Fecal contamination of water is often determined by the amount of Escherichia coli cells (​E . coli​) present in a water sample. In this study E. coli​ was used as indicator bacteria. Water samples from five different lakes, each with 0, 1 or 2 beach closures respectively, were tested for ​E. coli​ to determine if they were truly safe for swimming. It was hypothesized that if the beach had the highest number of closures, it would have the highest current E. number of coli. However, the results showed that the number of closures had no impact on E levels. coli​, with Highland Lake having the most closures and only 0.115 ​E. coli ​cells/mL compared to Sebago Lake without closures and 0.447 ​E. coli cells/mL. Even though four out of five samples tested positive for E. coli, the ​E. The quantities of coli cells/mL were all well within the 126 ​E standard. coli​/100 ml for safe levels of bacteria in freshwater lakes (EPA 2018). Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get the original essayIntroduction: Summers in Maine are short, so people take advantage of the warm weather and use the lakes for recreational activities. With this increase in the number of people swimming, it is crucial to perform weekly water testing to prevent the spread of enteric pathogens from contaminated water (MD 2018). This is particularly important since in recent years there have been numerous outbreaks of gastrointestinal illnesses associated with recreational waters (Yoder et al 2008). To help reduce public health risks associated with contaminated water, the EPA established water quality standards specifically for freshwater recreational areas in 1989. The standards were based on studies conducted by Cabelli and Dufour who demonstrated that Escherichia coli​, a fecal coliform, was found to be the best indicator of contamination levels in fresh water sources (Cabelli​ et al​ 1979). More recent findings have reaffirmed this assertion due to the ease with which E. coli​ are cultured and their positive correlation with fecal pathogens (Wade et al​ 2003). Coliforms are small Gram-negative bacteria that do not produce spores and are capable of fermenting lactose. These small bacteria are found both in the environment and in the feces of all warm-blooded animals (Cabelli et al 1982). For this reason, coliforms are used to indicate general contamination of water sources. However, because coliforms are found in many environments, a more specific indicator like E. coli ​is often used to isolate the cause of contamination. ​E. coli ​is a fecal coliform, which means that it is a type of coliform found in large quantities in the intestines of warm-blooded animals (Cabelli ​et al​ 1982). Thus the presence of ​E. coli​ in a sample indicates fecal contamination. However, most strains of E. coli ​are not harmful, but their presence indicates that other fecal pathogens may be present (Wade ​et al​ 2003). In this experiment, five lake water samples from public beaches in southern Maine were tested for fecal contamination. The methods used were an IDEXX COLISURE kit and the most probable number equation to determine the total number of E. coli cells. It is assumed that if the lake was closed most often during the summer due to unsuccessful water tests, itwould currently have the highest E level. coli​.Methods and Materials: Fifty milliliter (mL) samples of lake water were collected from five different public beaches in southern Maine. Samples were collected from Highland Lake Beach, Woods Pond Beach, Mousam Lake Beach, Crystal Lake Main Beach, and Raymond Beach on Sebago Lake. Each 50 ml sterile test tube used to collect the samples contained 0.05 ml of 10% sodium thiosulfate to prevent chlorine from killing microbes (Laboratory Manual 2018). Half of an IDEXX COLISURE package was added to each sample respectively and then shaken vigorously. Using half of a 96-well microplate, 44 wells were each filled with 200 microliters (uL) of a sample. Each sample had its own positive controls (2 wells containing 200 µl of sterile water inoculated with E. coli culture) and negative controls (2 wells containing 200 µl of sterile water). A total of 48 wells were used for each sample. This procedure was repeated for all five samples using a total of three 96-well microplates. All samples were incubated at 37 degrees Celsius for 4 days. Wells containing the sample were compared to positive and negative controls to determine the presence of coliforms and E. coli. coli​. When the nutrient indicator chlorophenol-​red-​B​-D-galactopyranoside (CPRG) is metabolized by coliforms, it releases a magenta product called chlorophenol red. Thus, the total number of wells containing a color change, from yellow to magenta, was counted and recorded. This was repeated for all samples. UV light was then used to observe the presence of E. coli ​in the sample. ​E. coli​ has a unique enzyme that allows it to metabolize 4-methylumbelliferyl-BD-glucuronide (MUG), releasing methylumbelliferone which is a fluorescent product. The total number of fluorescent wells was then counted and recorded. The 50 ml tube containing the original sample was also observed under UV light for fluorescence. This was repeated for all samples. The most probable number (MPN) equation was used to calculate the number of ​E. coli cells present in each sample. The MPN equation is: # cells per ml = (1/volume) x In (total number of wells/number of negative wells). A bar graph was then used to compare the total number of ​E. coli​ found in each lake and the number of times each beach was closed last summer. Results: The overall objective of this experiment was to measure the level of E. coli​ present in five different lake samples and determine if previous closures have had an impact on the current ​E. coli levels​. The five samples were all collected from public beach accesses within a one-hour radius of Portland. Lakes were deliberately selected based on the number of reported closures. To compare the effect of closures on the level of ​E. coli​, samples were collected from two lakes that were never closed, two lakes that were closed only once, and one lake that was closed twice. The figure below visually represents the experimental results. A bar graph illustrating E levels. coli​ found in each of the five lake samples relative to the number of closures. Figure 1. shows the overall level of ​E. coli​ found in each sample. The red number above each lake indicates the total number of times the lake was closed last summer. This highlights the correlation between ​E. coli levels and number of closures. Sebago Lake had the highest ​E. Coli levels of 0.447 cells/mL although it does not have.