and increasing in size. In liquid medium, students interpreted microbial growth as increasing cloudiness of the liquid. Some additional contaminating microorganisms appeared on the plates, raising some discussion about sterile technique.