Concept Check 2 MB351 Fall 2018 PDF

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TOPIC SECTION 2: CONCEPT CHECK This Topic Section 2 Concept Check must be completed using this Word form and uploaded to Moodle as a PDF. ONLY PDF documents will be accepted/graded. *Please see separate document on Moodle for dilution problem set*

Student Name: Carol Kowald

Scientists use a number of different methods to determine the concentration of microorganisms that are present in a given population. There are multiple methods that can be used to accomplish this goal including measuring the optical density of the population with a spectrophotometer, directly counting of microorganisms with microscopic counts, and serial dilution plating to count viable cells. There a re pros and cons for each method. The question being asked will dictate which method is best. Example Problem Set: You are the lead microbiologist at a large fast food company charged with ensuring food safety. Recently, many customers have become ill after dining at one of your company’s restaurants. Individuals developed symptoms consistent with an Escherichia coli infection. You are charged with determining what product is contaminated with this pathogen to stop others from being infected, as well as develop methods to prevent contamination, and potential human illness, in the future. Escherichia coli is a Gram negative, facultative anaerobe, that is catalase positive, and able to ferment lactose. Most strains of E. coli are naturally resistant to the antibiotic rifampicin. It is a common inhabitant of the lower intestine of warm-blooded animals (including humans); however, some pathogenic strains can cause severe disease in humans. In an effort to determine what food product(s) is/are the source of the E. coli outbreak, you decide to use a viable plate count on three different products you suspect to be the source, including shredded cheese, lettuce, and ground beef. You separately homogenize these products, perform serial dilutions, and plate on nutrient rich agar (For example: LB Media). Your dilution experiment results for each of the three food products are given as a separate PDF document on the Moodle Course Page. Use that document to answer the following questions. What are the concentrations of viable bacteria for each food product? You must include units in your answer. *Please see accompanying PDF document for dilution experiments. (2 points each) Cheese: 5.4 x 10^4 CFU/mL Lettuce 2.45 X 10^4 CFU/mL Beef: 2.85 X 10^7 CFU/mL

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For the following questions, you may use lecture content, and you may also use outside resources.

Questions: Describe at least three environmental conditions (i.e. temperature), you would use to incubate your dilution plates to detect E. coli, with a brief explanation. (3 points) Since E.Coli is a mesophile, I would incubate the plates at around its optimum growth rangearound 25-40 degrees Celcius. Since E.Coli is a faculative anaerobe, it can continue to grow without oxygen present like in the gut. I would still incubate with oxygen however because studies have shown it grows better with the presence of oxygen in the lab since it doesn't have to go through anaerobic fermentation. The last environmental condition would be the pH at which the dilution plate is kept at. The average human colon has a pH between 5.5 and 7. This lines up with the optimum range of E. coli as well. The bacterium can survive between 4.4 and 9.7

Are the CFU/mL numbers calculated above representative of ALL of the bacteria associated with these food products? Why/Why not? (2 points) No. In science, applying a fact to ALL of something is rarely appropriate. Other bacteria can be associated with these food products. In this sample, we are only looking at populations of E.Coli, and there could be plenty other bacteria associated that couldn't grow on the agar for some reason or were not selected for in the experiment. Explain two different growth medias you could employ to selectively determine the concentration of E. coli in these samples. Use the information above to help with this response. (2 points) MacConkey agar would be appropriate since it selects for Gram - bacteria. MAcConkey is a form of selective and differential media and selects for or against certain specific bacteria. One could also use another selective media, like an MRS agar, since you want to inhibit growth of every microbe except for the E.coli. As the problem stated above, the E.Coli should be gronw on nutrient rich agar so the E.Coli has the necessary nutrients to grow. With the experiment discussed in the problem set, can you identify what sample(s) is/are the likely source of contamination? Why/Why not? If not, what would you have to do experimentally to answer this question? (2 points) From the calculated concentrations of viable bacteria, it is shown that the beef has the highest amount of E.Coli concentration. For starters, E.Coli is shown to be able to live in beef very well and is an extremely common source of E.Coli contaminiation. From the calculations above, the E.Coli concentration in the beef is shown to be at the highest value, at 2.85 x 10^7 CFU/mL However, in the experiment, it is shown that E.Coli was present in the cheese, lettuce, and beef. It is impossible to tell which each E.Coli infection came from directly. Experimentally, you could test each ingredient and see if iit has the properitties capable of causing human illness. One needs to see which E.coli have a toxic strain in them that causes the illness. One can't determine which caused the illness based off numbers alone.

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