Abstract
The report presents the results of a microbiological experiment to confirm the hypothesis that as leftover food sits out over time, the number of bacteria in the food will increase. The study was undertaken to indicate the risk of bacterial contamination and the importance of proper storage technique. The study was performed using a liquid food (corn tortilla soup) which was exposed to the external environment at room temperature. The control group in our experiment was the same food stored in a zip lock bag and refrigerated.
The soup was tested over a period of time, which served as the independent variable, to check for bacterial growth i.e. the dependent variable. The samples were collected from the experimental and control group at 3 points in time separated by 30 minute intervals. The solutions were cultured at strengths 1/100 to 1/1000 using serial dilution technique and the bacterial colonies were counted. Pour plates were prepared to nurture and enumerate the bacterial colonies. The findings showed an increase in bacteria as time progressed thus validating the initial hypothesis identified.
Further improvements were suggested by using greater number of dilutions, greater period of time over which the experiment was conducted, using different foods and using more than one type of bacteria for the experiment to provide further insights.
Introduction
This study is an independent microbiological experiment with an objective to confirm the increasing bacteria in leftover food which is kept out over a period of time by counting the microbial population. The study is important to check the safety of food which has been allowed to sit out over a period of time and consecutively, the importance of proper storage (Alderson, 2011). The experiment was conducted using corn tortilla soup. A liquid food was chosen to allow for easy and effective dilution to facilitate the experiment.
For the purpose of our study we have tested the bacterial growth in the soup at 3 points in time – 30 minutes, 60 minutes and 90 minutes post preparation. We maintained a control group for our experiment which was corn tortilla soup placed in a zip lock bag refrigerated immediately post preparation.
The bacterial growth in the soup was counted at 1/100 and 1/1000 dilutions using serial dilution technique over the 3 points in time. Serial dilution technique was used to facilitate the counting of bacterial colonies (Alderson, 2011). We used pour-plate technique for identifying and quantifying the bacterial growth. We ensured that all variables other than exposure time remained constant throughout the experiment. The pour plate technique was used as its primary function is to allow enumeration or counting of bacteria (Alderson, 2011).
The experiment was performed using corn tortilla soup which is a food broth based soup. We maintained an experimental and a control group through the experiment. Post-preparation of the soup we took the first sample and immediately put it in a zip lock bag and then refrigerated it. The second sample was taken from the same soup and left out on the counter at room temperature. We used the same soup so that no other variables would affect the results of our experiment.
As part of the experiment we tested both the samples after 30, 60 and 90 minutes of preparation to check for bacterial growth. Exact interval of 30 minutes was maintained. The actual time of collection of samples for culture was 17.30, 18.00 and 18.30. For each test we sampled 1 oz of soup broth. The soup broth was collected using 1 oz metal shot glass and a metal spoon which had been sterilized through flaming. For each sampling the same glass and spoon were used but sterilized each time.
We used a serial dilution technique with dilution factor of 10 to prepare the solutions for our experiment. One part soup was mixed with nine parts water. The resulting solution was again mixed with 9 parts of water to arrive at a 1/100 concentration of the soup broth. Using the same technique we also prepared a solution at 1/1000 concentration. Low concentrations helped in easy counting of bacteria.
The cultures which were used for bacterial counts were prepared using pour-plate technique. We mixed the solutions prepared with melted agar to allow for the bacteria to spread evenly through the medium. The agar was then allowed to set on which the bacteria formed colonies. The assumption used is that each bacterium would form an individual colony and hence counting the colonies would give us the original number of bacteria present in the solutions and consecutively the soup. We used twelve pour plates per group. Out of theses plates we selected one plate for each condition to count. These plates were divided into 32 squares and we selected five random squares for counting. Based on the total count we calculated the average number of colonies (bacteria) per square. We then calculated the total colonies on a plate. The calculations can be detailed as follows (for number of colonies per plate) –
Let x1, x2, x3, x4 and x5 be the number of colonies in the 5 squares. Average colonies per square is x = (x1+x2+x3+x4+x5)/5. Total number of colonies on a plate is 32x.
Results
We compared the cultures of the soup at 30 minutes (1/1000), 60 minutes (1/100) and 90 minutes (1/1000). We have attached the pictures of the microbial cultures prepared from the solutions.
The findings of the experiment have been tabulated as below –
The findings indicate that greater the amount of time the food is left exposed to the external environment at room temperature, higher is the bacterial growth. This confirms the primary hypothesis that we had started out with.
Discussion
The findings of the experiment show an increase in the number of bacterial colonies (and hence bacteria) in the soup as time progresses. The results therefore help in validating the initial hypothesis we started out with, namely, that the bacterial growth is directly proportional to the time that food is exposed to the external environment at room temperature. The experiment indicates the importance of refrigeration of leftovers to prevent bacterial contamination.
However, we have also noted certain steps which would have helped us in improving the experiment. Firstly, the container used for storing and transporting the soup may have allowed for accidental contamination thus skewing the findings of bacterial counts. We could have used a different storage container for transport to prevent contamination. We could have also made more serial dilutions than the two chosen to allow for better comparison. Finally, the experiment itself could be strengthened by comparing two types of bacteria and their behavior over a period of time.
As a next step, the experiment should be repeated with different kinds of food to further validate the hypothesis. The findings should be more detailed with focus on the control group to quantify the benefits of refrigeration. The comparison should be spread across more dilutions and time. Also, identifying specific types of bacteria and using different mediums to encourage growth of varied bacteria would help in developing greater insights from the experiment.
References
Alderson, G. (2011). Microbiology - Experiments and Lab Techniques (13th ed., p. 59). Fountainhead Press
Alderson, G. (2011). Microbiology - Experiments and Lab Techniques (13th ed., p. 69). Fountainhead Press
Alderson, G. (2011). Microbiology - Experiments and Lab Techniques (13th ed., pp. 299-300). Fountainhead Press
