Ch. 3 - Observing Microorganisms Through a Microscope

Tortora - Microbiology: An Introduction 14th Edition

Tortora14th EditionMicrobiology: An IntroductionISBN: 9780138200398Not the one you use?Change textbook

Tortora 14th Edition

Ch. 3 - Observing Microorganisms Through a Microscope

Problem 9

Chapter 3, Problem 9

Fill in the following table regarding the Gram stain:

Verified step by step guidance

Step 1: Understand the purpose of the Gram stain, which is to differentiate bacteria into Gram-positive and Gram-negative groups based on the structure of their cell walls.

Step 2: Identify the key components of the Gram stain procedure: crystal violet (primary stain), iodine (mordant), alcohol or acetone (decolorizer), and safranin (counterstain).

Step 3: For each step in the table, describe what happens to Gram-positive bacteria: they retain the crystal violet-iodine complex and appear purple because of their thick peptidoglycan layer.

Step 4: For each step, describe what happens to Gram-negative bacteria: they lose the crystal violet-iodine complex during decolorization due to their thinner peptidoglycan layer and outer membrane, and then take up the safranin counterstain, appearing pink/red.

Step 5: Fill in the table by matching each step of the Gram stain with the expected color and structural changes in both Gram-positive and Gram-negative bacteria.

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Key Concepts

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

Gram Stain Procedure

The Gram stain is a differential staining technique used to classify bacteria into Gram-positive or Gram-negative based on their cell wall properties. It involves four steps: crystal violet staining, iodine treatment, alcohol decolorization, and counterstaining with safranin. Each step affects how bacteria retain or lose the stain.

Bacterial Cell Wall Structure

The difference in Gram stain results is due to variations in bacterial cell walls. Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet-iodine complex, appearing purple. Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane, causing them to lose the initial stain and take up the counterstain, appearing pink.

Interpretation of Gram Stain Results

Understanding how to interpret the colors and morphology after Gram staining is essential. Purple indicates Gram-positive bacteria, while pink indicates Gram-negative. This distinction helps in bacterial identification and guides antibiotic treatment decisions.

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Gram Stain

Related Practice

Textbook Question

Which of the following is not a modification of a compound light microscope?

a. Brightfield microscopy

b. Darkfield microscopy

c. Electron microscopy

d. Phase-contrast microscopy

e. Fluorescence microscopy

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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

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

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