1. Introduction Staphylococcus aureus is a pathogen that is common in our surrounding environment. It is “responsible for a wide range of human disease, including septicemia; endocarditis and pneumonia; and wound, bone, and joint infection (Feil et al 2003).” Staphylococcus aureus is commonly “associated with nosocomial infections (Caiazza and O’Toole 2003).” Group B Streptococcus is “the most common cause of bacterial infection in the newborn (Curtis et al 2003).” Streptococcus bovis (Group D Streptococcus) is important to understand because it is “capable of causing fulminant neonatal sepsis or meningitis that is indistinguishable clinically from that caused by group B streptococcus (Gavin et al 2003).” Escherichia coli is a common pathogen associated with food. “The presence of feces on meat products is a primary source of foodborne pathogens, such as Escherichia coli 0157:H7 (Ashby 2003).” As stated, all of these pathogens are capable of causing illness if the immune system is compromised or if present in large enough amounts. Therefore, it is helpful to gather information about their growth; in order to, develop ways to limit their presence in harmful numbers. By determining the pH range and optimal pH that these microorganisms survive in, we can hopefully produce cleaning products, food preparation methods, soaps, and other things which will limit the growth of these certain pathogens. Objective/ Hypothesis I plan to set up an experiment which will allow me to monitor the amount of growth of certain Staphylococcus, Streptococcus, and Escherichia species at varying pH. Through this, I want to determine the range of pH in which each organism will survive in. I also want to determine if the organisms have an optimal pH in which they experience maximum growth. I hypothesize that each organism will have a different optimal pH. Yet, their ranges will probably be similar. Figure 1: set up of material Materials and Methods The materials used can be seen in Figure 1 above. A set of tubes containing 3 mL of Tryptic Soy Broth (TSB) were treated with varying amounts of either HCl or NaOH to adjust the pH from pH 1 to pH 14. These tubes were then inoculated with either E. coli, S. aureus, or S. bovis. The tubes were then incubated at 36°C for 48 hrs. Later, the amount of growth in each tube was recorded. This information was used to set up another set of tubes identical to the first set, except the pH range between the tubes was narrowed. Once again, they were incubated and growth was recorded. Three trials of this procedure were performed for each of the three organisms. The temperature of incubation, incubation time, and broth composition (except pH) remained constant in all three trials. Results The data collected showed that Staphylococcus aureus grew at a pH range of 4.0 to 9.25 during the first two trials and a pH range of 4.5 to 9.25 during the last trial. The optimal pH of S. aureus was found to be around pH 7 in all trials. Streptococcus bovis only grew in a range very close to pH 7 in all three trials. Therefore, pH 7 is also considered the optimal pH of S. bovis in these results. Lastly, Escherichia coli experienced growth in a range of pH 7 to pH 9 in all three trials. In addition, it grew equally across this range in all three trials. Therefore, an optimal pH was unable to be determined with this particular data. The overall results from all three trials are shown for each organism in Table A below. In addition, graphs of the amount of growth of each organism over varying pH ranges are shown in Figure 2, 3, and 4. Table A: overall ranges and optimal pH for each organism Figure 2: S. aureus growth Figure 3: S. bovis growth Figure 4: E. coli growth Conclusion S. aureus preferred a pH range of 4.0 – 9.25 in trials 1and 2 and 4.5 – 9.25 in trial 3. The preferred optimal pH of S. aureus was around pH 7. This agrees, for the most part, with the work previously done Feeherry et al (2003) which states, “S. aureus growth occurs in the pH range 4.5 to 9.3 (7.2 optimum).” There is slight disagreement with the lowest (4.0) and highest (9.25) values of the pH range found in this experiment. This is probably due to experimental error. The pH paper used is not as accurate as a pH meter which was used in Feeherry’s experiment. Therefore, it can be assumed that in trial 1 and 2, the range is within the expected 4.5 – 9.3. S. bovis only grew at a range very close to pH 7. This agrees with Chamkha et al (2003) who studied a strain of Streptococcus (B7) which “grew optimally at pH 7.” E. coli grew equally well in the range of pH 7 to pH 9. Originally, it was thought that these results disagree with the previous work of Vinneras et al (2003) which states, E. coli is capable of growing in a “pH range of 2.5 – 9.3.”The explanation behind this disagreement is that in order “to survive in extremely acidic conditions, E. coli has evolved three adaptive acid resistance strategies to maintain internal pH (Gong et al 2003).” This acid tolerance is dependent on arginine. The growth broth used did not contain arginine. So, it was incapable of acid tolerance. In the hypothesis, I expected most organism would have similar ranges in which they prefer to grow. Yet, I expected each organism would have an optimal pH specific for that particular organism. This was disproved through the results of this experiment. The ranges varied greatly depending on the particular organism but the optimal pH was closely centered around 7 for most organisms. Effect of pH on the growth of Staphylococcus aureus, Streptococcus bovis, and Escherichia coli Pam Shelton Biology Undergraduate, Health Science Concentration Tennessee Technological University, Cookeville, TN 38505 pH rangeOptimal pH S. aureus4.5 – 9.25~7 S. bovis~77 E. coli7 – 9None seen