Microbiology labs

Lab 4 Selective Media & Agar” “BIO250L”

Student Name: Amy Bailey

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Access Code (located on the lid of your lab kit): AC-OU2JH9B

“Experiment 1- Selection and Differentiation of Body Inhabiting, Gram-Positive Bacteria”

“Table 4: Experiment 1 Growth Conditions”

“Surface Tested”

“Growth (Good or Poor)”

“Color (Red or Yellow)”

“Other Observations”

“LB/Nutrient Agar”

“MSA Agar”





White growth present too. Small Round spores.





Same growth as seen on the skin but in fewer quantities.





Spores are very few (2-5). Nutrient agar has a 1 large colony.





1 small round colony.

“Post-Lab Questions”

“1. What substance in MSA confers selectivity? Why?”

While MSA is both selective and differential, the high salt content is what makes it selective. This environment will not allow certain bacterial cells to thrive making the plate selective to certain bacteria.

“2. What substance makes MSA differential? Why?”

MSA is differential due to its sugar source—Mannitol. This opens the window to a few more bacteria making the plate differential too.

“3. What purpose does phenol red serve in MSA?”

The phenol is a pH level indicator. If the color is red, it is the bacteria is basic. If the color is yellow, it means fermentation has taken place and is now acidic.

“4. Why are the nutrient agar plates used in this experiment?”

As with the rest of the experiments, the nutrient agar provides a broad growth medium.

“Insert photo of your cultures after incubation with your name clearly visible in the background:”


Experiment 2- Selection and Differentiation of Gram-Negative Bacteria from Liquid Samples”

“Table 5: MacConkey Agar Results”



“Colony Color”





Lactose negative results because of the colorless results

Water dispenser



There is no gram-neg. organisms

Faucet water



There is no gram-neg. organisms

“Post-Lab Questions”

“1. What ingredient(s) makes MacConkey agar selective for Gram-negative bacteria?”

MacConkey’s agar contains two things that make it selective for gram-negative bacteria. It contains both bile salts and crystal violet that inhibits the growth of most gram positive bacteria.

“2. What types of bacteria are inhibited on MacConkey agar?”

Most Gram positive bacteria are inhibited.

“3. What ingredient(s) makes MacConkey agar differential?”

MacConkey agar has lactose as its sugar of choice and neutral red to detect acid production.

“4. Why is an indicator dye used in MacConkey agar?”

Like MSA, MacConkey uses the dye for a pH indicator. If the bacteria turn red, it is an acidic producer when they digest/ferment.

“5. What are some potentially pathogenic bacteria that are lactose fermenters that will grow on MacConkey agar?”

Escherichia coli, Klebsiella, and Enterobacter

“6. What are some potentially pathogenic bacteria that do not ferment lactose that will grow on MacConkey agar?”

Salmonella, Shigella, and Pseudomonas

“7. How would you verify that the colonies that grew on a MacConkey agar plate were Gram-negative?”

Due to the selective ingredients, the gram-positive organisms are inhibited from growth. The plate would show no color or are colorless for lactose negative results. Anything colored is lactose positive.

“8. Look up the formulation for MacConkey agar. Is this a chemically defined or complex media? Why is that important?”

MacConkey is a complex media. This is because the media is composed of a food sugar called lactose. However, we still do not know the complete makeup of the bacteria we are looking for and as such, this also makes it a complex media. Since we do not know the makeup, it is important to provide a wide source of nutrition.

“Insert photo of your cultures after incubation with your name clearly visible in the background:”


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