Quiz on Macronutrients and Media Composition

Which of the following is NOT a macronutrient that’s required by all cells?

• Nitrogen

• Phosphate

• Sulfur

• Sodium

Answer explanation

D. Remember the mnemonics “CHONPS”, sodium is not a requirement for ALL cells, but a nutrient that’s required by MOST cell.

Tags

NGSS.MS-LS2-1

NGSS.MS-LS2-3

Based on the media composition, what type of media is this? Peptone(20g/l), lactose(10h/l), NaCl(5g/l), Crystal Violet(0.001g/l), Neutral Red(0.03g/l), Bile Salts(1.5g/l), Agar(13.5g/l)

• defined, differential but non-selective

• defined, differential and selective

• complex, selective and differential

• complex, selective but non-differential

Answer explanation

This is the media composition of MacConkey Agar, which is a complex media (e.g. you won’t be able to write the chemical formula for peptone). It is both selective and differential. It selects for gram - bacteria, and the agar turns pink if bacteria is lactose-fermenting.

Which of the following media composition makes MacConkey Agar selective? (one or more answer)

• Crystal Violet

• Bile Salt

• Lactose

• Neutral Red

Answer explanation

D. Both crystal violet and bile salt make this media selective by inhibiting gram + bacteria by disrupting their membrane. For C, Lactose (together with PH indicator) reveals acid production from fermentation, making this media differential for lactose-fermenting bacteria

Which of the following statements correctly describes the difference between enrichment culture and direct isolation?

• Enrichment culture focuses on isolating a single pure colony, while direct isolation increases the abundance of a target microbe over time.

• Enrichment culture uses selective media to give a target microbe an advantage, while direct isolation does not.

• Direct isolation uses selective media to give a target microbe an advantage, while enrichment culture does not.

• Enrichment culture and direct isolation both use selective media to give a target microbe an advantage.

Answer explanation

B. Enrichment culture provides target microbe selective advantage, allowing it to be enriched in abundance over time (but might still have other bacteria in the mixture). Direct isolation uses selective and/or differential agar to isolate a single colony, obtaining a pure culture, usually through streak plate or drop plate technique.

What term best describes the procedure depicted in the image?

• Enrichment culture

• Direct counts

• Spread plating

• Streaking

Answer explanation

C. Spread plate is a common direct isolation method to obtain a pure culture. The stick that spreads the plate is strictly sterilized to avoid contamination.

What is the dilution factor for the tube with the correct viable count?

Answer explanation

B. Plate 3 has the correct viable count. Only count plates with 30-300 colonies. Plate 1 and 2 have too many colonies, while plate 4, 5, and 6 have too few colonies. 

Dilution = volume transferred from initial sample/total volume. Tube 1 is 1ml sample/(1ml sample+9 ml diluent)=10^-1, tube 2 is 10^-2, and tube 3 is 10^3.

Select the correct relationship between optical density, absorbance, cell density, and turbidity.

• A. Optical density and absorbance are inversely related to cell density and turbidity.

• B. Turbidity is measured as optical density, also called absorbance, which increases as cell density increases.

• C. Turbidity measures the clarity of the medium, which decreases as cell density increases.

• D. Absorbance increases as optical density decreases, indicating higher cell density and turbidity.

Answer explanation

B. Turbidity (light scattered by cells) is measured by spectrophotometry, which is measured as absorbance (measure of how much light is absorbed by a sample). Optical density and absorbance are used interchangeably. Absorbance will increase if the cell density increases. 

Tags

NGSS.HS-PS4-5