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Biological & Biomedical Sciences
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English (U.S.)
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Topic:

Primary Media for Isolation of Microorganisms and Pure Bacterial Colonies

Coursework Instructions:

Exercise 4: Pure bacterial colonies
1. When an agar plate is inoculated, why is the loop sterilized after the initial inoculum is put on?
2. Distinguish between a pure culture and a mixed culture.
3. Define a bacterial colony. List four characteristics by which bacterial colonies may be distinguished.
4. Why should a Petri dish not be left open for any extended period?
5. Why does the streaking method you used to inoculate your plates result in isolated colonies?
Exercise 5: Pour plate and streaking technique to obtain pure cultures
1. Discuss the relative convenience of pour- and streak-plate techniques in culturing clinical specimens.
2. How do you decide which colonies should be picked from a plate culture of a mixed flora?
3. Why is it necessary to make pure subcultures of organisms grown from clinical specimens?
4. What kinds of clinical specimens may yield a mixed flora in bacterial cultures?
5. When more than one colony type appears in pure culture, what are the most likely sources of extraneous contamination?
Exercise 3: Primary media for isolation of microorganisms
1. Define a differential medium and discuss its purpose.
2. Define a selective medium and describe its uses.
3. Why is MacConkey agar selective as well as differential? 4. Why is blood agar useful as a primary isolation medium?
5. What is the major difference between Modified Thayer-Martin (MTM) and chocolate agar? When would you use MTM rather than chocolate agar?

Coursework Sample Content Preview:

Microbiology
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Microbiology
Exercise 3: Primary Media for Isolation of Microorganisms
A differential medium refers to a cultured approach that supports the distinction of microorganisms based on colonies, color, shape or size, and appearance. The goal is to identify microbes and understand their interactions in medium dyes based on their metabolic properties. On the other hand, a selective medium is used to support the growth of specific microorganisms. For instance, if a given microbe is resistant to specific antibiotics, then the item can be added to a medium to prevent other microbes.
MacConkey agar is different due to its ability to ferment lactose, and certain species will produce colonies in varying appearances in the medium. The lactose fermenting species develop pink colonies, and this is a differentiation property (Jung & Hoilat, 2022). Also, MacConkey is selective by inhibiting the growth of most gram-positive microbes due to crystal violet. Blood sugar is a valuable isolation medium due to its general nutrients that cultivate fastidious organisms and define hemolytic capabilities
A modified Thayer Martin has the hemoglobin necessary to provide an X-factor hemin. Chocolate agar is non-selective and allows fastidious bacteria from a variety of specimens. On the contrary, Thayer-Martin gets used to isolate pathogens like Neisseria from a mixed flora of bacteria and fungi. Thus, MTM can be used instead of chocolate agar when selecting pathogenic Neisseria microorganisms.
Exercise 4: Pure Bacterial Colonies
Loop sterilization is necessary to avoid contamination. Sterilization is needed for the loop to be used giant without contaminated plate and risking spread of bacteria in thinner layers. Also, this process supports the goal of getting individual colonies.
In bacteriological cultural methods, pure culture refers to only one containing single types, while mixed one has more than two bacteria. Once a bacteria culture is in a media for a long time, cells consume available nutrients and release toxic metabolites. As a result, the entire culture or population becomes affected and dies.
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