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

question 77

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) Something is likely 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) A spectrophotometer can measure absorbance as an indicator of cell number, but can't discriminate between a live cell and a dead one. The presence of dead cells keeps the absorbance high even into the death phase.
• C) E. coli just grows slowly, and has a long generation time. After 16 hours the cells are still in stationary phase, so the cell numbers are expectedly constant.
• 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) There is no point in measuring the absorbance of the culture because the cells are bacterial. It would make more sense to measure would the transmission value.

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Definitions:

Materials Account

An account used in bookkeeping to track costs of raw materials during an accounting period.

FIFO Method

The FIFO (First-In, First-Out) method is an inventory valuation strategy where the oldest inventory items are sold or used first.

Weighted Average Method

The weighted average method is an inventory costing approach that calculates the cost of goods sold and ending inventory based on the average cost of all items available for sale during the period.

Weighted Average Method

An inventory costing method that calculates the cost of goods sold and ending inventory based on the weighted average cost of all goods available for sale during the period.