The purpose of this experiment was to compare the growth rates of three types of bacteria at four incubation temperatures.
I became interested in this idea because I’ve always been fascinated with bacteria and health. I have read a lot about bacteria and their growing temperatures and I really enjoyed it so I decided to do a project on it this year.
The information gained from this experiment would help food preparers determine better temperatures to store food. It would also help hospitals, and homes know temperatures that would protect them from various bacteria.
→ My first hypothesis was that Staph Aureus would grow best at 37∞C.
→ My second hypothesis was that E. coli would grow best at 37∞C.
→ My third hypothesis was that the Group B Strep would grow best at 37∞C.
→I base my hypotheses on a quote from Marie Clark, a microbiologist at Memorial Hospital, who stated, “These three bacteria normally grow best at 37∞ C.“ Human body temperature is about 37∞C and I thought this would be the optimal.
The constants in this study were:
- Testing
procedures
- Incubator,
incubator temperature, and humidity
- Equipment
type
- Magnifying
glass used to view colonies
- Time
exposed to the specific temperature
- Light
- Turbidity
of each test tubes contents
The
manipulated variables were the temperatures (5∞, 20∞, 37∞, and 42∞ C) and the
three types of bacteria ( Staph Aureus, E coli, and G. B Strep.)
The responding variable was the growth rate of the bacteria.
To measure the responding variable I counted the number of different bacteria colonies visible to the naked eye at each temperature.
QUANTITY
|
ITEM
DESCRIPTION
|
4
|
Incubators
|
12
|
Plastic test tubes
|
48
|
Petri dishes
|
1
|
Sample of Staph Aureus
|
1
|
Sample of E. coli
|
1
|
Sample of G. B. Strep
|
1
|
Data sheet
|
2
|
Pair of latex or vinyl
gloves
|
1
|
Fluid impermeable lab
coat
|
1
|
Nephlometer (turbidity
meter)
|
2
|
Micro-liter measurers (10
and 20 uLs)
|
3
|
Bacteria spreading loops
|
PROCEDURES
1. Place
12 test tubes in a tray in four rows of three.
2. Label the tubes in the front row 1, 2, 3, 4.
3. Pump 1.8ml. distilled water into the first test tube in the first row.
4. Use a cotton swab to remove some bacteria from sample containing Staph Aureus.
5. Swish the swab around in the water of the first test tube in the first row.
6. Measure and adjust the turbidity of the test tube contents on an electric density measurer so that it has 80% light transmission.
7. Remove 20 micro liters from first test tube and place it in second test tube of that same row.
8. Add 1.8 ml. to the second test tube in the first row (making a dilution of 1/10 of the first tube.)
9. Take 20 uL of solution from second test tube and put it in the third test tube of that same row.
10. Add 1.8 ml to the solution in the third test tube of that same row. This is a dilution of 1/100 of the first tube.
11. Take 20 uL of solution in the third test tube and put it in the fourth test tube of that same row.
12. Add 1.8 ml to the solution in the fourth test tube of that same row. This is a dilution of 1/1000 of the first tube. The first tube now has the most bacteria and the fourth now has the least.
13. Repeat steps 3-10 on the remaining two rows using E. coli and Group B Strep.
14. Sort 48 Petri dishes into 4 groups of 12 labeling the first 12 with the first incubation temperature.
15. Divide the 12 dishes into 3 groups of 4; label each dish in a group with one of the 3 bacteria types and the numbers 1, 2 ,3, or 4.
16. For each different incubation temperature, take another 12 and do as in steps 14 and 15 but label accordingly.
17. From the fourth test tube of row #1 (having least amount of bacteria) draw 10 uL of the diluted bacteria and place it on one Petri dish labeled with that bacteria name and the number “4”.
18. Spread bacteria evenly with a sterile loop in three directions - vertically, horizontally and diagonally.
19. Repeat steps 17-18 except use tube #3 for this bacteria and label the Petri dish accordingly.
20. Repeat step 19 except for tubes #2 and #1.
21. Repeat steps 17-20 with the other types of bacteria labeling the Petri dishes accordingly.
22. Place each of the 48 dishes in the specified incubators (12 for each incubator) for three days.
23. Take the dishes out after the time allotted.
24. Count the number of colonies grown on each dish at each temperature for each bacteria type.
25. Record the results on a data sheet.
26. After the tests have been conducted kill all of the bacteria. The bacteria laden plates and test tubes must be autoclaved.
The original purpose of this experiment was to determine the growth rates of three types of bacteria at four incubation temperatures.
The results of the experiment were that at 42∞ C. Staph Aureus grew the most overall colonies at 352 colonies; E. coli had only 320 colonies; and Group B Strep had only 64 colonies. At 37∞ C. Staph Aureus again had 604 colonies; E. coli was next at 280 colonies; and Group B Strep had 120 colonies grown. At 20∞ C. E. coli had 431 colonies, Staph Aureus had 272 colonies and GB Strep had 162 colonies. At 5∞ C. E. coli had 4 colonies, Staph Aureus had 1 colony, and GB Strep had none.
Overall the three types of bacteria grew the best at 37∞ Celsius with an average of 335 colonies grown. The next most productive temperature was 20∞ Celsius with 228 colonies. At 42∞ Celsius 245 colonies grew. The last was 5∞ Celsius that grew only 2 colonies.
From all three temperatures the bacteria that grew the most were Staph Aureus at 307 colonies. E coli was second and grew 258 colonies. Then it was Group B Strep with 86 colonies.
My first hypothesis was Staph Aureus would grow best at 37∞C. The results indicate that this hypothesis should be accepted.
My second hypothesis was E coli would grow best at
37∞C. This hypothesis should be rejected because E coli grew the best at
20˚C.
My third hypothesis was Group B Strep would grow best at
37∞C.This hypothesis should also be rejected because GB Strep grew the
best at 20˚C.
Because of the results of this experiment, I wonder if the results would change if I kept the same temperatures but saw if they grew better in a sugar or a starch environment. I also wonder if the results would have been different if I had used 32˚C or 27˚C incubation temperatures but had kept the same type of environment.
If I were to conduct this project again I would use less bacteria in the tubes so the bacteria growth would’t be as dense as it was from tubes 2, 3, and 4 (which had colonies numbering in the thousands to ten-thousands and had to be estimated as to the number of colonies grown.) As an alternative, I would then calculate the area covered by the bacteria colonies rather than count the individual colonies themselves.
Researched
by ---- Rachel E
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