Question: How does the pH of household vinegar on agar affect the growth of Staphylococcus epidermidis?Background information: Staphylococcus epidermidis is the most common bacteria found on the hands of humans. This bacteria was initially regarded as a microorganism found on the human skin that has no effect on the human body; however, after doing further research, scientists have concluded that it is in fact the most common source of infections on medical devices (“NNIS System Report”). This comes from the fact that S. epidermidis is a permanent colonizer of the human skin (Uckay, et. al). Though the bacteria rarely turns into a life-threatening disease, the frequency of S.

epidermidis found on the skin, along with the fact that they are so difficult to treat, raises concern for the health system. The costs that are brought up with the infections that may arise as a result of S.epidermidis are estimated to be around $2 billion annually, solely in the US (Costerton, Kloos W, Kloos WE, et. al).

This fact encourages further experiments to be conducted to determine what environments best inhibits bacterial growth. One example of a physical environment that inhibits bacterial growth is temperature. Most bacteria have a minimum, optimum, and maximum temperature for best growth.

These bacteria are often sorted into different groups based on their minimum, optimum, and maximum temperature. If a temperature is set outside of the optimum temperature range, which for S.epidermidis is 30-37 degrees celsius, the growth would not be at its maximum (Kaiser, 2006). Since that range of temperature is slightly above room temperature, it would be rather hard to control the temperature and keep it the same for all of the trials. Thus, it will be easier to store it in a refrigerator that will be set to a certain temperature.Adding on, another factor that affects bacterial growth is their oxygen requirement. Different bacteria grow best at their ideal oxygen requirement, which is why there are five groups that grow best at different oxygen requirements. The first is obligate aerobes, which are bacterial cells that only grow in the presence of oxygen because it gains most of their energy through aerobic respiration.

Contrary to obligate aerobes, there are obligate anaerobes, which are bacterial cells that only grow in the absence of oxygen and gain its energy through anaerobic respiration and/or fermentation. A type of anaerobe would be facultative anaerobes, which can essentially grow in the presence and/or absence of oxygen, but generally grows best in environments with oxygen. Most bacterial cells (including S.epidermidis) are considered to be facultative anaerobes (Kaiser, 2006).

Moving on to the topic of pH, from prior experiments that have been conducted, it can be inferred that as the vinegar pH (becomes more basic increases outside of the optimal pH range), the bacterial growth decreases. As the vinegar becomes more acidic (lower pH), it inhibits bacterial growth better, similarly to how it is inhibited when the vinegar becomes more basic (Wu, 2002).Statement of personal interest: I am personally interested in seeing if common household vinegar can affect the bacterial growth because I get sick very often, which my parents usually say is a result of me not washing my hands often enough with soap. Due to this, I think it would be very interesting to see how common household vinegar would inhibit bacterial growth on my skin, then further explore soaps that claim to kill 99.9% of germs and see if they have any of the similar components to household vinegar within the soap.

Adding on, since the bacteria is so commonly found on the hands, it would be very applicable to everyday life and will allow us to determine which soap would perhaps be the best for removing most of the S. epidermidis from the skin to prevent viral infections.Independent and Dependent Variables: The IV is the pH of the vinegar. The pH of the vinegar would be varied by combining it with bases (such as NaOH) to increase the pH. To decrease the pH of the vinegar (make it more acidic), apple cider vinegar (with the highest percentage of acetic acid) will be combined with common household vinegar. The DV is the bacterial growth, which will be measured by counting the number of colonies of bacteria (“How to Determine Bacterial Growth?”) Controlled Variables: The temperature, this will remain constant by placing all of the petri dishes inside the fridge to make sure that all petri dishes are kept at the exact same temperature. The volume of vinegar that will be placed in each petri dish, which will be kept constant by measuring the amount of vinegar that will be poured on the agar plate using a graduated cylinder. The type of light and the amount of light is another variable that will be kept constant by placing all the petri dishes in the same fridge with similar light bulbs.

The type of agar and the amount of agar used will be kept constant by using the same agar and measuring the mass of the agar in the petri dish, and then measuring the petri dish alone and subtracting the two to see the amount of agar in each petri dish.The amount of time the bacteria spends on the agar with vinegar will stay constant by checking the results every 24 hours exactly.Hypothesis:As the pH of the vinegar on the agar increases outside of the optimal pH range, the bacterial growth will decrease.

The optimal pH range for S.epidermidis is 5.5-7.0, which means it grows better in a more acidic than a basic or neutral environment (Korting et. al, 1992). Once the pH values go outside of the optimal pH range, it will inhibit further bacterial growth, given that the constants remain the same. Due to S.epidermidis being considered a neutrophile (bacteria growing best at neutral pH values), it will not grow as well if the pH were to increase outside of the optimal pH range or decrease outside of the optimal pH range.


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