Introduction

I wanted to conduct an experiment which would place two types of bacteria together and would be able to allow me to find out which bacteria was doing better. I chose the two bacteria because they had color and could easily be told apart. I started experimenting with Serratia marcescens and Sarcinae lutea. The question was brought about by the presence of many types of bacteria on a petri dish. The bacteria all lived there, but would one type of bacteria dominate? That is the question I wanted to answer. I figured that experimenting with only two types of bacteria would be a good manageable number. I had no clear hypothesis. This is because I had never seen the two types of bacteria in direct competition. I did assume, however, that one type of bacteria would manage to win out over the other type.

Materials

1. Sarcina lutea
2. petri dishes
3. Serratia marcescens
4. bunsen burners
5. nutrient rich agar
6. distilled water
7. inoculating loop
8. slides

Procedure

I experimented with varies ways of testing competition between bacteria. First, I experimented with making a dish with which had one streak of each bacteria, one streak of both bacteria, and one streak of each bacteria in an "X." Then I tried to switch the order of the streaks which made the "X" to see if that had a different effect. Another method of testing the bacteria which I tried was juxtaposition. This required me to make a streak of one bacteria and then put dots of the other right next to the streak. The final method, which I used as the basis for my experiment, was the streak plate method. This method required many steps. First, I had to get a petri dish with nutrient rich agar and divide the dish into three equal sectors. I drew lines on the bottom of the dish and numbered the sectors ("1", "2", and "3"). Then I had to light the bunsen burner and sterilize my loop in the flame. Then I used the loop to place a drop of water on the slide. Then I sterilized the loop again and used it to take some bacteria from a culture of S. marcescens and smear it in the water drop on the slide. I sterilized the loop again and took some S. lutea and smear that on the slide. The slide then contained a mixed culture. I sterilized the loop and used it to obtain one loop of the mixture. I streaked it back on forth in Sector One without streaking the same place twice. I sterilized the loop and streaked through Sector One in a line. Then I used the loop to streak Sector Two in the same manner as I did Sector One. After sterilizing the loop, I made a line with the loop through Sector Two, and used it to streak Sector Three(Figure One). I then incubated the dish at 30 degrees The results were then very easy to obtain (Figure Two). I merely had to count the number of colonies of each bacteria in each sector. Then I could get numerical results to back up my claim that one bacteria did better than another bacteria.

Results

I conducted five trials using my final method. I decided that the best way to get results would be to count the number of each type of bacteria and compare those numbers. Whenever there was a sector which was all one bacteria, I estimated that about 1000 colonies inhabited that area if it was Sector One, 500 if it was Sector Two, and 250 if it was Sector Three. This made it easier to count the bacterial colonies. For trial one, I had 1124 colonies of S. marcescens compared 127 colonies of S. lutea, a ratio of about 9 to 1. In trial two, the numbers were 1610 and 57, a ratio of about 28 to 1. In the third trial, the numbers were 1533 and 41, a ratio of nearly 37 to 1 . In trial four the numbers were 1750 and zero. Finally, in the fifth trial, the numbers were 1568 and 17, with a ratio of 92 to 1. There is no common ratio of one bacteria to the other. Also, during the course of my experiment I tried to place two trials in the incubator at 45 degrees and they didn't produce any results.

Discussion

The main purpose of my experiment was to find out which bacteria would do better in direct competition. I came up with a method for experiment, carried it out, and obtained good results.
The results of my experiment are very conclusive. There can be no doubt that when the two types of bacteria are placed together and allowed to grow, the S. marcescens is able to thrive while the S. lutea is comparatively scarce. Table I shows the totals and it is clear that the S. marcescens has more colonies. The results prove that one bacteria does better than another, but they don't tell me why that happens. I have, however, come up with a couple of theories on my own.
The first theory is the "more needy bacteria theory." This theory proposes that the Serratia marcescens needs more elements from the environment in order to thrive. I believe that it take up more water and more of the nutrients from the agar to survive. I believe that for this reason, the S. marcescens manages to leave nothing on which the S. lutea can live. The second is the "more advanced bacteria theory." I theorize that the Serratia marcescens is a more advanced bacteria and has developed systems for coping with competition. I think it might be able to naturally produce some chemical which it releases to kill off other types of bacteria. This might work like an antibiotic. This isn't as practical as my first theory, however, because I have observed the two types of bacteria living side by side. The two trial which I had placed in the incubator at 45 degrees never grew. I think that this may have happened for two reasons. The first is that the temperature was too high and it dehydrated and killed the bacteria. The second is that there was a light bulb in the incubator. I believe that the light emitted by the bulb may have inhibited the growth of the bacteria.
I observed that the optimum conditions for growth were darkness, 30 degrees Celcius, and about 48 to 72 hours of incubation. Whenever I deviated from that, I got very few or no results.

Conclusion

In direct competition, or at least in the way which I tested competition, the bacteria Serratia marcescens is able to thrive over the bacteria Sarcinae lutea. The S. marcescens reproduces more than the S. lutea. The problem with my experiment was that there were many different ways to show visually that the S. marcescens is able to do better than the S. lutea, but I had trouble finding a numerical way to prove it.

The method I used finally provided numerical results, but it left me with the problem of why the results turned out the way they did. Had I been able to observe the bacteria under a microscope for a lengthy period of time I might have been able to tell why. There was no possible way for me to do that so my only option was to theorize to attempt to explain.

I think the use of such contrasting bacteria was really good because it makes the counting of the different colonies much easier. I don't think replication would have been beneficial because the five trial were all in accordance.