Serial Dilution Lab Report

Figure 1 is an illustration of lab 1, the serial dilution performed for bacteria. This table is describing the number of colony forming units and the observations of the plates per dilution after a week. We didn’t stain the bacteria because time was limited and we would have only determined if they were gram positive or negative.

Lab 2.

After 72-hours of incubation at 37°C the fungi plates were refrigerated for a week. They were then observed, and a plate count was performed. A geometric mean was used to summarize bacteria data because the data can have so much variability. Bacteria can grow at an exponential rate under the right conditions. The geometric mean value will not be influenced by large fluctuations from between one data point and the next (Table 2). After establishing the CFU’s, the fungi was then extracted from the petri dish, stained, and then observed through microscopy. Identified were mycelium, septate hyphae, non-septate hyphae, sporangiophore, bacteria, sporangium, and spores (Figures 1-16).

Table 2 illustrates the data from the second part of the lab, for fungal microorganism.

This table is the data of the serial dilution performed per mL, it describes the CFU’s, the geometric mean, and observations.

The results for both labs were to determine the actual amount plated. This is the amount of dilution used to make the particular pour plate or spread plate. In order to do this we needed to use the following equation (Table 3):

Table 3 is the data from both labs determining the plate count per mL for both bacteria and for fungi. While doing lab 1 for bacteria we only tested each of the five dilutions on one plate; where as, for the second lab for fungi we tested four dilutions on two plates

each.

Discussion
Most of the time is is extremely difficult to determine the actual number of microorganisms in a sample. This is why scientists calculate the number of microorganisms by diluting the sample. In this experiment, we used the method serial dilution and spread plate technique to obtain the isolated colonies of bacteria and fungi that exists in local Connecticut soil. Due to of their very small size, counting the number of bacteria in soil can be difficult if not impossible. Therefore, using serial dilutions and pour plate technique was critical to determine the number of bacteria colony in the solution. A wide range of dilutions of the original solution were used to get countable numbers of bacteria in the sample. The solution is diluted to the dilution factors of 102, 103, 104 and 105. The high number of bacterial cells can be reduced by each additional dilution and easily counted. From the results, the solution culture that contain the sample have significant changes in number or organisms as dilutions get greater. The solution culture became less cloudy from the 102 diluted factor to 105 diluted factor. This means that the concentration of the bacteria in solution culture is reduced. This shows that the number of bacteria has reduced by using the serial dilution, this is proven in the data. The pour plate method were the same for the two labs; but different agar was poured. For the bacteria, a standard nutrient agar was used while for the fungi a phenol red agar was preferred. Selective and differential media are used to isolate or identify particular organisms. Selective media allow certain types of organisms to grow, and inhibit the growth of other organisms. The pour plates allowed the bacteria and fungi to grow on the surface to be observed. Refer to the lab results, the pour plate method shows that there where significant decline in numbers, there were about 143 viable cells/mL in the dilution 104, while about 67 viable cells/mL in the dilution 105. This is because there are more microorganisms in the dilution of 104, which causes too many colonies competing with each other for nutrients and space. Thus, making the colonies less visible for counting; although the colonies are visible in all the dilutions, they are not suitable to be used in calculations or data. The reason they are not suitable is because the range for colonies acceptable for statistical counting are between 30 and 300 (Pepper et al., 2015). The results of the pour plate method show that there are more than 300 colonies, TNTC for the dilutions of 102 and 103.