You Take Absorbance Readings on a Spectrophotometer Across a 6-Hour

question 16

Multiple Choice

You take absorbance readings on a spectrophotometer across a 6-hour culture of

• 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.
• 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?

---

Definitions:

Cuff Deflated

Refers to the process of releasing air from a cuff, commonly associated with blood pressure measurement devices, to lower its pressure.

Palpatory Method

Systolic blood pressure measured by using the sense of touch. This measurement provides a necessary preliminary approximation of the systolic blood pressure to ensure an adequate level of inflation when the actual auscultatory measurement is made.

Brachial Artery

An artery that provides a palpable pulse and audible vascular sounds in the antecubital space (the bend of the elbow).

Stethoscope

A medical instrument used for listening to the internal sounds of an organism's body, typically used for assessing heart, lung, and other organ sounds.