Microbiology worksheet
Exercise 8
Questions
1. Define a culture medium.
A culture media are agar gelatinous or liquid medium nutritive substances such as microorganisms, animal cells, plant cells, or tissues and are cultivated for scientific purposes.
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company.
2. Discuss some of the physical and chemical factors involved in the
composition, and in the preparation, of a culture medium.
Nutrient ingredients:
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pH and buffering:
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Heat (to reconstitute):
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Heat (to sterilize):
heating in an autoclave at 121°C for 15 min. kills all living organisms, including spores for the media
glassware heated at 160 °C for 2 hours
Other:
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3. At what temperature does agar solidify?
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Page 1 of 5
From Laboratory Manual & Workbook in Microbiology Applications to Patient Care (9th ed.). By
Josephine A. Morello, Helen Eckel Mizer, and Paul A. Granato Copyright © 2006 The McGraw-
Hill Companies, Inc. Reprinted with permission of The McGraw-Hill Companies, Inc.
BIO2071_Microbiology Laboratory
At what temperature does agar melt?
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4. What would happen to plates poured with agar that is too hot?
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Could they be used?
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5. What would happen to plates poured with agar that is too cool?
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Could they be used?
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6. Why are culture media sterilized before use?
7. Discuss the relative value of broth and agar media in isolating bacteria from
mixed cultures.
8. Are nutrient broths and agars, as you have prepared them, suitable for
supporting growth of all microorganisms pathogenic for humans? Explain your
answer.
Page 2 of 5
From Laboratory Manual & Workbook in Microbiology Applications to Patient Care (9th ed.). By
Josephine A. Morello, Helen Eckel Mizer, and Paul A. Granato Copyright © 2006 The McGraw-
Hill Companies, Inc. Reprinted with permission of The McGraw-Hill Companies, Inc.
BIO2071_Microbiology Laboratory
Exercise 9
Questions
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?
6. Why is it necessary to isolate individual colonies from a mixed growth?
7. Why was a blood agar, rather than a nutrient agar, plate used for the
culture from your mouth?
8. Are the large numbers of microorganisms found in the mouth cause for
concern? Explain.
9. How do microorganisms find their way into the mouth?
Exercise 10
Questions
1. Discuss the relative convenience of pour- and streak-plate techniques in
culturing clinical specimens?
2. Why are plate cultures incubated in the inverted positions?
3. How do you decide which colonies should be picked from a plate culture
of a mixed flora?
4. Why is it necessary to make pure subcultures of organisms grown from
clinical specimens?
5. How can you determine whether a culture or subculture is pure?
6. What kinds of clinical specimens may yield a mixed flora in bacterial
cultures?
Page 3 of 5
From Laboratory Manual & Workbook in Microbiology Applications to Patient Care (9th ed.). By
Josephine A. Morello, Helen Eckel Mizer, and Paul A. Granato Copyright © 2006 The McGraw-
Hill Companies, Inc. Reprinted with permission of The McGraw-Hill Companies, Inc.
BIO2071_Microbiology Laboratory
7. When more than one colony type appears in pure culture, what are the
most likely sources of extraneous contamination?
Exercise 16
Questions
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. How can one distinguish E. coli from P. aerugionosa on
Nutrient agar?
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Blood agar?
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MAC agar?
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6. What is the major difference between Modified Thayer-Martin (MTM) and
chocolate agar? When would you use MTM rather than chocolate agar?
7. If you wanted to isolate S. aureus from a pus specimen containing a mixed
flora, what medium would you choose to get results most rapidly? Why?
8. What is the value of making a Gram stain directly from a clinical
specimen?
9. Why is aseptic technique important in the laboratory? In patient care?
Page 4 of 5
From Laboratory Manual & Workbook in Microbiology Applications to Patient Care (9th ed.). By
Josephine A. Morello, Helen Eckel Mizer, and Paul A. Granato Copyright © 2006 The McGraw-
Hill Companies, Inc. Reprinted with permission of The McGraw-Hill Companies, Inc.
BIO2071_Microbiology Laboratory
Exercise 17.2 and 17.3
Questions
1. What is the color of phenol red at an acid pH?
2. What is the function of a Durham tube?
3. Why is iodine used to detect starch hydrolysis?
4. Name one indole-positive organism?
5. How is indole produced in SIM medium? How is it detected?
6. How is hydrogen sulfide demonstrated in this medium?
7. Name two methods of determining bacterial motility.
8. Why is it essential to have pure cultures for biochemical tests?
9. Could a pH-sensitive color indicator be used to reveal the presence of a
contaminant in a fluid that should be sterile? Explain.