### Determine the concentration of your overnight culture

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1) Assume that you measure the OD of an overnight E.coli culture. The spectrophotometer gives you a reading of 2.8. What would you do to determine the concentration of your overnight culture and why?

2) The OD of an E.coli culture is 0.23. You transfer 300 µl of this culture into 29.7ml of medium. You continue this same dilution one more time. You transfer 50 µl of your last dilution onto an LA plate. After 24 hours of incubation, you observe 11 colonies on this last plate. A) Did you expect this result? If not, name at least three reasons for this result.

3) A friend of yours asks you to help him/her draw a bacterial growth curve. He/she gives you the following information: the TA started a fresh culture at 8:30am (the OD of this fresh culture was 0.14). During class the following OD readings were obtained: 0.19 at 9:50am, 0.28 at 10:10am, 0.29 at 10:30, 0.68 at 10:50 am, and 1.1 at 11:30am. At 11:30 am the OD of a 1:10 dilution of the stock culture was 0.29. A) Draw the graph. B) What was the generation time of this culture?

4) For a different experiment you are growing up Saccharomyces cerivisiae instead of E.coli. The measured OD is 0.89. A) In order to calculate the concentration of the Saccharomyces cerivisiae culture, can you use the equation that you use to calculate the concentration of an E.coli culture? If yes or no, give a detailed explanation. B) Does E.coli or Saccharomyces cerivisiae have a shorter generation time?

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