Question 1

Which of the following best describes the difference between bacteriophage lytic and lysogenic cycles?

Accepted Answer

The lytic cycle results in immediate destruction of the host cell, while the lysogenic cycle involves integration of the phage genome into the host DNA without immediate lysis.

Question 2

What is the primary purpose of making dilutions of the T4 virus in a viral plaque assay?

Accepted Answer

To ensure that the number of plaques formed is countable and statistically reliable.

Question 3

Given a viral plaque assay where 0.1 mL of a $$10^{-5}$$ dilution produces 50 plaques, what is the titer of the original sample in plaque-forming units per mL (PFU/mL)?

$$	ext{Titer} = \frac{	ext{Number of plaques}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}}$$

Accepted Answer

$$1 	imes 10$$^{7}$$ PFU/mL

Question 4

Which of the following best differentiates between macroscopic and microscopic appearance of mold versus yeast?

Accepted Answer

Molds are filamentous and fuzzy macroscopically, while yeasts are smooth and creamy; microscopically, molds show hyphae and spores, while yeasts show budding cells.

Question 5

A microbiologist observes a colony with a fuzzy surface and pigmented reverse on Sabouraud agar. Which organism is most likely present?

Accepted Answer

A mold such as Aspergillus species

Question 6

Which of the following is a distinguishing microscopic feature of Streptococcus pyogenes compared to Streptococcus pneumoniae?

Accepted Answer

S. pyogenes forms chains of cocci, while S. pneumoniae forms pairs (diplococci).

Question 7

A throat swab is streaked on blood agar and produces clear, complete hemolysis around colonies. Which type of hemolysis is this, and which Streptococcus group is most likely responsible?

Accepted Answer

Beta hemolysis; Group A Streptococcus

Question 8

Which test would best differentiate between Staphylococcus aureus and Streptococcus pyogenes in a clinical specimen?

Accepted Answer

Catalase test: S. aureus is catalase positive, S. pyogenes is catalase negative.

Question 9

Which of the following best explains why MacConkey agar is used for the identification of Gram-negative bacteria in a mixed culture?

Accepted Answer

It inhibits the growth of Gram-positive bacteria and differentiates lactose fermenters from non-fermenters.

Question 10

A Gram-negative rod is isolated from a urine sample. It grows pink colonies on MacConkey agar and is indole positive. Which organism is most likely present?

Accepted Answer

Escherichia coli

Question 11

Which of the following is a key difference between Enterobacteriaceae and non-Enterobacteriaceae Gram-negative rods?

Accepted Answer

Enterobacteriaceae ferment glucose, while non-Enterobacteriaceae do not.

Question 12

A clinical isolate is oxidase positive, does not ferment lactose, and grows on MacConkey agar. Which organism is most likely?

Accepted Answer

Pseudomonas aeruginosa

Question 13

Which of the following best describes the use of a dichotomous key in bacterial identification?

Accepted Answer

It is a stepwise tool that uses a series of paired choices to identify organisms based on their characteristics.

Question 14

A patient sample is streaked on CNA agar and MacConkey agar. Growth is observed only on CNA agar. What does this indicate about the organism?

Accepted Answer

It is likely a Gram-positive bacterium.

Question 15

Which of the following is a correct interpretation of the catalase test?

Accepted Answer

Bubbling upon addition of hydrogen peroxide indicates a catalase-positive organism.

Question 16

A Gram stain of a clinical specimen shows Gram-positive cocci in chains. The colonies are beta-hemolytic on blood agar. Which is the most likely organism?

Accepted Answer

Streptococcus pyogenes

Question 17

Which of the following best explains why correct spelling and biochemical association are important when identifying Streptococcus species in the lab?

Accepted Answer

Because different Streptococcus species have similar names but different clinical implications and biochemical properties.

Question 18

Which of the following is a correct statement about the use of selective and differential media in microbiology?

Accepted Answer

Selective media inhibit the growth of certain organisms, while differential media distinguish organisms based on biochemical reactions.

Question 19

A laboratory technician observes alpha hemolysis on blood agar. Which organism is most likely responsible?

Accepted Answer

Streptococcus pneumoniae

Question 20

Which of the following best describes the difference between lactose fermenters and non-fermenters on MacConkey agar?

Accepted Answer

Lactose fermenters produce pink colonies, while non-fermenters produce colorless colonies.

Final Lab Practical Study Guide: Microbiology Lab Identification and Differentiation

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