exam 2

Question 1

What is the purpose of differential media, and what does a color change indicate?

Answer 1

Differential media, such as ChromAgar and Blood Agar, help distinguish between bacteria based on metabolic characteristics, often resulting in color changes or halos due to fermentation.

Question 2

What does the yellow color in a Durham fermentation tube indicate?

Answer 2

Yellow color indicates the presence of pH-lowering acids due to fermentation of glucose by bacteria like E. coli.

Question 3

What does a blue color on a citrate slant indicate?

Answer 3

A blue color indicates that the bacterium can use citrate as a carbon source, leading to an increase in pH, typical of organisms like Enterobacter

Question 4

What does a clear halo around colonies on Blood Agar indicate?

Answer 4

A clear halo indicates beta hemolysis, which is the complete lysis of red blood cells, seen in organisms like Streptococcus pyogenes and Staphylococcus aureus.

Question 5

What is indicated by a metallic green sheen on EMB agar?

Answer 5

The metallic green sheen typically indicates lactose fermentation by E. coli.

Question 6

What does the presence of bubbles in a catalase test indicate?

Answer 6

The presence of bubbles indicates the enzyme catalase, which breaks down hydrogen peroxide into oxygen and water, a characteristic of staphylococci.

Question 7

What is the purpose of the oxidase test, and what does a blue/black color indicate?

Answer 7

The oxidase test identifies organisms that use aerobic respiration. A blue/black color indicates the presence of cytochrome oxidase, as seen in Pseudomonas.

Question 8

What do blue colonies on an ampicillin plate indicate in a transformation experiment?

Answer 8

Blue colonies indicate successful transformation, where the bacteria express beta-galactosidase and are resistant to ampicillin.

Question 9

What does it mean if white colonies appear on an ampicillin plate?

Answer 9

White colonies represent satellite colonies that are not transformed but grow near transformed colonies producing beta-lactamase.

Question 10

What does growth on SM110 or MSA agar indicate?

Answer 10

Growth indicates that the bacteria are halophiles, such as Staphylococcus aureus, which thrive in high salt concentrations.

Question 11

What does a yellow color on MSA indicate?

Answer 11

A yellow color indicates mannitol fermentation, resulting in acid production, seen in Staphylococcus aureus

Question 12

What type of organism grows best at pH 12?

Answer 12

Alkalinophiles thrive at pH 12.

Question 13

Which type of bacteria has an optimum growth temperature of 55°C?

Answer 13

Geobacillus stearothermophilus has an optimum growth temperature of 55°C.

Question 14

What does a halo around an antibiotic disk indicate?

Answer 14

: A halo indicates that the bacterium is sensitive to the antibiotic, meaning the antibiotic worked and inhibited growth.

Question 16

What is the difference between a disinfectant and an antiseptic?

Answer 16

A disinfectant kills germs on non-living surfaces, while an antiseptic kills microorganisms on living tissues like skin

Question 17

What is the purpose of selective media like EMB or MSA?

Answer 17

Selective media inhibit the growth of certain bacteria while allowing others to grow. EMB selects for Gram-negative bacteria, while MSA selects for halophiles.

Question 18

What type of media is Blood Agar, and what does it differentiate?

Answer 18

Blood Agar is both enriched and differential. It differentiates bacteria based on their hemolytic properties (beta, alpha, or gamma hemolysis)

Question 19

What does a red color after adding reagents to a nitrate test indicate?

Answer 19

A red color indicates that nitrate has been reduced to nitrite, confirming anaerobic respiration in the bacterium.

Question 20

What does a positive catalase test look like, and what does it indicate?

Answer 20

A positive catalase test results in bubbling upon the addition of hydrogen peroxide. This indicates the presence of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen, as seen in Staphylococcus species.

Question 21

What does a gas bubble in a Durham tube indicate?

Answer 21

A gas bubble indicates that the bacterium is producing gas as a byproduct of fermentation, typical of organisms like E. coli.

Question 22

What type of bacteria grows best at 25°C, and what is an example?

Answer 22

Psychrotolerant bacteria grow well at 25°C. An example is Serratia marcescens.

Question 23

Which type of bacteria thrives in temperatures between 45°C and 80°C?

Answer 23

Thermophiles thrive in these temperatures. An example is Geobacillus stearothermophilus.

Question 24

What does it mean if there is no clear zone (halo) around an antibiotic disk?

Answer 24

No halo means the bacteria are resistant to the antibiotic, and the antibiotic was not effective in inhibiting bacterial growth.

Question 25

What does a clear zone around an antibiotic disk signify?

Answer 25

A clear zone indicates that the bacteria are sensitive to the antibiotic, meaning the antibiotic successfully inhibited bacterial growth.

Question 26

How do you differentiate between Gram-positive and Gram-negative bacteria using selective media like EMB or CNA?

Answer 26

EMB is selective for Gram-negative bacteria and inhibits Gram-positive bacteria, whereas CNA is selective for Gram-positive bacteria and inhibits Gram-negative bacteria.

Question 27

What result would you expect from a bacterium that can use nitrate as a terminal electron acceptor in anaerobic respiration?

Answer 27

A positive nitrate reduction test, indicated by the red color after adding reagents, confirms that nitrate has been reduced to nitrite.

Question 28

What does a red color change after adding methyl red in an MR test indicate?

Answer 28

: A red color indicates that the bacteria are mixed acid fermenters and have produced stable acidic byproducts, lowering the pH below 4.4.