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 8

Chapter 3, Problem 8

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

Verified step by step guidance

Step 1: Understand the Gram stain principle. Gram-positive bacteria retain the crystal violet stain and appear purple or blue, while Gram-negative bacteria do not retain the crystal violet and instead take up the counterstain (usually safranin), appearing red or pink.

Step 2: Identify the morphology and color described. Red cocci are spherical bacteria that appear red, indicating they are Gram-negative cocci. Blue rods are rod-shaped bacteria that appear blue, indicating they are Gram-positive rods.

Step 3: Recognize that different shapes and different Gram stain colors suggest the presence of more than one bacterial type or species in the sample.

Step 4: Consider that a single species typically has a consistent Gram reaction and morphology, so seeing both red cocci and blue rods means you are likely observing two different species.

Step 5: Conclude that the correct interpretation is that you have two different species present, corresponding to option (b).

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Key Concepts

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

Gram Staining and Color Interpretation

Gram staining differentiates bacteria into Gram-positive (purple/blue) and Gram-negative (pink/red) based on cell wall structure. Gram-positive bacteria retain crystal violet dye, appearing blue or purple, while Gram-negative bacteria take up the counterstain safranin, appearing red or pink.

Bacterial Morphology

Bacteria are classified by shape, such as cocci (spherical) and rods (bacilli). Observing different shapes in the same field suggests the presence of multiple bacterial types or species, as morphology is a key identifying feature.

Interpreting Mixed Staining Results

Seeing red cocci and blue rods together usually indicates two different bacterial species with distinct Gram reactions. This is not typically due to staining error or cell age, but rather a mixed culture or sample containing diverse bacteria.

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Guided course

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Introduction to Staining

Related Practice

Textbook Question

What is the purpose of a counterstain in the acid-fast stain?

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Textbook Question

Which of the following pairs is mismatched?

a. Capsule—negative stain

b. Cell arrangement—simple stain

c. Cell size—negative stain

d. Gram stain—bacterial identification

e. None of the above

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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

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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Textbook Question

Fill in the following table regarding the Gram stain:

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