Microbiology Lab Mid Term Study Guide
Questions with Verified Solutions
What is the purpose of inoculating a microbiological medium with a sample?
✔✔ To introduce microorganisms for growth and observation.
What does the term "aseptic technique" refer to in microbiology?
✔✔ Methods used to prevent contamination during handling of cultures and samples.
Why is it necessary to flame the neck of a culture tube before and after use?
✔✔ To sterilize the opening and prevent contamination from airborne microorganisms.
What is the purpose of a streak plate technique?
✔✔ To isolate individual bacterial colonies from a mixture.
Why is it important to use fresh bacterial cultures when performing a Gram stain?
✔✔ To ensure accurate results since older cultures may not retain the Gram-positive reaction.
1
,What does the presence of a yellow color in a phenol red broth indicate?
✔✔ That the bacteria have fermented the carbohydrate and produced acid.
What is the purpose of the oxidase test?
✔✔ To detect the presence of cytochrome c oxidase in bacteria.
What does a positive result in the catalase test look like?
✔✔ Bubbling or fizzing when hydrogen peroxide is added to the bacterial culture.
What type of media is MacConkey agar, and what does it differentiate?
✔✔ It is a selective and differential medium that differentiates lactose fermenters from non-
fermenters.
What is the difference between a selective and a differential medium?
✔✔ Selective media inhibit the growth of certain microorganisms, while differential media allow
for the differentiation of organisms based on specific characteristics.
Why do you use a Durham tube in a fermentation test?
2
,✔✔ To capture gas produced by bacteria during fermentation.
What is the purpose of the endospore stain?
✔✔ To identify bacteria that can form endospores, a structure resistant to harsh conditions.
What color will endospores appear after an endospore stain?
✔✔ Green, as the primary stain malachite green stains the spores.
What is the purpose of using an agar slant in microbiology?
✔✔ To provide a solid growth surface for the long-term cultivation and storage of
microorganisms.
How do you determine the motility of a bacterium using a SIM medium?
✔✔ By observing whether the bacteria spread throughout the agar, indicating movement.
What type of bacteria would show a positive result for hydrogen sulfide production on SIM
medium?
✔✔ Bacteria that produce hydrogen sulfide will form black precipitates in the medium.
3
, Why are bacterial cultures incubated at specific temperatures?
✔✔ To provide optimal growth conditions for different types of bacteria, based on their
preferred temperature range.
What is the primary function of a biosafety cabinet in a microbiology lab?
✔✔ To provide a controlled environment that minimizes the risk of contamination and exposure
to hazardous materials.
What is the purpose of using a Bunsen burner in the microbiology lab?
✔✔ To sterilize tools, like inoculating loops, and create an aseptic environment by producing a
sterile flame.
What is the main purpose of performing a Gram stain in microbiology?
✔✔ To classify bacteria as Gram-positive or Gram-negative based on differences in cell wall
structure.
What is the difference between a selective and a differential test?
4
Questions with Verified Solutions
What is the purpose of inoculating a microbiological medium with a sample?
✔✔ To introduce microorganisms for growth and observation.
What does the term "aseptic technique" refer to in microbiology?
✔✔ Methods used to prevent contamination during handling of cultures and samples.
Why is it necessary to flame the neck of a culture tube before and after use?
✔✔ To sterilize the opening and prevent contamination from airborne microorganisms.
What is the purpose of a streak plate technique?
✔✔ To isolate individual bacterial colonies from a mixture.
Why is it important to use fresh bacterial cultures when performing a Gram stain?
✔✔ To ensure accurate results since older cultures may not retain the Gram-positive reaction.
1
,What does the presence of a yellow color in a phenol red broth indicate?
✔✔ That the bacteria have fermented the carbohydrate and produced acid.
What is the purpose of the oxidase test?
✔✔ To detect the presence of cytochrome c oxidase in bacteria.
What does a positive result in the catalase test look like?
✔✔ Bubbling or fizzing when hydrogen peroxide is added to the bacterial culture.
What type of media is MacConkey agar, and what does it differentiate?
✔✔ It is a selective and differential medium that differentiates lactose fermenters from non-
fermenters.
What is the difference between a selective and a differential medium?
✔✔ Selective media inhibit the growth of certain microorganisms, while differential media allow
for the differentiation of organisms based on specific characteristics.
Why do you use a Durham tube in a fermentation test?
2
,✔✔ To capture gas produced by bacteria during fermentation.
What is the purpose of the endospore stain?
✔✔ To identify bacteria that can form endospores, a structure resistant to harsh conditions.
What color will endospores appear after an endospore stain?
✔✔ Green, as the primary stain malachite green stains the spores.
What is the purpose of using an agar slant in microbiology?
✔✔ To provide a solid growth surface for the long-term cultivation and storage of
microorganisms.
How do you determine the motility of a bacterium using a SIM medium?
✔✔ By observing whether the bacteria spread throughout the agar, indicating movement.
What type of bacteria would show a positive result for hydrogen sulfide production on SIM
medium?
✔✔ Bacteria that produce hydrogen sulfide will form black precipitates in the medium.
3
, Why are bacterial cultures incubated at specific temperatures?
✔✔ To provide optimal growth conditions for different types of bacteria, based on their
preferred temperature range.
What is the primary function of a biosafety cabinet in a microbiology lab?
✔✔ To provide a controlled environment that minimizes the risk of contamination and exposure
to hazardous materials.
What is the purpose of using a Bunsen burner in the microbiology lab?
✔✔ To sterilize tools, like inoculating loops, and create an aseptic environment by producing a
sterile flame.
What is the main purpose of performing a Gram stain in microbiology?
✔✔ To classify bacteria as Gram-positive or Gram-negative based on differences in cell wall
structure.
What is the difference between a selective and a differential test?
4
