Q: A student prepares a serial dilution and plates 0.1 mL of the $$10^{-4}$$ dilution. If 32 colonies are counted, what is the original concentration of bacteria per mL?

$$32 \times 10$$^{5}$$ = 3,200,000$$ bacteria/mL

Question 1

Which of the following statements best explains why agar is commonly used as a solidifying agent in microbiological media?

Accepted Answer

Agar liquefies above 100 ºC and solidifies below 40 ºC, and most organisms cannot degrade it.

Question 2

A microbiologist wants to isolate Salmonella typhi from a fecal sample containing many other bacteria. Which type of medium should be used and why?

Accepted Answer

Selective medium, because it suppresses unwanted bacteria and encourages growth of Salmonella typhi.

Question 3

Which of the following best describes a complex medium?

Accepted Answer

A medium containing extracts or digests of yeasts, meat, or plants, with an unknown exact chemical composition.

Question 4

A student prepares a serial dilution and plates 0.1 mL of the $$10^{-4}$$ dilution. If 32 colonies are counted, what is the original concentration of bacteria per mL?

Accepted Answer

$$32 	imes 10$$^{5}$$ = 3,200,000$$ bacteria/mL

Question 5

Which of the following media would best allow you to distinguish between lactose fermenters and non-fermenters among Gram-negative enteric bacteria?

Accepted Answer

MacConkey Agar

Question 6

Why are reducing media necessary for the cultivation of anaerobic bacteria?

Accepted Answer

They combine with dissolved oxygen and remove it from the medium.

Question 7

Which of the following is a key difference between selective and differential media?

Accepted Answer

Selective media suppress unwanted microbes, while differential media allow visual differentiation of colonies.

Question 8

A water quality lab filters 10 liters of lake water and finds 55 coliform colonies on the membrane. What is the concentration of coliform bacteria per liter?

Accepted Answer

$$\frac{55}{10} = 5.5$$ bacteria/L

Question 9

Which of the following organisms cannot be grown on artificial media due to their obligate intracellular lifestyle?

Accepted Answer

Rickettsia species

Question 10

A microbiologist wants to culture a microaerophile. Which of the following methods would best provide the appropriate environment?

Accepted Answer

Use of a candle jar to reduce oxygen and increase CO$$_2$$

Question 11

Which of the following best describes the principle behind using a spectrophotometer to estimate bacterial growth?

Accepted Answer

The amount of light absorbed by a bacterial suspension is directly proportional to the number of bacteria present.

Question 12

Which of the following is a characteristic of a chemically defined medium?

Accepted Answer

The exact chemical composition and quantity of each ingredient is known.

Question 13

Why is it important to serially dilute a bacterial sample before plating for colony counts?

Accepted Answer

To obtain a countable number of colonies (30-300) on a plate.

Question 14

Which of the following best explains why Sabouraud’s Dextrose Agar is used to culture fungi?

Accepted Answer

Its low pH inhibits bacterial growth while allowing fungi to grow.

Question 15

A microbiologist is using a direct microscopic count with a special slide. If the average number of cells per large square is 14 and the volume of fluid over the square is $$1/1,250,000 mL$$, what is the concentration of bacteria per mL?

Accepted Answer

$$14 	imes 1,250,000 = 17,500,000$$ bacteria/mL

Question 16

Which of the following best describes an enrichment culture?

Accepted Answer

A culture that provides nutrients and conditions favoring the growth of a specific organism present in low numbers.

Question 17

Which of the following is a correct formula for calculating the number of bacteria per mL in a sample using plate counts?

Accepted Answer

$$	ext{Number of colonies} 	imes 	ext{dilution factor} \div 	ext{volume plated (in mL)}$$

Question 18

Which of the following best explains why some bacteria, such as Mycobacterium leprae, cannot be grown on artificial media?

Accepted Answer

They are obligate intracellular parasites and rely on host cells for nutrients.

Question 19

Which of the following is a reason why metabolic activity can be used as an indirect method to estimate bacterial numbers?

Accepted Answer

The amount of metabolic product is directly proportional to the number of bacteria.

Culture Media and Quantification of Microbial Growth

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