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Ch. 3 - Observing Microorganisms Through a Microscope
All textbooksTortora 14th EditionCh. 3 - Observing Microorganisms Through a MicroscopeProblem 3.9a
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Chapter 3, Problem 3.9a

Fill in the following table regarding the Gram stain:
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Identify the purpose of the Gram stain, which is to differentiate bacterial species into two groups: Gram-positive and Gram-negative, based on the structural differences in their cell walls.
Understand the basic steps of the Gram staining procedure: application of a primary stain (crystal violet), addition of a mordant (iodine), decolorization with alcohol or acetone, and counterstaining with safranin.
Recognize the characteristics of Gram-positive bacteria, which retain the crystal violet stain and appear purple due to their thick peptidoglycan layer.
Recognize the characteristics of Gram-negative bacteria, which do not retain the crystal violet stain and appear pink or red after counterstaining with safranin, due to their thinner peptidoglycan layer and outer membrane.
Fill in the table by categorizing the bacteria based on their Gram stain results, noting the color they appear after the staining process and any relevant structural features.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Gram Staining Procedure

The Gram staining procedure is a differential staining technique used to classify bacteria into two groups: Gram-positive and Gram-negative. It involves applying a series of dyes, including crystal violet, iodine, alcohol, and safranin, to bacterial cells. The outcome depends on the cell wall structure; Gram-positive bacteria retain the crystal violet stain, appearing purple, while Gram-negative bacteria do not and take up the counterstain, appearing pink.
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Cell Wall Structure

The cell wall structure of bacteria is crucial for the Gram stain's effectiveness. Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain, while Gram-negative bacteria possess a thinner peptidoglycan layer surrounded by an outer membrane. This structural difference is key to understanding why the Gram stain differentiates these two groups and influences their susceptibility to antibiotics.
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Clinical Relevance of Gram Staining

Gram staining is clinically significant as it aids in the identification of bacterial pathogens and informs treatment decisions. The results can guide the choice of antibiotics, as Gram-positive and Gram-negative bacteria often respond differently to various drugs. Rapid identification through Gram staining can be critical in managing infections and improving patient outcomes.
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Related Practice
Textbook Question

Assume you stain Clostridium by applying a basic stain, carbolfuchsin, with heat, decolorizing with acid-alcohol, and counterstaining with an acidic stain, nigrosin. Through the microscope, the endospores are 1, and the cells are stained 2.

a. 1—red; 2—black

b. 1—black; 2—colorless

c. 1—colorless; 2—black

d. 1—red; 2—colorless

e. 1—black; 2—red

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Textbook Question

What is the purpose of a decolorizer in the Gram stain? In the acid-fast stain?

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Textbook Question

Assume that you are viewing a Gram-stained field of red cocci and blue rods through the microscope. You can safely conclude that you have

a. made a mistake in staining.

b. two different species.

c. old bacterial cells.

d. young bacterial cells.

e. none of the above

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Textbook Question

In 1996, scientists described a new tapeworm parasite that had killed at least one person. The initial examination of the patient’s abdominal mass was most likely made using

a. brightfield microscopy.

b. darkfield microscopy.

c. electron microscopy.

d. phase-contrast microscopy.

e. fluorescence microscopy.

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Textbook Question

DRAW IT Label the components of the direct and indirect FA tests in the following situations. Which test is direct? Which test provides definitive proof of disease?

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Textbook Question

NAME IT Van Leeuwenhoek was the first to see this budding microbe with a nucleus and cell wall; although humans have used it since before the beginning of recorded history, Louis Pasteur was the first to figure out what it does.

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