Microbiology Lab Practicum Review Key Terms and Definitions Study Guide 2026–2027 Updated: Laboratory Skills, Techniques and Microbial Concepts

Microbiology Lab Practicum Review Key
Terms and Definitions Study Guide 2026–
2027 Updated: Laboratory Skills, Techniques
and Microbial Concepts

Microbiology Lab Practicum Review Exam nb nb nb nb




Instructions: Choose the single best answer for each question.
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1. A student observes a pink/red rod-
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shaped bacterium after performing a Gram stain. Which of the following is the most like
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ly identification of this organism?
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• A) Gram-positive Staphylococcus
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• B) Gram-negative Bacillus
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• C) Gram-positive Streptococcus
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• D) Gram-negative Escherichia coli
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Correct Answer: D) Gram-negative Escherichia coli
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(Rationale: Gram-negative bacteria lose the crystal violet-
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iodine complex during decolorization and take up the safranin counterstain, appearing pink/red.
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Rod-shaped indicates a bacillus; E. coli is a gram-negative bacillus.)
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2. The primary purpose of the heat fixation step in preparing a bacterial smear is to:
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• A) Kill the bacteria and adhere them to the slide.
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• B) Enhance the uptake of the primary stain.
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• C) Prevent the bacteria from washing off during staining.
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• D) Both A and C.
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Correct Answer: D) Both A and C.
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(Rationale: Heat fixation kills the organisms (making them safe) and coagulates the proteins, caus
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ing the cells to stick firmly to the glass slide so they are not washed away during the staining proc
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edure.)

,3. Which of the following is an example of a selective medium?
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• A) Blood Agar (BA)
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• B) MacConkey Agar (MAC)
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• C) Nutrient Agar (NA)
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• D) Chocolate Agar
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Correct Answer: B) MacConkey Agar (MAC)
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(Rationale: MacConkey Agar contains crystal violet and bile salts which inhibit the growth of Gra
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m-positive organisms, thus selecting for Gram-
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negative bacteria. Blood Agar is differential, Nutrient Agar is general-
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purpose, and Chocolate Agar is enriched.)
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4. The aseptic technique of passing the inoculating loop through the flame of a Bunsen
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burner until it is red-hot is called:
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• A) Sterilization
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• B) Disinfection
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• C) Sanitization
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• D) Pasteurization
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Correct Answer: A) Sterilization
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(Rationale: Sterilization is the complete destruction of all microbial life, including spores. The inten
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se heat of the flame incinerates any contaminants on the loop, achieving sterilization.)
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5. You are using a microscope and the image is blurry. You use the coarse focus knob an
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d the fine focus knob but the image remains blurry. Which of the following components
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should you check next?
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• A) The objective lens
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• B) The condenser
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• C) The ocular lens
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• D) The iris diaphragm
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,Correct Answer: A) The objective lens
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(Rationale: While all are important for image quality, if the focus knobs are not resolving the ima
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ge, the most common issue is that the objective lens is dirty. Cleaning the lens is the next logical s
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tep.)



6. The term "resolution" (or resolving power) in microscopy refers to the ability to:
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• A) Magnify an image.
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• B) Distinguish two adjacent points as separate entities.
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• C) Increase the brightness of the image.
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• D) Increase the contrast between the specimen and the background.
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Correct Answer: B) Distinguish two adjacent points as separate entities.
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(Rationale: Resolution is the ability to see fine detail and distinguish between two very close object
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s. Magnification makes things bigger, but resolution makes them clearer.)
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7. Which type of media is used to determine the oxygen requirements of a microorganis
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m?

• A) Broth culture
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• B) Agar slant
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• C) Thioglycollate broth
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• D) Differential plate
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Correct Answer: C) Thioglycollate broth
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(Rationale: Thioglycollate broth is a reducing medium that creates an oxygen gradient. The locati
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on of growth within the tube (top, middle, or bottom) indicates whether the organism is an obliga
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te aerobe, anaerobe, facultative anaerobe, or microaerophile.)
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8. When performing a streak plate for isolation, what is the purpose of flaming the loop
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between each quadrant? nb nb




• A) To cool the loop down.
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• B) To kill the bacteria on the loop to dilute the sample for the next quadrant.
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• C) To sterilize the loop and prevent contamination of the pure culture.
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, • D) To add nutrients to the agar.
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Correct Answer: B) To kill the bacteria on the loop to dilute the sample for the next qua
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drant.
(Rationale: Flaming between quadrants ensures that only a small number of cells are carried over
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to the next section, allowing for the isolation of individual colonies. This dilutes the sample.)
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9. You are performing an acid-
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fast stain on a sputum sample. The primary stain is carbolfuchsin. After applying the dec
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olorizer (acid-alcohol), the acid-fast organisms will:
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• A) Lose the primary stain and need a counterstain.
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• B) Appear blue.
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• C) Remain red.
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• D) Be washed away from the slide.
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Correct Answer: C) Remain red. nb nb nb nb




(Rationale: Acid- nb




fast bacteria (like Mycobacterium) have a waxy mycolic acid layer in their cell wall that resists the
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bdecolorizer, trapping the primary stain (carbolfuchsin). They remain red.)
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10. An instrument that uses steam under pressure to achieve sterilization (e.g., for media
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) is called a(n):
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• A) Incubator
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• B) Autoclave
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• C) Centrifuge
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• D) Hot air oven
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Correct Answer: B) Autoclave nb nb nb




(Rationale: An autoclave uses moist heat (steam) under pressure at 121°C for 15 minutes to sterili
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ze. This is the standard method for sterilizing microbiological media and equipment.)
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11. What is the function of the oil immersion objective (100x)?
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• A) To stain the specimen more effectively.
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• B) To reduce the refraction of light and increase resolution.
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