BIOL123

Lab 4 Selective Media & Agar BIO250L”

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“Pre-Lab Questions”

1. What is the difference between chemically defined and chemically complex media? Give either a

clinical or environmental research example for which each media type would be the most

appropriate choice for culturing microorganisms.

In chemically defined media, the ingredients that make up the chemical media are know

and can be identified and quantity known such a medium is use to determine the least

amount of nutrients that a microbe needs to survive In chemically complex media the

chemical composition is unknown.the ingredients in this media have an organic origin for

instance, blood, and animal or human tissue can be added to a culture to enable a

pathogen to grow. The reason these organic ingredients are added is because the

pathogen is already know to grow in an animal or a human being

2. Why is differential media typically inoculated with isolated colonies that have been previously

cultured on general growth media?

Differential media is used in previously isolated colonies to identify specific microbes on

basis of the characteristics of that mircorogranism. One example is how agar can

differentiate between hemolytic and non hemolytic bacteria-

3. Use a textbook or a reputable internet source (such as www.cdc.gov) to research and describe a

scenario in a lab or clinical setting in which a selective and/or differential test would be

necessary.

In the case where a microbe is resistant to an antibiotic, selective test can and have

been used on a culture that is already resistant to anti-biotic.this allows for the cells that

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Lab 4 Selective Media & Agar BIO250L” have already grown a resistance to the antibiotic to be identified and marked. This was

done to cells that were resistant to neomycin. In a differential test, pin points the

presenceof a certain organism this makes it possible to eliminate this organism as soon

as it has been spoted https://bio.libretexts.org/Bookshelves/Microbiology/Book

%3A_Microbiology_(Boundless)/6%3A_Culturing_Microorganisms/6.3%3A_Culturing_B

acteria/6.3C%3A_Selective_and_Differential_Media

“EXPERIMENT 1: Bioprospecting for Starch Degrading Bacteria

Data Tables Table 3: Starch Agar and Gram Iodine Results

Sample Growth on Starch Agar? (Yes or No)

Clear area around colonies?

Do these bacteria contain amylase?

A Yes no y

B Yes yes- yes-

C Yes y- -yes

D No no no

Post-Lab Questions 1. Why is cow manure used as a potential source of starch-degrading bacteria? (If you are not

familiar with the process of digestion in cows, use a reputable internet source to inform your

answer.)

Because of the presence of Amylase which is needed to digest starch in the mouth

2. What are some other potential sources of starch-degrading bacteria?

potential sources of starch degrading bacteria can be found in fermenting food like hops

in the process of beer making, kombucha, and milk kefir

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Lab 4 Selective Media & Agar BIO250L” 3. What component makes starch agar selective for starch-degrading bacteria?

Because starch agar usually has in it the bacteria that degrades starch, it makes a good

selective media because it is will act as a selective nutrient to promote the growth of this

degrading bacteria

4. Why were each of the following steps performed in this experiment?

a. Serial dilution:

To act as a dilutant of microbes so that they it is possible to count them The

stock solution has too many microbes to be effective in the other process of

microbe identification thought isolation.-

b. Growth on the nutrient agar plates:

this was for purpose of bacteria isolation then transfer a single type of bacteria

hence being able to identify them

c. Streak on the starch agar plates:

to introduce a different media that would identify the presence of starch

degrading bacteria

Insert a photo of your plates. Include your name and access code handwritten in the background of

your photo.

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Lab 4 Selective Media & Agar BIO250L”

“EXPERIMENT 2: Selection and Differentiation of Body Inhabiting Gram-Positive Bacteria

Data Tables Table 4: Experiment 2 MSA Growth Observations

Surface Tested

Growth (Yes or No) –

Nutrient Agar

Growth (Yes or No) – MSA Agar

MSA color around colonies (Red or

Yellow)

Other Observations

Face Yes Yes Yellow Cfogged up –

nose Yes Yes Yellow Thick and droppy-

Controller Yes Yes Yellow Many organisms

–

Control No No Select one: clear –

Post-Lab Questions 1. What chemical in MSA confers selectivity? How?

The HIGH concentration of sodium chloride allows for the growth of microgranisms like

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Lab 4 Selective Media & Agar BIO250L” staphylococcus which while other types of this organism cannot ferment MSA, The

staphylococcus does

2. What chemical makes MSA differential? How?

Sugar mannitol is the differential chemical presence of organisms that can use sugar

mannitol as a source of nutrition will ferment it causing to become acidic which in turn

change the ph .This change in ph accounts for color change when this kind of bacteria is

present.-

3. Why are the nutrient agar plates used in this experiment?

The agar plates are serve to help compare the growth rates in MSA and Agar

4. Was there any growth on you MSA plates? Did any of the colonies change the color of the MSA?

What does this tell you about the bacteria taken from each area of your body?

yes there was a change in color which serves as confirmation that there were bacteria

that allowed for MSA to ferment hence making the set up acidic. The acidity is what

made the co

Insert a photo of your plates. Include your name and access code handwritten in the background of

your photo.

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Lab 4 Selective Media & Agar BIO250L”

EXPERIMENT 3: Selection and Differentiation of Body Inhabiting Gram-Negative Bacteria from

Liquid Samples

Data Tables Table 5: MacConkey Agar Results

Sample Growth?

(Yes or No) Colony Color (Clear or Red)

Analysis

CTap water- Yes Select one: Cpretty clean-

Spring water – Yes Select one: C pure and seems extremely safe –

Pond water – Yes Select one: extremely dirty-

Experiment 3 Post-Lab Questions 1. What types of bacteria are inhibited on MacConkey agar? What ingredient(s) in MacConkey agar

selects against those bacteria?

MacConkey agar are inhibited by Gram positive bacteria the ingredients

that allow for for selection are crystal violet and salts .

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Lab 4 Selective Media & Agar BIO250L” 2. What ingredient(s) makes MacConkey agar differential?

MacCoknkey agar is a differential Because it contains lactose. When bacteria works on

lactose to break it down, there is a release of acidic waste which changes the ph and

gives it the red-pink look

3. Using a textbook and a reputable online source (such as the CDC), research and describe some

potentially pathogenic members of the intestinal bacteria family Enterobacteriaceae. Which

pathogenic species are lactose fermenters that will grow on MacConkey agar?

Salmonella typhimurium- i will ferment lactose that is on the MacConkey agar.E-coli is

also a lactose fermenting bacteria.

4. Use a reputable internet source to research and describe some potentially pathogenic intestinal

bacteria that do not ferment lactose that will grow on MacConkey agar.

C-Legionella, Bordetella, Helicobacter are non lactose fermenting bacteria

5. Use a reputable internet source to research and describe what variations of MacConkey agar can

be used to detect other species of bacteria.

there is a variation of MacConkey Agar without Crystal Violet it detects Gram-Positive

cocci.- There there is MacConkey Agar that has Violet it is used to control t bacteria

like Proteus which interfere with other results.:

6. How would you verify that the colonies that grew on a MacConkey agar plate were Gram-

negative?

Crystal violet and bile salts usage helps identify gram negative colonies because they

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Lab 4 Selective Media & Agar BIO250L” stop the growth of gram-positive bacteria thus all that is left in the sample are gram

negative bacteria

7. Look up the formula for MacConkey agar either in a microbiology textbook or online. Is this a

chemically defined or a chemically complex media? Why is that important?

MacConkey agar is a complex chemically defined media because within it there

ingredients that propagate the growth of some bacteria while other ingredients inhibit the

growth of othes.

Insert a photo of your plates. Include your name and access code handwritten in the background of

your photo.

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Lab 4 Selective Media & Agar BIO250L”

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