Introduction

It is common knowledge among scientists that most bacteria grow more rapidly in warmer, damper climates. However, I was curious as to the effect of heat on antibiotics. So I set up an experiment whereby I could test the difference in the resistance of four different types of antibiotics to one type of bacteria in two different temperatures.

I hypothesized that as the temperature increased, the resistance of the antibiotics would decrease. I figured that if an antibiotic offers a certain amount of resistance to the growth at a certain dosage, it would not be able to counter the increased rate of growth that would result from the higher temperature of the environment.

I selected S. Marcescens as my experimental bacteria because of its red color and subsequent visibility. I selected the four antibiotics that had the greatest effect on the bacteria during an earlier group experiment. They were; streptomycin, kanamycin, tetracycline, and neomycin. After sterilizing a metal transfer loop, I distributed S. Marcescens among eight petri dishes that had already been filled with nutrient agar. The agar assists the growth of the bacteria by providing a constant source of nutrition. I then equally distributed the four different antibiotics, two dishes to each type. The antibiotics come in pellet form and are released by a plastic dispenser. I placed four of the dishes, each containing a different antibiotic pellet, into an incubator set at 300C, and the other four dishes into an incubator at 250C.

After letting the cultures grow for overnight, or sometimes longer, I collected data. I measured the radii of the spaces around the antibiotic pellet in which the bacteria did not grow. I used a small ruler and took the measurements in millimeters. The greater the number, the greater the degree of resistance of the antibiotic.

The results from the first trial were tainted as a result of a glitch and a subsequent change in the experimental design. So, for convenience purposes, I will only deem the data from the second and third trial relevant to this experiment.

The first results collected from the second trial both agree and disagree with my hypothesis. The Kanamycin yielded measurements of 7mm at 300C and 10mm at 250C. On the contrary, the Neomycin measured 4mm at 300C and 2mm at 250C. The icing on the cake was the negative results of both tetracycline cultures, there was no resistance in either of the two specimens. Two days later, the measurements remained pretty constant except for the change in the 250C neomycin culture from 2mm to 3.5mm of resistance.

The third trial incorporated a slight change in the experimental design. The incubator had a slight temperature gradient problem, so the 250C section became a 270C section. As a result, both the neomycin and kanamycin measurements saw noticeable decreases. The kanamycin dropped from 10mm to 7mm, and the neomycin dropped from about 3mm to 1.5mm. The figures for both the second and third trial are attached. During the second trial, I noticed that there was a color gradient among the dishes: some were darker shades of red than others. So, I conducted a side-experiment to find out if the color gradient was an inherent trait or not. I placed a sample of light bacteria and a sample of dark bacteria in each of the two temperature areas in the incubator. The results showed that both of the 250C cultures were dark while both of the 300C cultures were light.

After conducting this experiment, I have reached the conclusion that temperature affects some antibiotic behavior, depending on the type of antibiotic that is being used. Temperature did not seem to greatly affect streptomycin, while there were noticeable differences in the kanamycin and neomycin cultures. Tetracycline, in the meantime, did not resist the S. Marcescens growth under either temperature.

I believe that the cause of the inconsistency of the results is the inconsistency of the source cultures. The S. Marcescens samples for the second and third experiments were taken from various dishes that had been previously used either for my experiment or somebody else's. A specific example is the unusually weak resistance of the kanamycin at 300C in the third trial. After considering possible reasons, I concurred that it is a result of my using the 300C kanamycin culture of the second trial as a source culture for the third. Perhaps the bacteria of that culture had developed a slight immunity to kanamycin. Also, there was a slight temperature change in the incubator (250C to 270C), which may have affected the results.

There were many possible sources of error in this experiment. First, it is nearly impossible to create identical cultures of bacteria using such a crude and imprecise method as smearing. This lack of consistent appearance hinders the credibility of a comparison between cultures. It adds an extra, unwanted variable in the experiment which makes it very difficult to produce viable data. Second, there was a 20C temperature change in the incubator between trial 2 and 3 of the experiment. Again, this makes it hard to make a fair comparison of results between trials. Finally, there was the issue of the pigment levels of the S. Marcescens. The lighter shades were harder to measure accurately because the borders of the bacteria were less distinct. These errors, as well as some others make it impossible for me to guarantee the accuracy of these results and the conclusions that I draw from them. The worst thing about these experimental glitches was that there was very little that I could do to avoid them; to my knowledge, there were no other means of distributing the bacteria; I did not have enough time to conduct the multitude of experiments to solve the pigment dilemma; there was only one main incubator, so we had to make do with its flaws. Despite the bind that I found myself in, I still made a minor attempt to get to the bottom of the color gradient problem.

The side experiment that I described in the "Results" section was conducted to solve the dilemma: what is the cause of the pigment differences among various cultures of S. Marcescens? The results show that the two cultures in 260C incubation both produced dark pigment, and the two dishes at 300C produced light pigment. From this data, I have concluded that the color gradient of S. Marcescens is a result of temperature, not an inherent mutation trait. However, I have both pigments that were produced by both temperatures, so my conclusion is obviously wrong. However, I also theorize that the presence of certain antibiotics in certain temperatures may be another cause of the color gradient.

From the data collected in this experiment, I have derived what I feel to be a fair, and considerable conclusion. Temperature does not have an overall effect on antibiotic behavior. Rather, its effects vary as types of antibiotics and bacteria vary. Each case is different, as was illustrated during this experiment; streptomycin was not greatly affected by temperature, as opposed to kanamycin which showed a considerable difference between the results achieved at 250C (or270C) and those achieved at 300C. Tetracycline, unlike all of the others, did not offer any resistance to the S. Marcecsens at either temperature. It is obvious that the effect of temperature is as unique and individual as the antibiotics themselves.

I conclude that temperature does not have an overall effect on antibiotics. There are many factors which constitute the bacteria-fighting capabilities of antibiotics. Some of these include: type of antibiotic, type of bacteria, agar, temperature, and incubation period. As for S. Marcescens, the pigment variations may be the result of temperature, presence of antibiotic, or some other unknown factor. In order to produce more accurate and feasible data, one might try conducting a similar experiment using numerous types of bacteria in a more stable, and perhaps a more diverse incubator. I would strongly advise that someone who wishes to conduct an experiment similar to mine first conduct preliminary experiments to find a method by which he or she can equally distribute bacteria that will produce the same pigment. By eliminating some of these major experimental errors that I encountered, one might be able to achieve results that could truthfully answer the question: What are the effects of temperature on antibiotic resistance to bacteria?