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 3

Chapter 3, Problem 3

Carbolfuchsin can be used as a simple stain and a negative stain. As a simple stain, the pH is:
a. 2
b. Higher than the negative stain
c. Lower than the negative stain
d. The same as the negative stain

Verified step by step guidance

Understand the role of carbolfuchsin in staining: it can be used both as a simple stain and as a negative stain, but the pH conditions differ depending on the staining method.

Recall that a simple stain typically uses a basic dye, which means the pH is usually higher (alkaline) to allow the dye to bind to negatively charged cell components.

Recognize that a negative stain uses an acidic dye, which means the pH is lower (acidic) so that the background is stained and the cells remain unstained.

Compare the pH of carbolfuchsin when used as a simple stain (basic pH) versus when used as a negative stain (acidic pH).

Conclude that the pH of carbolfuchsin as a simple stain is higher than the pH when it is used as a negative stain.

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Key Concepts

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

Carbolfuchsin as a Stain

Carbolfuchsin is a basic dye commonly used in microbiology for staining bacterial cells. It binds to negatively charged components like nucleic acids and cell walls, making cells visible under a microscope. Its staining properties depend on the pH of the solution, which affects its charge and interaction with cells.

Simple Stain vs. Negative Stain

A simple stain colors the cells directly, usually with a basic dye, highlighting cell morphology. In contrast, a negative stain colors the background, leaving cells unstained and visible as clear shapes. The pH of the stain influences whether the dye acts as a basic or acidic stain, affecting which part is stained.

Effect of pH on Dye Charge and Staining

The pH of a staining solution determines the ionization state of the dye molecules. At low pH, dyes like carbolfuchsin carry a positive charge, acting as basic stains that bind to negatively charged cell components. At higher pH, the dye may lose its positive charge, altering its staining behavior, such as in negative staining.

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Simple Staining Basic & Acidic Dyes

Related Practice

Textbook Question

Assume you stain Bacillus by applying malachite green with heat and then counterstain with safranin. Through the microscope, the green structures are

a. Cell walls

b. Capsules

c. Endospores

d. Flagella

e. Impossible to identify

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Textbook Question

Three-dimensional images of live cells can be produced with:

a. Darkfield microscopy

b. Fluorescence microscopy

c. Transmission electron microscopy

d. Confocal microscopy

e. Phase-contrast microscopy

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Textbook Question

The maximum magnification of a compound microscope is (a) ________; that of an electron microscope, (b) ________. The maximum resolution of a compound microscope is (c) ________; that of an electron microscope, (d) ________. One advantage of a scanning electron microscope over a transmission electron microscope is (e) ________.

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Textbook Question

Looking at the cell of a photosynthetic microorganism, you observe the chloroplasts are green in brightfield microscopy and red in fluorescence microscopy. You conclude:

a. Chlorophyll is fluorescent

b. The magnification has distorted the image

c. You’re not looking at the same structure in both microscopes

d. The stain masked the green color

e. None of the above

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Textbook Question

Calculate the total magnification of the nucleus of a cell being observed through a compound light microscope with a 10x ocular lens and an oil immersion lens.

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Textbook Question

Which type of microscope would be best to use to observe each of the following?

a. A stained bacterial smear

b. Unstained bacterial cells: the cells are small, and no detail is needed

c. Unstained live tissue when it is desirable to see some intracellular detail

d. A sample that emits light when illuminated with ultraviolet light

e. Intracellular detail of a cell that is 1μm long

f. Unstained live cells in which intracellular structures are shown in color

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