Question 1

Which of the following best describes the function of EMB (Eosin Methylene Blue) agar?

Accepted Answer

It is selective for Gram-negative bacteria and differentiates lactose fermenters.

Question 2

A microbiologist streaks a sample on Mannitol Salt Agar (MSA) and observes yellow colonies. What does this indicate about the organism?

Accepted Answer

The organism ferments mannitol and is likely Staphylococcus aureus.

Question 3

Which result would you expect from a catalase test performed on Streptococcus pyogenes?

Accepted Answer

No bubble formation indicating a negative result.

Question 4

A student performs an oxidase test and observes a dark blue color on the test strip. Which organism is most likely present?

Accepted Answer

Micrococcus species

Question 5

On a blood agar plate, a clear zone around bacterial colonies is observed. What type of hemolysis is this and which organism is most likely responsible?

Accepted Answer

Beta hemolysis; Streptococcus pyogenes

Question 6

A Durham tube fermentation test for lactose shows a yellow color and a gas bubble in the tube. What does this result indicate?

Accepted Answer

The organism ferments lactose to acid and gas.

Question 7

Which IMViC test detects the ability of bacteria to produce stable mixed acids from glucose catabolism, and what is the positive result?

Accepted Answer

Methyl red test; red color throughout the medium

Question 8

A Simmon’s Citrate Agar slant turns prussian blue after incubation. What does this indicate about the organism?

Accepted Answer

The organism produces citrase and can metabolize citrate.

Question 9

After adding iodine to a starch agar plate, a clear zone appears around the bacterial growth. Which enzyme is present and which organism is most likely responsible?

Accepted Answer

Amylase; Bacillus subtilis

Question 10

A gelatinase test tube remains liquid after refrigeration. What does this result indicate?

Accepted Answer

The organism produces gelatinase and hydrolyzes gelatin.

Question 11

Which of the following organisms is urease positive and would turn urea agar bright pink within 24 hours?

Accepted Answer

Proteus vulgaris

Question 12

Why do Gram-positive bacteria retain the crystal violet stain after decolorization in the Gram stain procedure?

Accepted Answer

They have a thick peptidoglycan layer that retains the stain.

Question 13

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

Accepted Answer

Crystal violet, iodine, alcohol, safranin

Question 14

A patient’s wound culture grows Bacillus anthracis. Which staining procedure would you use to confirm the presence of endospores, and what colors would you expect for spores and vegetative cells?

Accepted Answer

Schaeffer-Fulton stain; spores green, cells red or pink

Question 15

Which of the following best explains why basic dyes are commonly used to stain bacterial cells?

Accepted Answer

Bacterial cells are slightly negatively charged and attract the colored cation of basic dyes.

Question 16

A microbiologist tests a bacterial isolate for casein hydrolysis and observes a clear zone around the colony on milk agar. Which organism is most likely responsible?

Accepted Answer

Bacillus subtilis

Question 17

Which of the following is a plausible reason for a false positive result in the catalase test?

Accepted Answer

Contamination with red blood cells from blood agar.

Question 18

A student streaks Escherichia coli on EMB agar. What colony appearance should they expect and why?

Accepted Answer

Green metallic sheen, because E. coli ferments lactose vigorously.

Question 19

Which of the following best describes the principle behind the use of enrichment media such as blood agar?

Accepted Answer

It supports the growth of fastidious organisms and differentiates hemolytic activity.

Question 20

A clinical isolate is tested for antibiotic sensitivity using the disk diffusion method. The zone of inhibition around the kanamycin disk is 20 mm. According to the sensitivity table, how should this organism be classified?

Accepted Answer

Sensitive

Microbiology Laboratory Techniques: Media, Biochemical Tests, and Staining

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