Microbiology Lab Review: Chapter 16

Lab Review

Chapter 16: Quantifying Microbial Growth - Turbidity and Standard Curve

1. [CH-16-01] What is turbidity and why is it appearing in bacterial culture?

Cloudiness of a solution caused by scattering and absorption of light in the sample. Can be measured by transmittance or absorbance.

2. [CH-16-02] Why is it important to stir up the cell suspension before recording the turbidity of the suspension?

Cells may have settled on the bottom making the solution appear less turbid than it actually should be.

3. [CH-16-03] Explain the differences between transmittance and absorbance.

Transmittance = Measure of the amount of light that passes through a sample.

Absorbance = Measure of the amount of light that is scattered/absorbed by the sample.

4. [CH-16-04] What are the differences in the transmittance and absorbance scales on the spectrophotometer? What units are used to express transmittance and absorbance?

Transmittance scale: 0 – 100% (% of light that passes through)

Absorbance scale: 0 – 2 (Expressed in optical density units, OD)

5. [CH-16-05] If a solution is clear to the human eye or according to spectrophotometer’s reading, can it be assumed that the solution is sterile? Yes, No. Explain your answer.

No, extremely dilute solutions with very small cell concentrations can appear clear to the eye and are not detectable by the spectrophotometer.

6. [CH-16-06] What is the range of optical density that can be used to reliably convert absorbance to cell concentration?

The range of optical density is 0.1 OD – 0.6 OD.

7. [CH-16-07] What is the minimum cell concentration reliably detected by the spectrophotometer?

Minimum cell concentration = 10⁶ cells

8. [CH-16-08] Explain how a standard curve is built.

Using data of absorbance at different cell concentrations (calculated via hemocytometer), plot points on the graph (cell concentration vs absorbance). Then, draw the line of best fit for the points = standard curve.

9. [CH-16-09] What is the purpose of making the standard curve?
   The curve can be extrapolated to values not yet calculated. Allows for easy readings of cell concentration with just absorbance.

10. [CH-16-10] Can the same standard curve be used for all microbial species? Yes or No, explain your answer.
    No, different microbial species have different levels of absorbance owing to their cell walls and intracellular contents. Each microbial culture needs a new standard curve.