EXERCISE 1

 

Bacteria in the Environment

 

Supplies Required: 1 tube containing 5 ml of a sterilized broth medium referred to as tryptic soy (T.S.) broth, 1 plate of solidified tryptic soy broth, sterile swabs.

 

Protocol:

 

PERIOD 1

 

1.         Label your tube and plate of tryptic soy medium (use a Sharpie or similar pen). Label everything you incubate with your name, experiment number and any identifying marks required (e.g., name of organisms if a pure culture is used, location of inoculum, etc.). Label Petri plates on the bottom (the half containing the solidified medium), not the top. Tape plates together if you are incubating more than one plate at a time.

 

2.         Inoculate your tube and plate by exposing them to objects in your immediate environment. Some suggestions follow:

 

            a.         Add some dust to the plate or tube

            b.         Cough or sneeze on the plate

c.         Swab a door knob or table top, or inside the incubator and place the swab in the broth medium or swab across the plate

            d.         Add some soil to either medium

            e.         Place your thumb on the Petri plate

            f.          Comb your hair over an exposed plate

            g.         Press your moist lips onto the exposed plate

h.        Swab the sinks in a nearby lavatory, or swab the surface of your skin and place the swab in the sterile broth

            i.         Place a coin on the agar surface

            j.         Use your imagination!

 

3.         Incubate your tube and plate in the incubator in the laboratory at 32oC (89.2oF). This temperature feels pleasantly warm. Do NOT screw the cap on your tube of broth down tightly. Leave it "cracked" a little to allow air (O2) to penetrate.

 

After 48 hours incubation your samples will be refrigerated.

 

PERIOD 2

 

1.         Retrieve your samples and check for microbial growth. How does the tube of broth appear as compared with what you started with (sterile broth)? Are there any visible "masses" (colonies) of bacteria or fungi on your Petri plate? Are the colonies the same in appearance or do they vary from one colony to another? If so, in what ways? What is the significance of a cloudy (turbid) broth tube? Why is the broth turbid? Compare your results with your neighbors and the class as a whole.

 

2.         When you are through with your materials remove all writing from any glassware. Plastic Petri plates do not need to have writing removed. Discard materials as instructed.