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
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“Pre-Lab Questions”
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.
Why is differential media typically inoculated with isolated colonies that have been previously cultured on general growth media?
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
“EXPERIMENT 1: Bioprospecting for Starch Degrading Bacteria
Data Tables
Table 3: Starch Agar and Gram Iodine Results
Post-Lab Questions
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.)
What are some other potential sources of starch-degrading bacteria?
What component makes starch agar selective for starch-degrading bacteria?
Why were each of the following steps performed in this experiment?
Serial dilution:
Growth on the nutrient agar plates:
Streak on the starch agar plates:
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“EXPERIMENT 2: Selection and Differentiation of Body Inhabiting Gram-Positive Bacteria
Data Tables
Table 4: Experiment 2 MSA Growth Observations
Post-Lab Questions
What chemical in MSA confers selectivity? How?
What chemical makes MSA differential? How?
Why are the nutrient agar plates used in this experiment?
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?
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EXPERIMENT 3: Selection and Differentiation of Body Inhabiting Gram-Negative Bacteria from Liquid Samples
Data Tables
Table 5: MacConkey Agar Results
Experiment 3 Post-Lab Questions
What types of bacteria are inhibited on MacConkey agar? What ingredient(s) in MacConkey agar selects against those bacteria?
What ingredient(s) makes MacConkey agar differential?
Using a textbook and a reputable online source (such as ), 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?
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.
Use a reputable internet source to research and describe what variations of MacConkey agar can be used to detect other species of bacteria.
How would you verify that the colonies that grew on a MacConkey agar plate were Gram-negative?
Look up the formula for MacConkey agar either in a microbiology textbook or . Is this a chemically defined or a chemically complex media? Why is that important?
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Student Name:
Access Code (located on the underside of the lid of your lab kit):
“Pre-Lab Questions”
What determines if a bacterial cell is Gram-positive or Gram-negative?
In this lab, both viruses and prions were introduced as acellular organisms. Do some research and describe one other type of acellular organism. What characteristics about this organism classify it as acellular?
Bacteria have many different shapes that often determine their class. Research and form a hypothesis on the evolutionary reasons for so many different bacterial morphologies.
Do a search online or look in your textbook for 1-2 antibiotics that affect Gram-positive bacteria and list them. On what part of the cell do the antibiotics usually work? List one or two antibiotics that affect Gram-negative bacteria? On what part of the cell do the antibiotics usually work? (Be sure to cite your sources in your answer.)
Why do you think it is important to identify a bacterial disease in a patient before prescribing any antibiotic treatments? (Be specific.)
“EXPERIMENT 1: Staining
Data Tables
Table 1: Experiment 1 Staining Observations
Post-Lab Questions
How does crystal violet enhance the visualization of microbial features?
What are some of the limitations of simple staining?
Give an example of a situation in a lab or medical setting in which simple staining would be utilized.
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“EXPERIMENT 2: Negative Staining
Data Tables
Table 2: Experiment 2 Staining Observations
Post-Lab Questions
After visualizing the stained samples either using your microscope or by looking at the sample images provided, describe what physical/visual characteristics you were able to observe after performing the negative staining vs. after performing the simple stain.
So far in this lab, you have used one type of simple stain and one type of negative stain, yet there are many other simple and negative dyes available. Pick one simple dye and one negative dye, and discuss how those dyes differ from the ones you used in this lab. Give a scenario in which their use would be appropriate.
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EXPERIMENT 3: Gram Staining
Data Tables
Table 3: Experiment 3 Staining Observations
Post-Lab Questions
What color are the Gram-positive bacteria after Gram staining? Gram-negative bacteria?
What different characteristic(s) exist between the two groups that account for the different staining conditions?
Why was the Gram iodine added to the Gram staining procedure?
Why is a counterstain (safranin) added to the Gram staining procedure?
What are the advantages of performing a Gram stain vs. a simple stain for visualizing bacteria?
Using either a textbook or a reputable online resource, research some of the typical characteristics of bacteria, and discuss why it might be important for a researcher or a hospital technician to be able to differentiate between Gram-positive and Gram-negative bacteria
Did you experience any technical difficulties or atypical results during this experiment? If so, what happened, and how could you avoid these issues in the future?
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