Mod 4 Test

Question 1

True or False: Growth media is best suited for distinguishing between two similar species of bacteria.

Answer 1

False
Growth media is designed to simply support (and not restrict) microbial growth.

Question 2

A researcher is asked to determine which of two vials contains E coli and which contains salmonella. Knowing both are Gram-negative while only one of them is capable of fermenting lactose, which type of media would be best suited?

Answer 2

Differential media

Question 3

What are the requirements of a fastidious microbe?

Answer 3

A fastidious microbe is an organism with complex growth requirements such that if absent it will not grow. Enriched medias thus contain these specific and essential nutrients required for the growth of a particular subset of microorganisms.

Question 4

True or False: LB agar is classified as a non-selective, non-differential media.

Answer 4

True
LB agar is the most basic type of agar and like LB media supports the growth of virtually all microbes without restriction.

Question 5

What is agar used for in microbiology?

Answer 5

Agar is used to create a solid, smooth surface on which microbes can grow.

Question 6

Blood agar is which type of medium? Select all that apply.
Differential
Selective
Enriched
Selective and Differential

Answer 6

Differential
Enriched

Question 7

Match the following hemolytic class with its description of activity.

1. Alpha hemolysis A. No hemolytic activity
2. Beta hemolysis B. Incomplete hemolytic
3. Gamma hemolysis C. Complete hemolytic

Answer 7

1B
2C
3A

Question 8

Columbia CNA agar is used to isolate:

Answer 8

Gram-Positive
CNA agar restricts Gram-Negative microbes therefore only Gram-Positive microbes will grow.

Question 9

True or False: Chocolate agar gets its brown color from cocoa to produce an enriched media.

Answer 9

False
Chocolate (cocoa) is never added to the media. The name is derived simply based on the color that actually comes from the presence of ‘cooked’ (lysed) red blood cells in the media.

Question 10

An unknown microbe is streaked onto a MacConkey agar plate. After an overnight incubation at 37°C, growth is observed. Would a Gram stain be necessary? Why or why not?

Answer 10

No. A Gram stain would not be necessary, as only Gram-Negative microbes will grow on MacConkey agar.

Question 11

In an attempt to detect the presence of the pathogenic strain of E. coli O157:H7, a researcher spread a culture onto a MacConkey agar with failed results. What type of agar should they (correctly) try next? Based upon what you know about the formulation of differential plates, explain why you chose this specific agar.

Answer 11

The microbe should be plated on SMAC (Sorbitol-MacConkey agar) as it is specifically formulated to detect O157:H7. Pathogenic E. coli (O157:H7) cannot ferment sorbitol while non-pathogenic E. coli can ferment both soribitol and lactose. Therefore, colonies that ferment (acidic conditions; non-pathogenic) can be differentiated from non-fermenters (neutral to basic conditions; pathogenic).

Question 12

What is the Gram status (positive or negative) of microbes growing on Eosin Methylene Blue (EMB) agar plates?

Answer 12

Gram-Negative. EMB plates specifically restrict the growth of Gram-Positive bacteria.

Question 13

Mannitol salt agar will turn what color in the presence of the pathogenic strain Staphylococcus aureus?

Answer 13

Yellow

Pathogenic Staph aureus will turn the agar from red to yellow.

Question 14

What is the process of spreading a bacterial culture onto a petri dish called?

Answer 14

Plating

Question 15

In order to visual individual colonies of bacteria would you culture your sample in a liquid media or on a solid (agar) media? Why?

Answer 15

Solid (agar) media. The primary advantage is that cells are held into place. When grown in a nutrient broth, bacterial cells can multiply but are free to move around in solution. When grown on agar within a petri dish the fixed in such as way as to form colonies.

Question 16

True or False: The visualization of colonies on a petri dish represents bacterial cells that have multiplied ~2-3 times.

Answer 16

False
When an individual colony is visible on agar the bacterial cell has multiplied often a million times over.

Question 17

True or False. The purpose of a quadrant streak is to produce individual colonies of a bacterial population.

Answer 17

True
The purpose of the quadrant streak is to generate individual colonies such that a single (pure) bacterial sample can be isolated.

Question 18

Which of the following are correct in order to be considered a pure culture? Select all that apply.

Obtained by a serial dilution method
Obtained by a 4-phase dilution streak method
Multiple species
Free from contaminants
Obtained by pour plate serial dilution method
Long lifespan
Single organism

Answer 18

Obtained by a serial dilution method
Obtained by a 4-phase dilution streak method
Free from contaminants
Obtained by pour plate serial dilution method
Single organism

Question 19

When performing a quadrant streak, the sample is spread across the plate in such as way as to form what?

Answer 19

A dilution gradient is formed. The resulting gradient should always contain within it the growth of individual colonies.

Question 20

In what phase of a dilution streak would you expect to find the lowest concentration of bacteria, P2 or P4?

Answer 20

P4 (Phase 4) would contain the lowest concentration of bacteria. The phases rank (from highest to lowest), P1 > P2 > P3 > P4.

Question 21

True or False. When performing a dilution streak a new (or sterilized) loop must be used for each phase.

Answer 21

True
Failure to do so would prevent the establishment of a dilution gradient, as the same bacterial concentration would be spread across both phase regions.

Question 22

What might a researcher do to encourage growth of a microbe with a long generation time that has already been streaked on an agar plate?

Answer 22

Increase incubation time

Question 23

True or False. To restrict the growth of a pathogenic microbe a researcher might decrease an incubator from 37°C to 25°C.

Answer 23

True
Pathogenic strains of bacteria tend to grow faster than non-pathogenic strains at 37°C, so researchers may set incubators at 25°C to restrict its growth.

Question 24

When given an unknown bacterial sample the first step is to expand the current bacterial population. Which form of media best suites this need? Why?

MSA agar
LB media containing ampicillin and neomycin
MacConkey agar
Blood agar

Answer 24

Blood agar. All other options (A, B and C) are all forms of selective media, meaning they may potentially inhibit the growth of the unknown sample. Although blood agar is considered a differential media, it is, most importantly, a non-selective media. Given the alternatives, this is the best option.