You Take Absorbance Readings on a Spectrophotometer Across a 6

question 39

Multiple Choice

You take absorbance readings on a spectrophotometer across a 6 hour culture of E. coli cells growing in tryptic soy broth (TSB) . Your absorbance readings clearly indicate a lag phase, a log phase, and a stationary phase. You come back in and take readings at 8, 10, 12, 14, and 16 hours, but the absorbance number remains the same. Shouldn't it start coming down as the closed batch culture enters death phase? What's the most likely thing that is happening?

• A)Clearly, something is wrong with the spectrophotometer and it isn't measuring the correct values.Perhaps something is on the detector, making it register falsely high absorbance numbers.
• B)When we establish a growth curve, we should actually plot the log of the number of viable cells vs.time.However, a spectrophotometer can only measure absorbance.Absorbance is NOT the same as the number of viable cells.Many of the cells in the tube are most likely dead, but the machine can't discriminate between a live cell and a dead one.This keeps the absorbance high even into the death phase.
• C)It'll happen-E.coli just grow slowly.We haven't gotten to the end of the stationary phase yet.Be patient!
• D)Perhaps there's a big smudge of something on the tube that is blocking some of the light.This would lead to an elevated reading for every timepoint after the smudge was placed on the tube.

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Definitions:

Helping Behaviour

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Qualities or characteristics of a person or thing that evoke interest, desire, or pleasure in others.

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