Q: A student is using a compound light microscope with a 10x ocular lens and a 40x objective lens. What is the total magnification, and how does this relate to the concept of resolution?

The total magnification is $$10 \times 40 = 400$$; magnification increases the size of the image, but resolution determines the ability to distinguish two close points as separate.

Question 1

A student is using a compound light microscope with a 10x ocular lens and a 40x objective lens. What is the total magnification, and how does this relate to the concept of resolution?

Accepted Answer

The total magnification is $$10 	imes 40 = 400$$; magnification increases the size of the image, but resolution determines the ability to distinguish two close points as separate.

Question 2

Which of the following best describes the purpose of using aseptic technique in a microbiology laboratory?

Accepted Answer

To prevent contamination of cultures, the environment, and the experimenter by unwanted microorganisms.

Question 3

During a Gram stain, a student forgets to apply the iodine mordant. What is the most likely result when observing Gram-positive bacteria under the microscope?

Accepted Answer

The Gram-positive bacteria will appear pink/red because the crystal violet is not fixed and is washed out by alcohol.

Question 4

Which of the following correctly matches a medium with its primary purpose and the organism it is designed to select or differentiate?

Accepted Answer

Mannitol Salt Agar: selective for Staphylococcus species and differentiates based on mannitol fermentation.

Question 5

A microbiologist is testing for the presence of catalase enzyme in a bacterial isolate. Which of the following results and interpretations is correct?

Accepted Answer

Bubbling upon addition of hydrogen peroxide indicates a positive catalase test, meaning the bacteria produce catalase.

Question 6

Which of the following best describes the difference between basic and acidic dyes used in bacterial staining?

Accepted Answer

Basic dyes have a positively charged chromophore and bind to negatively charged bacterial cell surfaces, while acidic dyes have a negatively charged chromophore and stain the background.

Question 7

A patient sample is stained using the acid-fast technique. The cells appear bright pink/red under the microscope. What is the most likely identity of the pathogen, and which dye is responsible for this color?

Accepted Answer

Mycobacterium species; carbol fuchsin dye.

Question 8

A microbiologist performs a spore stain on a culture of Bacillus subtilis. After staining, what colors should the spores and vegetative cells appear, and which dye is used to stain the spores?

Accepted Answer

Spores appear green (malachite green), vegetative cells appear pink/red (safranin).

Question 9

A blood agar plate shows colonies surrounded by a clear, colorless zone. What type of hemolysis is this, and what does it indicate about the organism?

Accepted Answer

Beta hemolysis; the organism completely lyses red blood cells.

Question 10

A student is interpreting results from an EMB agar plate. Colonies of E. coli appear metallic green. What does this indicate about E. coli's metabolic capabilities?

Accepted Answer

E. coli ferments lactose vigorously, producing acid that causes the metallic green sheen.

Question 11

A serial dilution is performed as follows: 1 mL of culture is added to 9 mL of diluent (first tube), then 1 mL from this tube is added to another 9 mL of diluent (second tube), and so on for a total of 3 dilutions. If 0.1 mL from the final tube is plated and 50 colonies are counted, what is the original cell concentration in CFU/mL?

Accepted Answer

The dilution factor is $$10^{-3}$$ for the third tube, and $$0.1 mL $$was plated, so the original concentration is $$\frac{50}{0.1 	imes 10$$^{-3}$$} = 5 	imes 10$$^{5}$$ CFU/mL.

Question 12

Which of the following best explains why Sabouraud agar is used for culturing fungi rather than bacteria?

Accepted Answer

Sabouraud agar has a low pH and high sugar content, which inhibits most bacteria but allows fungi to grow.

Question 13

A clinical sample is tested using the Kirby-Bauer disk diffusion method. After incubation, a clear zone of 20 mm is observed around an antibiotic disk. What does this zone of inhibition indicate?

Accepted Answer

The organism is sensitive to the antibiotic; the larger the zone, the more effective the antibiotic.

Question 14

Which of the following is a correct sequence of steps for the Gram stain procedure?

Accepted Answer

Crystal violet, iodine, alcohol (decolorizer), safranin.

Question 15

A microbiologist observes rod-shaped bacteria arranged in chains under the microscope. Which term best describes this arrangement and shape?

Accepted Answer

Streptobacilli.

Question 16

Which of the following media is both selective and differential, and what is its primary use?

Accepted Answer

EMB agar; selects for Gram-negative bacteria and differentiates lactose fermenters.

Question 17

A student is asked to identify an unknown bacterium. The organism is Gram-negative, rod-shaped, grows on EMB agar with a metallic green sheen, and ferments lactose. Which organism is most likely?

Accepted Answer

Escherichia coli.

Question 18

Which extracellular enzyme is detected by a clear zone around colonies on starch agar after adding iodine, and what does this indicate?

Accepted Answer

Amylase; the organism hydrolyzes starch.

Question 19

A clinical isolate is tested on Mannitol Salt Agar. The colonies grow and turn the medium yellow. What does this result indicate?

Accepted Answer

The organism is likely Staphylococcus aureus and ferments mannitol.

Question 20

Which of the following best describes the difference between selective and differential media?

Accepted Answer

Selective media inhibit the growth of some organisms while allowing others to grow; differential media distinguish organisms based on metabolic properties.

Microbiology Lab Final Exam Study Guide: Microscopy, Staining, Media, and Bacterial Identification

Study Guide - Practice Questions

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