Question

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A medium containing substance that cause some bacteria to look differently than others is called a differential medium. MSA is a medium that changes colors when S. aureus is present because that species can ferment the mannitol in the medium and cause a pH change. T or F

Calculation of Bacterial Counts from a Dilution Plate: A total of 145 colonies were counted on the plate that received 1.0 mL of the 1:10,000 dilution of a specimen. The bacterial count is obtained by multiplying the number counted on the plate multiplied by the dilution factor. What is the bacterial count for the original specimen?

1,450 colonies/mL

14,500 colonies/mL

145,000 colonies/mL

1,450,000 colonies/mL

14,500,000 colonies/mL

Indicate which of the following observations can be made when observing growth on Nutrient Agar.

opacity/clarity of colonies

color of colonies

shape and /or pattern of growth

none of the above

all the above

Macroscopic Observations: Colony characteristics on NA plates include the vertical dimension of a colony, which is referred to as its elevation. The terms crateriform, umbonate, convex, flat, and raised are all used to describe the elevate of a colony. T or F

Purpose of SIM (sulfur, indole, motility) Medium: What can be detected by using SIM medium?

motility

tryptophan degradation

H2S production

all the above

none of the above

Many bacteria can move using flagella. Motility can be observed by using Wet Mounts. Another way to differentiate motile versus nonmotile organisms is through the inoculation SIM medium. SIM medium is inoculated with bacteria in a straight line within the culture tube. Diffuse turbidity in the medium are indicative of motile organisms, and nonmotile bacteria will only grow along that line of inoculation. Motility is usually observed by using a _______ medium.

liquid

hard solid

semisolid

Tryptophan Hydrolysis Results: In the SIM test, Tryptophan hydrolysis results in the production of indole, which is detected by the addition of Kovac’s reagent. T or F

You have inoculated a SIM tube and 48 hours later the following results are recorded. The medium is black, turbid all over, and there is no red ring when you add Kovak’s reagent. Which of the following represent the correct interpretation of these results?

H2S +, Motile, Indole –

H2S +, Nonmotile, Indole +

H2S -, Nonmotile, Indole +

Mannitol Fermentation +, Nonmotile, Indole +

Mannitol Fermentation -, Motile, Indole +

Answer 1

1) True

This is because MSA is a medium that changes colors when S. aureus is present. This is due to the fermentation of mannitol on the MSA plate which produces an acidic byproduct that causes the pH indicator (phenol red) to turn yellow. The species which cannot ferment mannitol cannot change the pH and hence the colour remains red.

2) The correct answer is-

d- 1,450,000 colonies/mL

The bacterial count is obtained by multiplying the number counted on the plate multiplied by the dilution factor. Since the dilution facter is 1:10,000, the bacterial count = (number of colonies in the plate of 1:10,000) * 10,000= 1450000 colonies/ml

3) The correct answer is-

E- all the above

This is because when we observe growth on Nutrient Agar, we can see opacity/clarity of colonies, color of colonies, shape and /or pattern of growth of colonies.

4) True

The five elevation of colonies are-

a. flat
b. raised
c. convex
d. pulvinate (very convex)
e. umbonate (raised in the center)

5) The correct answer is-

d- all the above

SIM medium tests the ability of an organism to do several things: reduce sulfur (H2S production), produce indole (tryptophan degradation) and swim through the agar (motility).

6) The correct answer is-

c- semisolid

SIM is a semi-solid medium designed to differentiate species based on their ability to produce hydrogen sulfide, to split indole from tryptophan, and to exhibit motility.

7) True.

In the SIM test, Tryptophan hydrolysis results in the production of indole which is combined with Kovac's Reagent (which contains hydrochloric acid and p-dimethylaminobenzaldehyde in amyl alcohol) and turns the solution from yellow to cherry red.

8) The correct answer is-

a- H2S +, Motile, Indole –

This is because H2S positive will turn yellow to black colour, motility is present as the culture is turbid and indole is absent as the solution did not have cherry red ring on addition of Kavoc's reagent.