Question 1

Which of the following best describes a 'defined' culture medium?

Accepted Answer

A medium in which the exact chemical composition is known and all components are specified.

Question 2

Which of the following ingredients would make a culture medium 'complex'?

Accepted Answer

Peptone, beef extract, and yeast extract

Question 3

A microbiologist wants to isolate a pure culture from a mixed sample. Which technique should they use, and what is the principle behind it?

Accepted Answer

Streak plate; it dilutes the sample across the agar surface to separate individual cells that grow into isolated colonies.

Question 4

What is the main purpose of using immersion oil with the 100x objective lens in light microscopy?

Accepted Answer

To increase the numerical aperture and reduce light refraction, improving resolution.

Question 5

Why is 'resolution' considered more important than 'magnification' in microscopy?

Accepted Answer

Resolution determines the ability to distinguish two close points as separate, while magnification only enlarges the image.

Question 6

Which of the following best explains why the Gram stain is considered a differential stain?

Accepted Answer

It distinguishes bacteria based on differences in their cell wall structure, resulting in different colors for Gram-positive and Gram-negative bacteria.

Question 7

What are the typical colors observed after Gram staining for Gram-positive and Gram-negative bacteria, respectively?

Accepted Answer

Purple for Gram-positive, pink/red for Gram-negative

Question 8

Which stain is specifically used for Mycobacterium tuberculosis, and why is the Gram stain not effective for this species?

Accepted Answer

Acid-fast stain; Mycobacterium has a waxy cell wall rich in mycolic acids that resists Gram staining.

Question 9

What is the main difference between a negative stain and a simple (basic) stain?

Accepted Answer

A negative stain colors the background, leaving cells unstained, while a simple stain colors the cells directly.

Question 10

Which of the following best describes 'heat-fixing' in the preparation of a bacterial smear?

Accepted Answer

Passing the slide through a flame to kill and adhere the bacteria to the slide.

Question 11

A microbiologist is culturing an obligate (strict) anaerobe. Which of the following methods would best reduce oxygen levels in the culture environment?

Accepted Answer

Using an anaerobic jar with a gas-generating packet to remove oxygen.

Question 12

Which category of microorganism can grow in the presence or absence of oxygen, but grows better when oxygen is available?

Accepted Answer

Facultative anaerobes

Question 13

A tube of thioglycollate broth shows dense growth at the bottom and none at the top. Which type of microorganism is most likely present?

Accepted Answer

Obligate (strict) anaerobe

Question 14

Which of the following best describes a microaerophile?

Accepted Answer

An organism that requires oxygen at lower concentrations than are present in the atmosphere.

Question 15

Which laboratory method can be used to increase the oxygen available to microbes growing in broth?

Accepted Answer

Shaking the broth culture to aerate it.

Question 16

A culture medium contains bile salts and crystal violet to inhibit Gram-positive bacteria, and lactose plus neutral red to differentiate lactose fermenters. Which type of medium is this?

Accepted Answer

Selective-differential medium (e.g., MacConkey agar)

Question 17

Fanny Hesse made a significant contribution to microbiology by introducing which substance as a solidifying agent for culture media?

Accepted Answer

Agar

Question 18

A microbiologist observes isolated colonies on a streak plate. What does this indicate about the culture?

Accepted Answer

Each isolated colony likely arose from a single cell or a small group of identical cells.

Question 19

Which of the following equations correctly represents the relationship between resolution ($$d$$), wavelength ($$\lambda$$), and numerical aperture ($$NA) in $$light microscopy?

Accepted Answer

$$d = \frac{0.5 	imes \lambda}{NA}$$

Question 20

A student is preparing a bacterial smear for Gram staining. Which step is essential to prevent the cells from washing off during staining?

Accepted Answer

Heat-fixing the smear by passing it through a flame.

Microscopy and Cultivation Study Worksheet – Guided Review

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