Q: Which of the following equations would you use to calculate the average diameter of Micrococcus luteus if you measured five cells with diameters $$d_1$$, $$d_2$$, $$d_3$$, $$d_4$$, $$d_5$$?

$$\text{Average diameter} = \frac{d_1$ + d_2$ + d_3$ + d_4$ + d_5$}{5}$$

Question 1

Which of the following best explains why Nigrosin is used in negative staining of bacteria?

Accepted Answer

Nigrosin is acidic and negatively charged, so it is repelled by the negatively charged bacterial surface.

Question 2

What is the primary advantage of using a negative stain over a heat-fixed stain when observing bacterial morphology?

Accepted Answer

Negative staining avoids distortion of cell size and shape caused by heat fixing.

Question 3

A student prepares a negative stain but accidentally heat fixes the slide. What is the most likely outcome?

Accepted Answer

The bacteria will be distorted due to dehydration from heat fixing.

Question 4

Which of the following best describes the appearance of bacteria in a properly prepared negative stain?

Accepted Answer

Bacteria appear transparent against a dark background.

Question 5

Why is it important to hold the second slide at a 30-degree angle when spreading the Nigrosin-bacteria mixture?

Accepted Answer

To create a thin, even smear for optimal microscopic observation.

Question 6

Which of the following statements about Micrococcus luteus is correct based on the study materials?

Accepted Answer

It is a Gram-negative coccus that forms tetrads and produces a yellow pigment.

Question 7

Which of the following best describes Bacillus cereus as seen in a negative stain?

Accepted Answer

Gram-positive rods often forming chains (strepto).

Question 8

A student measures the length and width of Bacillus cereus using an ocular micrometer. Why is it important to average measurements from multiple cells?

Accepted Answer

To account for variation in cell size and obtain a representative value.

Question 9

Which of the following is a critical step that must NOT be performed during negative staining?

Accepted Answer

Heat fixing the slide.

Question 10

If a student observes a slide with transparent round cells in groups of four against a dark background, which organism is most likely present?

Accepted Answer

Micrococcus luteus

Question 11

Which of the following equations would you use to calculate the average diameter of Micrococcus luteus if you measured five cells with diameters $$d_1$$, $$d_2$$, $$d_3$$, $$d_4$$, $$d_5$$?

Accepted Answer

$$	ext{Average diameter} = \frac{d_1$ + d_2$ + d_3$ + d_4$ + d_5$}{5}$$

Question 12

Which of the following best explains why negative staining is useful for observing bacterial capsules?

Accepted Answer

Capsules repel the stain and appear as clear halos around the cells.

Question 13

A student accidentally blots the slide after applying Nigrosin. What is the most likely result?

Accepted Answer

The bacteria will be washed away, resulting in few or no cells visible.

Question 14

Which of the following is the correct order of steps for preparing a negative stain?

Accepted Answer

Label slide, add Nigrosin, mix bacteria, spread with second slide, air dry.

Question 15

Why is oil immersion used when examining bacteria under the microscope after negative staining?

Accepted Answer

To achieve higher magnification and resolution for observing small bacterial cells.

Question 16

Which of the following best describes the arrangement of Bacillus cereus as seen in the provided images?

Accepted Answer

Rods in chains (streptobacilli)

Question 17

If a student observes both tetrads of cocci and chains of rods on the same slide, what is the most likely explanation?

Accepted Answer

The slide contains a mixture of Micrococcus luteus and Bacillus cereus.

Negative Staining: Principles, Procedure, and Morphology of Micrococcus luteus and Bacillus cereus

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