Does light effect bacterial growth?
By Randy Abend
Question:
    Does the Addition of light in the incubator in any way increase the growth of bacteria?
Hypothesis:
    I thought that the addition of light in the incubator would indeed increase bacteria growth. I felt that light would increase growth because it often does increase growth processes. In plant life especially, light is vital for survival in most cases. I thought that this would hold true for bacteria as well.
Materials:
  1. Large supply of agar plates( number varies depending on how many types of bacteria and how many trials)
  2. incubator
  3. low-mid watt light bulb(no more than 60 watts)
  4. Bacteria cultures: Bacillus cereus, Bacillus megaterium, Bacillus subtilis, Escherichia coli, Sarcinae lutea, Serratia marcecsens
  5. Bunsen burner
  6. inoculating loop
  7. black marker
  8. labels
  9. aluminum foil or black construction paper
Methods:
    I started by taking two agar dishes and with a marker created four even sections on the bottom of each agar dish. Next, I located a culture of E. coli and placed five dots in each section of each dish. For a total of Twenty dots per dish. The way I transferred the bacteria onto the dish was by using the tip of the inoculating loop. First, I would sterilize the loop over the Bunsen burner, then take a small sample of bacteria-and finally put five dots in one section. After every five dots I would sterilize and take a new sample. Once I had two cultures with bacteria, I would label each (bacteria, date, temp, and either light or no light), put aluminum foil over one of them, and place them both in the incubator. I made sure that they were both equidistant from the light source to avoid altered results. I performed this procedure every class, changing the bacteria each time. At the beginning of each class I would remove the overnight cultures and rate the growth according to a predetermined system*. After rating the cultures, I would record the results and then create a new set of cultures.
Results:
    When recording the results, I gave each dot a number (0 to 5) based on its growth. I took the average rating for each section and then came up with an average rating for each dish. The results were as follows:
Bacteria

E.coli
S. mar
B.sub
B.meg
S.lut

 Light

1.65
3.45
1.82
4.70
1.50

 No Light

1.15
5.00
4.60
4.70
3.00
Discussion:
    As I began recording the results of my experiment, I did not see an immediate trend. The first stage had the non-lit culture(E.coli) grow more and the second day, the culture exposed to light grew more by a small margin(S.mar). From then on, the culture not exposed to light grew more every time. At first, I thought the added growth occurred because the aluminum foil might have created somewhat of an oven effect and caused higher temperatures for its culture. I tested and disproved this by putting a thermometer both inside the aluminum foil culture and the exposed culture. They both read 450c.

    I continued to do trial after trial and the covered culture continued to grow more. After talking to Mr. Reiblein, he mentioned the ultraviolet experiment(One that I had not been familiar with because I was out of school when the class did it). In that experiment the class concluded that ultraviolet does not penetrate a solid in increments of either five or ten minutes; allowing bacteria in a petri dish to survive for those times. The class did not however; expose ultraviolet rays for a prolonged period of time. It is very possible that for times such as a minimum of twenty-four hours the ultraviolet penetrates the dish enabling or stunting bacteria. The reason why the covered cultures did better is that the aluminum foil provided added protection against the ultraviolet the light bulb created.

    I ran into some problems over the course of the experiment. After coming up with a theory of why my results were what they were, I needed an answer to explain why S. marcecsens grew more in the exposed culture than the covered culture( even if it was only a .67 difference). So, I created another set of cultures for that bacteria and incubated it overnight. The new culture also had limited growth, but the covered culture had more. The source culture must have been lacking because in both cases the new cultures grew poorly. Although I am not sure it would change things, if re-creating the experiment, I might want to incubate all cultures for a set amount of time. This was impossible because sometimes I had to leave a culture for a weekend instead of just one night. I don't think things would change because the same types of bacteria were always incubated for the same time.

    Another problem that was suggested to me was the fact that my results were bias. What I mean is, once I began to see a trend I rated the dots to fit my trend. I don't believe I did this but, I think the problem is very sub-conscious. One way to combat this problem is by doing a blind test. I could ask a classmate to hand me the two cultures, having removed the foil so that I don't know which is which(only that person does). Next rate the two cultures, and have the classmate then tell you which is what.

Conclusion:
    After performing all of my trials, I have concluded my original hypothesis was incorrect. In periods of 1-3 days, the bacteria which I used in my experiment grow more when covered than they do when exposed to light in the incubator.
Go Back To The Main Page
Go Back To The Bacteria Page