Q: A student is using a compound light microscope with a 40X objective lens and a 10X ocular lens. What is the total magnification, and which formula should be used to calculate the limit of resolution ($$D) $$for this microscope?

Total magnification is $$400\times$$; $$D = \frac{\lambda}{NA_{\text{condenser}} + NA_{\text{objective}}}$$

Question 1

Which of the following best describes the main difference between prokaryotic and eukaryotic cells as observed under a light microscope?

Accepted Answer

Eukaryotic cells are generally larger and contain membrane-bound organelles, while prokaryotic cells lack these features.

Question 2

A microbiologist is attempting to isolate a single species of bacteria from a mixed culture. Which technique and medium combination would be most appropriate for isolating Gram-negative enteric bacteria that ferment lactose?

Accepted Answer

Streak plate method on MacConkey Agar

Question 3

A sample is streaked on Hektoen Enteric (HE) Agar. After incubation, yellow colonies are observed. What does this result indicate about the organism?

Accepted Answer

The organism is a Gram-negative enteric that ferments one or more of the sugars in the medium, producing acid.

Question 4

Which of the following best explains why a pure culture is essential for accurate identification of a bacterial species?

Accepted Answer

Because biochemical test results from mixed cultures are composites and can lead to misidentification.

Question 5

A microbiologist observes a bacterial colony with the following characteristics: large, gray, granular, irregular shape, and a 'mousy' smell. Which genus is most likely responsible for this colony morphology?

Accepted Answer

Bacillus

Question 6

A student is using a compound light microscope with a 40X objective lens and a 10X ocular lens. What is the total magnification, and which formula should be used to calculate the limit of resolution ($$D) $$for this microscope?

Accepted Answer

Total magnification is $$400	imes$$; $$D = \frac{\lambda}{NA_{	ext{condenser}} + NA_{	ext{objective}}}$$

Question 7

During a Gram stain, a student forgets to add the decolorizer. What is the most likely appearance of both Gram-positive and Gram-negative cells under the microscope?

Accepted Answer

Both will appear purple.

Question 8

Which of the following best describes the principle behind the catalase test?

Accepted Answer

Catalase converts hydrogen peroxide ($$H_2O_2) $$into water and oxygen gas: $$2H_2O_2 \xrightarrow{	ext{catalase}} 2H_2O + O_2$

Question 9

A clinical isolate is tested on Mannitol Salt Agar (MSA). The organism grows well and turns the medium yellow. What is the most likely identity of the organism, and what does this result indicate?

Accepted Answer

Staphylococcus aureus; it ferments mannitol and is likely pathogenic.

Question 10

A patient sample is inoculated into Fluid Thioglycollate Medium. After incubation, growth is observed only at the bottom of the tube. Which term best describes the organism's oxygen requirements?

Accepted Answer

Obligate anaerobe

Question 11

A microbiologist is using the API 20 E system to identify an unknown Gram-negative rod. After scoring the results, the seven-digit code is entered into the Analytical Profile Index. What is the primary purpose of this code?

Accepted Answer

To identify the organism by matching the code to a database of known biochemical profiles.

Question 12

Which of the following best explains the appearance of black colonies on Salmonella-Shigella (SS) Agar?

Accepted Answer

The organism reduces sulfur to H$$_2S$$, which reacts with ferric citrate to form a black precipitate.

Question 13

A water sample is tested using the Colilert® method. After incubation, the sample is yellow and fluoresces under UV light. What does this indicate?

Accepted Answer

Both total coliforms and E. coli are present in the sample.

Question 14

A microbiologist performs a standard plate count and obtains 45 colonies on a plate inoculated with 0.1 mL of a $$10^{-4}$$ dilution. What is the original cell density (OCD) in CFU/mL?

Accepted Answer

$$4.5 	imes 10^6$ CFU/mL

Question 15

Which of the following best describes the function of the enzyme luciferase in bioluminescent bacteria?

Accepted Answer

It catalyzes the oxidation of reduced flavin mononucleotide (FMNH$$_2) in $$the presence of oxygen and a long-chain aldehyde, resulting in light emission.

Question 16

A patient presents with a rapidly progressing necrotic wound after a traumatic injury. Laboratory culture reveals a Gram-positive, anaerobic, endospore-forming rod. Which organism is most likely responsible, and what is a key virulence factor?

Accepted Answer

Clostridium perfringens; alpha toxin (lecithinase)

Question 17

Which of the following statements about the nitrogen cycle is correct?

Accepted Answer

Nitrogen-fixing bacteria convert $$N_2 to$$ $$NH_3$$, making nitrogen available to plants and other organisms.

Comprehensive Study Notes: Microbiology Laboratory Techniques, Microbial Diversity, and Diagnostic Methods

Study Guide - Practice Questions

Study Guide - Flashcards