How can you tell a tiger from a wolf? By observing the characteristics of the animal. In the same way, each species of bacteria exhibits characteristics that, when taken together, can be used to differentiate the bacterial species in question from other species of bacteria.
There are different bacterial characteristics you will study throughout the semester. These include cultural characteristics, cellular characteristics, and biochemical characteristics. The simplest of these to observe are the cultural characteristics. Learning the cultural characteristics of the organisms we use in the lab will help you greatly, especially when working on your morphological unknown.
To observe the cultural characteristics (also referred to as colonial morphology) of an organism, you will need a plate on which there are isolated colonies of the bacteria to be studied. A stereo microscope (dissecting scope) or hand lens will also be helpful.
Select one or two isolated colonies to observe. Be sure the colonies you select look similar to the other colonies growing on the plate.
Cultural characteristics involve a host of factors:
Some people even use the smell of an organism to help determine its identity.
Keep in mind that different types of media, which contain different nutrients, can affect the cultural characteristics of bacteria. Some types of media are much more nutritive and will encourage hearty growth. Some types of media may restrict growth.
Colonial morphology may also be affected by the temperature at which the bacteria is incubated. Some bacteria grow better at body temperature and grow weakly at room temperature, or vice versa. Some bacteria express certain characteristics, such as the formation of pigment, more strongly at some temperatures than at others.
The length of time for which the bacteria is incubated may also affect colonial characteristics such as colony size.
The presence of other organisms can interfere with bacterial growth. Also, if the initial inoculum is too concentrated or too diluted, colonial morphology can be affected.
Here's a table you can use to record cultural characteristics of the bacteria you study. This information will help you greatly when solving your unknowns.
Here's a list of the organisms we use in our lab.