Where can we find bacteria?
by Liz LaRosa
In some states, you may not be able to do this lab any more, check with your science supervisor for safety
to take bacterial swabs from various places in the school
to inoculate a petri dish with a bacterial culture
to count bacterial colonies
to find out where bacteria hang out
to determine what kind of environmental conditions influence bacterial growth
Pre Lab Questions:
Where do you think you will find the highest amount of bacteria?
What do you think the petri dish will look like in 3-4 days?
Materials: (per group of 4 students)
1 petri dish with agar (2 students use each half of the dish)
2 cotton Q-tips
2 index cards with sample location
an incubator or really warm place (such as the school's boiler room!)
Choose an index card to determine your sample location
You will share one petri dish
Turn the dish upside down and using your marker, draw a line down the middle of the dish so that you have two equal halves.
Place your initials, date and sample location along the bottom perimeter of the dish, NOT in the middle.
When you and your partner are ready, come up and get your sterile Q-tip. Be very careful not to touch the side that will collect your sample, your hands can contaminate the Q-tip and alter your results.
Go to your assigned area and come back quickly!
Carefully open your jar (like Pac Man) and lightly rub your Q-tip across the agar on your side of the dish
Tape the dish shut and draw what your dish looks like in Figure 1.
Place your petri dish upside down on the tray.
We will examine the dishes in a few days and you will draw your finding in Figure 2.
Figure 1 : Draw your petri dish as soon as you have placed your sample on the agar (1/2 page)
Figure 2: Draw you Petri Dish after 3 - 4 days (1/2 page)
Table 1: Number of Colonies on petri dish (whole page)
Number of Colonies
How many clusters of bacteria appear to be growing in each petri dish?
Which perti dish had the most growth? The Least?
Why was the agar sterilized before this investigation?
What kind of environmental conditions seem to influence where bacteria are found?
How can you control the amount of bacteria that you will encounter?
2-3 sentences on what you learned.
Prepare the petri dishes a few days ahead of time. They are time consuming and require sterile procedures. Follow agar directions.
Leave one petri dish open on your desk all day long. Explain that there is bacteria floating in the air and landing on the dish RIGHT NOW! Close the dish and place it in a warm place. Show them the dish in a few days and they will be amazed!
Discuss the importance of keeping the lids on the petri dish and to only open the dish enough to place the Q-tip in and take out. Use the "Pac Man" technique.
Make a list of the places that the kids can go to for a sample. Have them pick an index card, at random, and that is where they will get their sample. The index card also acts as a hall pass. Students are allowed a few minutes to go and come back.
Some places to go are: water fountain, bathroom toilet handle, door knobs, secretary's phone, nurse's office bed, banisters, gym locker room benches or floors, cafeteria tables, computer keyboards, student desks, etc....
Fuzzy Colonies = Fungus not Bacteria
When going over the samples, place the all the dishes on a large table and see if one class had more bacteria then another class but from the same location. For example, if you have an 8 am class and a 2 pm class, there is more likely chance of bacteria accumulating through the day as more people are around. You may also notice patterns, that certain colony types (same color for example) may be from certain locations.
Dispose of the petri dishes carefully! If possible, autoclave them and place them in biohazard bags or multiple bag layers.
Pouring Plates - good animation of petri dish