Question 1

Which of the following results would you expect if you inoculate E. coli on an EMB plate and incubate it overnight?

Accepted Answer

Growth with a metallic green sheen, indicating rapid lactose fermentation

Question 2

During a Gram stain, what is the primary purpose of the 95% ethanol step?

Accepted Answer

To decolorize Gram-negative cells by dissolving the outer membrane

Question 3

A student observes yellow color in both the open and oil OF-dextrose tubes after incubation. What does this indicate about the organism's metabolism?

Accepted Answer

The organism is capable of both oxidative and fermentative metabolism

Question 4

Which of the following best describes the result of a starch hydrolysis test where a clear zone appears around the bacterial growth after adding Gram's iodine?

Accepted Answer

The organism produces the exoenzyme amylase and hydrolyzes starch

Question 5

If a bacterium grows on MSA but the medium remains red, what can you conclude?

Accepted Answer

The bacterium is salt tolerant but does not ferment mannitol

Question 6

Which step in the Gram stain procedure is most likely to cause all cells to appear pink if performed for too long?

Accepted Answer

95% ethanol

Question 7

A student streaks a starch agar plate with B. subtilis and E. coli. After adding iodine, only the B. subtilis side shows a clear zone. What is the most likely explanation?

Accepted Answer

B. subtilis produces amylase, E. coli does not

Question 8

Which of the following equations best represents the fermentation of glucose by bacteria?

Accepted Answer

$$\mathrm{C_6H_{12}O_6$} \rightarrow 2\ \mathrm{C_2H_5OH} + 2\ \mathrm{CO_2$}$$

Question 9

A microbiologist observes colorless colonies on an EMB plate. What is the most likely interpretation?

Accepted Answer

The organism does not ferment lactose

Question 10

Why is it important to heat-fix a bacterial smear before Gram staining?

Accepted Answer

To kill the bacteria and adhere them to the slide

Question 11

A student forgets to add mineral oil to one of the OF-dextrose tubes. What is the most likely consequence for interpreting the results?

Accepted Answer

Both tubes will test for oxidative metabolism only

Question 12

Which of the following best describes the function of Gram's iodine in the Gram stain procedure?

Accepted Answer

It acts as a mordant, forming a complex with crystal violet

Question 13

A microbiologist observes two different colony types on a streak plate. What is the best next step to identify each organism?

Accepted Answer

Subculture each colony type to separate media for further testing

Question 14

Which of the following is a correct interpretation of a yellow medium on MSA after incubation?

Accepted Answer

The organism is salt tolerant and ferments mannitol

Question 15

If a Gram stain is performed on S. aureus, what result should be expected under the microscope?

Accepted Answer

Purple cocci in clusters

Question 16

Which of the following best explains why starch must be hydrolyzed before bacteria can utilize it?

Accepted Answer

Starch is too large to enter the cell and must be broken down by exoenzymes

Question 17

A student observes that after Gram staining, their cells appear both purple and pink. What is the most likely explanation?

Accepted Answer

The smear was too thick or decolorization was uneven

Question 18

Which of the following best describes the metabolic test performed by OF-dextrose tubes?

Accepted Answer

It differentiates between oxidative and fermentative metabolism of glucose

Question 19

Why is it important to check for bacterial growth before interpreting the results of a starch hydrolysis test?

Accepted Answer

A clear zone without growth could be due to lack of inoculation, not starch hydrolysis

Question 20

Which of the following best explains why EMB agar inhibits the growth of Gram-positive bacteria?

Accepted Answer

The dyes eosin and methylene blue are toxic to Gram-positive cell walls

Microbiology Lab Techniques: Gram Staining, Plate Reading, and Carbohydrate Catabolism

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