Q: A student prepares a series of tenfold serial dilutions of a bacterial culture and plates 0.1 mL of each dilution. If the plate from the $$10^{-4}$$ dilution has 150 colonies, what is the estimated concentration of viable cells in the original culture (in cells/mL)?

$$1.5 \times 10$$^{7}$$ cells/mL

Question 1

Which of the following best describes a defined culture medium?

Accepted Answer

A medium with precise amounts of highly purified inorganic and organic chemicals; exact composition is known

Question 2

Why is aseptic technique critical in microbiological laboratory work?

Accepted Answer

It prevents contamination of the medium and maintains pure cultures

Question 3

A microbiologist wants to isolate a pure culture from a mixed sample. Which method is most appropriate?

Accepted Answer

Plating the sample on solid agar and selecting an isolated colony

Question 4

Which of the following is a limitation of microscopic cell counts?

Accepted Answer

Cannot distinguish between live and dead cells without special stains

Question 5

What is the main difference between total cell counts and viable cell counts?

Accepted Answer

Viable cell counts measure only cells capable of division and colony formation

Question 6

Which type of medium would you use to grow a nutritionally demanding (fastidious) microbe?

Accepted Answer

Complex medium enriched with blood or serum

Question 7

A researcher uses a spectrophotometer to measure the turbidity of a bacterial culture. What does an increase in optical density (OD) indicate?

Accepted Answer

An increase in the number of cells

Question 8

Why are results from plate counts often expressed as colony-forming units (CFUs) rather than actual cell numbers?

Accepted Answer

To account for cell clumps and viable cells that may not form visible colonies

Question 9

Which of the following best explains the 'Great Plate Count Anomaly'?

Accepted Answer

Microscopic counts reveal more organisms than are recoverable on plates due to differences in nutritional needs and growth conditions

Question 10

A student prepares a series of tenfold serial dilutions of a bacterial culture and plates 0.1 mL of each dilution. If the plate from the $$10^{-4}$$ dilution has 150 colonies, what is the estimated concentration of viable cells in the original culture (in cells/mL)?

Accepted Answer

$$1.5 	imes 10$$^{7}$$ cells/mL

Question 11

Which of the following is a disadvantage of using OD measurements for estimating cell numbers?

Accepted Answer

OD can be inaccurate if cells do not grow in uniform suspensions or attach to vessel sides

Question 12

Which medium would you use to distinguish between two bacterial species based on their metabolic activity?

Accepted Answer

Differential medium containing an indicator dye

Question 13

A sample contains both motile and non-motile bacteria. What must be done before performing a microscopic cell count?

Accepted Answer

Immobilize or kill motile cells to prevent movement during counting

Question 14

Which of the following is NOT a reason why plate counts may underestimate the actual number of cells in a natural sample?

Accepted Answer

Dead cells are included in plate counts

Question 15

Which of the following best describes a microbial colony?

Accepted Answer

A mass of cells arising from a single cell, visible on solid media

Question 16

A researcher wants to selectively grow Gram-negative bacteria from a mixed sample. Which type of medium should be used?

Accepted Answer

Selective medium containing compounds that inhibit Gram-positive bacteria

Question 17

Which of the following is a correct statement about the use of agar in solid media?

Accepted Answer

Agar is a gelling agent used at 1-2% concentration to restrict movement and allow colony formation

Question 18

Which of the following is a real-world application of differential media?

Accepted Answer

Distinguishing pathogenic bacteria in clinical diagnostic labs

Question 19

A scientist measures the OD of a bacterial culture at 540 nm and finds it to be 0.8. If the standard curve shows that $$OD = 0.8$$ corresponds to $$4 	imes 10$$^{8}$$ cells/mL, what is the cell concentration?

Accepted Answer

$$4 	imes 10$$^{8}$$ cells/mL

Question 20

Which of the following is a key advantage of using OD measurements over plate counts for monitoring bacterial growth?

Accepted Answer

OD measurements are quick, non-destructive, and allow repeated sampling of the same culture

Culturing Microbes: Media, Techniques, and Cell Counting Methods L 19 E 2

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