1
Q
What system of measurement is used to measure microorganisms?
A
The metric system.
2
Q
What is a major advantage of the metric system?
A
Units relate to each other by factors of 10.
3
Q
How many decimeters, centimeters, and millimeters are in 1 meter?
A
10 decimeters
100 centimeters
1000 millimeters
4
Q
Why is the U.S. system of measurement less convenient than the metric system?
A
It does not use easy conversions based on factors of 10.
5
Q
What units are commonly used to measure microorganisms?
A
Micrometers (µm) and nanometers (nm).
6
Q
What is a micrometer (µm) equal to in meters?
A
0.000001 m (10⁻⁶ m).
7
Q
What does the prefix “micro” mean?
A
Divided by 1 million (10⁶).
8
Q
What is a nanometer (nm) equal to in meters?
A
0.000000001 m (10⁻⁹ m).
9
Q
What unit was previously used to measure 10⁻¹⁰ meters?
A
Angstrom (Å).
10
Q
What has replaced the Angstrom unit?
A
Nanometers.
11
Q
What does the prefix kilo mean?
A
1000.
12
Q
How many meters are in 1 kilometer (km)?
A
1000 meters (10³ m).
13
Q
What is the standard unit of length in the metric system?
A
Meter (m).
14
Q
What does the prefix deci mean?
A
1/10.
15
Q
What does the prefix centi mean?
A
1/100.
16
Q
What does the prefix milli mean?
A
1/1000.
17
Q
What does the prefix micro mean?
A
1/1,000,000.
18
Q
What does the prefix nano mean?
A
1/1,000,000,000.
19
Q
What does the prefix pico mean?
A
1/1,000,000,000,000.
20
Q
How many nanometers are in 10 mm?
A
10,000,000 nanometers.
21
Q
What must you be able to diagram for Learning Objective 3-2?
A
The path of light through a compound microscope.
22
Q
What two terms must be defined for Learning Objective 3-3?
A
Total magnification and resolution.
23
Q
What type of microscope did van Leeuwenhoek use?
A
A simple microscope with one lens.
24
Q
What was van Leeuwenhoek’s maximum magnification?
A
300×.
25
Who is credited with making the first compound microscope?
Zaccharias Janssen.
26
Why were early compound microscopes ineffective?
Poor quality lenses prevented viewing bacteria.
27
Who developed a significantly better microscope around 1830?
Joseph Jackson Lister.
28
What is light microscopy?
The use of visible light to observe specimens.
29
What type of light does a compound light microscope use?
Visible light.
30
What forms the image in a compound light microscope?
A series of finely ground lenses.
31
What is the illuminator?
The light source of the microscope.
32
What is the function of the condenser?
Directs light rays through the specimen.
33
What are objective lenses?
Lenses closest to the specimen that magnify the image.
34
What is the ocular lens?
The eyepiece that further magnifies the image.
35
How is total magnification calculated?
Objective lens magnification × ocular lens magnification.
36
What magnification do most ocular lenses provide?
10×.
37
What is the total magnification using a 40× objective and 10× ocular?
400×.
38
What is the total magnification with oil immersion?
1000×.
39
What is resolution (resolving power)?
The ability to distinguish fine detail and structure.
40
What does resolution specifically refer to?
The ability to distinguish two points a specified distance apart.
41
How does wavelength affect resolution?
Shorter wavelength = greater resolution.
42
What is the smallest structure a compound light microscope can resolve?
About 0.2 µm.
43
What limits the magnification of compound light microscopes?
Resolution limits due to wavelength of light.
44
What are microscopes used for?
To magnify small objects.
45
Why does specimen size matter in microscopy?
It determines which microscope can be used effectively.
46
What do size bars on micrographs indicate?
The actual size of the specimen.
47
What does a red icon on a micrograph indicate?
The image has been artificially colorized.
48
What must specimens do to be seen clearly under a compound microscope?
Contrast sharply with their medium.
49
What is the refractive index?
A measure of the light-bending ability of a medium.
50
How is contrast increased in specimens?
By staining them to change refractive index.
51
What happens to light when it passes between materials with different refractive indices?
It bends (refracts).
52
Why is immersion oil used at high magnification?
To prevent loss of light rays and improve resolution.
53
Why does immersion oil improve image quality?
It has the same refractive index as glass.
54
What is brightfield microscopy?
A light microscopy technique that uses visible light and stained specimens viewed against a bright background.
55
What is a limitation of brightfield microscopy?
Poor contrast when specimens are unstained or very small.
56
What is darkfield microscopy?
A microscopy technique where light does not pass directly through the specimen, producing a bright specimen on a dark background.
57
What is darkfield microscopy used for?
Viewing live, unstained microorganisms.
58
How does darkfield microscopy compare to brightfield?
It provides better contrast for unstained specimens than brightfield.
59
What is phase-contrast microscopy?
A technique that enhances contrast by converting differences in refractive index into differences in brightness.
60
What is phase-contrast microscopy used for?
Observing live, unstained cells and internal structures.
61
How does phase-contrast microscopy compare to brightfield?
It allows visualization of unstained specimens that would be nearly invisible in brightfield.
62
What is differential interference contrast (DIC) microscopy?
A microscopy technique that uses polarized light to produce high-contrast images with a three-dimensional appearance.
63
What is DIC microscopy used for?
Viewing fine structural details in live, unstained specimens.
64
How does DIC compare to brightfield?
It provides sharper, more detailed images with apparent depth.
65
What is fluorescence microscopy?
A technique that uses fluorescent dyes and ultraviolet light to visualize specific structures.
66
What is fluorescence microscopy used for?
Identifying specific molecules or organisms labeled with fluorescent dyes.
67
How does fluorescence microscopy compare to brightfield?
It allows highly specific visualization rather than general cell structure.
68
What is confocal microscopy?
A fluorescence microscopy technique that produces sharp images by eliminating out-of-focus light.
69
What is confocal microscopy used for?
Producing detailed, three-dimensional images of specimens.
70
What is two-photon microscopy?
A fluorescence technique that uses long-wavelength light to excite fluorophores deep within specimens.
71
What is two-photon microscopy used for?
Imaging living tissues with reduced damage and deeper penetration.
72
What is scanning acoustic microscopy?
A microscopy technique that uses sound waves instead of light.
73
What is scanning acoustic microscopy used for?
Studying the physical properties of cells and tissues.
74
What type of energy does electron microscopy use?
Beams of electrons.
75
How does electron microscopy differ from light microscopy?
It uses electrons instead of light and has much higher resolution.
76
Why does electron microscopy have higher resolution?
Electrons have much shorter wavelengths than visible light.
77
Can live specimens be viewed with electron microscopy?
No, specimens must be fixed and placed in a vacuum.
78
What does a transmission electron microscope (TEM) do?
Passes electrons through a thin specimen.
79
What is TEM used for?
Viewing internal structures of cells in high detail.
80
What does a scanning electron microscope (SEM) do?
Scans electrons across the surface of a specimen.
81
What is SEM used for?
Viewing surface details and three-dimensional structure.
82
What are scanned-probe microscopes?
Microscopes that use a physical probe to scan a specimen’s surface.
83
What are scanned-probe microscopes used for?
Imaging surfaces at atomic or molecular resolution.
84
What microorganism is associated with peptic ulcers in the clinical case?
Helicobacter pylori.
85
Why is microscopy important in identifying H. pylori?
It allows visualization of bacteria associated with disease.
86
What measurement system is used to measure microorganisms?
The metric system.
87
What does the prefix kilo- mean?
1,000.
88
How many meters are in 1 kilometer (km)?
1,000 meters (10³ m).
89
What is the U.S. equivalent of 1 kilometer?
0.62 miles (1 mile = 1.61 km).
90
What is the meter (m)?
The standard unit of length in the metric system.
91
What is the U.S. equivalent of 1 meter?
39.37 inches, 3.28 feet, or 1.09 yards.
92
What does the prefix deci- mean?
1/10.
93
How many meters are in 1 decimeter (dm)?
0.1 meters (10⁻¹ m).
94
What is the U.S. equivalent of 1 decimeter?
3.94 inches.
95
What does the prefix centi- mean?
1/100.
96
How many meters are in 1 centimeter (cm)?
0.01 meters (10⁻² m).
97
What is the U.S. equivalent of 1 centimeter?
0.394 inches (1 inch = 2.54 cm).
98
What does the prefix milli- mean?
1/1,000.
99
How many meters are in 1 millimeter (mm)?
0.001 meters (10⁻³ m).
100
What does the prefix micro- mean?
1/1,000,000.
101
How many meters are in 1 micrometer (µm)?
0.000001 meters (10⁻⁶ m).
102
What unit is commonly used to measure bacteria?
Micrometers (µm).
103
What does the prefix nano- mean?
1/1,000,000,000.
104
How many meters are in 1 nanometer (nm)?
0.000000001 meters (10⁻⁹ m).
105
What structures are commonly measured in nanometers?
Viruses and molecules.
106
What does the prefix pico- mean?
1/1,000,000,000,000.
107
How many meters are in 1 picometer (pm)?
0.000000000001 meters (10⁻¹² m).
108
Which is larger: a micrometer or a nanometer?
A micrometer is larger.
109
Put these in order from largest to smallest: mm, µm, nm
mm > µm > nm.
110
Why are micrometers and nanometers important in microbiology?
Microorganisms and cellular structures are too small to measure in meters or centimeters.
111
Why is the metric system preferred in science?
Units are related by powers of 10, making conversions easier.
112
What is the path of light through a compound light microscope, in order?
Illuminator → condenser → specimen → objective lens → ocular lens (eyepiece).
113
What is the function of the illuminator in a compound microscope?
It is the light source that provides visible light.
114
What is the role of the condenser?
It focuses and directs light rays through the specimen.
115
What happens to light after it passes through the specimen?
It enters the objective lenses, where the image is magnified.
116
What does the ocular lens (eyepiece) do?
It magnifies the image again for viewing.
117
What is total magnification?
The product of the objective lens magnification multiplied by the ocular lens magnification.
118
How do you calculate total magnification?
Objective lens power × ocular lens power.
119
What is the typical magnification of the ocular lens?
10×.
120
What are common objective lens magnifications in microbiology?
10× (low power), 40× (high power), and 100× (oil immersion).
121
What are the total magnifications for each objective lens?
Low power: 100×; High power: 400×; Oil immersion: 1000×.
122
What is resolution (resolving power)?
The ability of lenses to distinguish fine detail and distinguish two points that are close together.
123
What does a resolving power of 0.4 nm mean?
Two points must be at least 0.4 nm apart to be seen as separate.
124
How does wavelength affect resolution?
Shorter wavelengths produce greater resolution.
125
What is the resolution limit of a compound light microscope?
About 0.2 µm.
126
Why is magnification limited in light microscopy?
Because resolution is limited by the wavelength of visible light.
127
What is brightfield microscopy best used for?
Viewing stained specimens using visible light.
128
What is darkfield microscopy used for?
Viewing live, unstained specimens that are difficult to see with brightfield microscopy.
129
How does darkfield differ from brightfield microscopy?
The specimen appears bright against a dark background instead of dark on a bright background.
130
What is phase-contrast microscopy used for?
Viewing live, unstained cells by enhancing differences in refractive index.
131
How does phase-contrast compare to brightfield microscopy?
It provides better contrast without staining.
132
What is differential interference contrast (DIC) microscopy used for?
Viewing live specimens with enhanced three-dimensional appearance.
133
How does DIC differ from brightfield microscopy?
It produces images with greater depth and contrast.
134
What is fluorescence microscopy used for?
Detecting specific structures using fluorescent dyes or labels.
135
How does fluorescence microscopy differ from brightfield microscopy?
It uses fluorescent light rather than visible light illumination.
136
What is confocal microscopy used for?
Producing sharp images of thick specimens by eliminating out-of-focus light.
137
How does confocal microscopy improve upon brightfield microscopy?
It provides clearer, more detailed images at specific depths.
138
What is two-photon microscopy used for?
Imaging living tissues with reduced damage and deeper penetration.
139
What is scanning acoustic microscopy used for?
Studying the mechanical properties of cells using sound waves.
140
What type of radiation does electron microscopy use?
Electron beams.
141
How does electron microscopy differ from light microscopy?
It uses electrons instead of visible light and has much greater resolution.
142
Why does electron microscopy have better resolution than light microscopy?
Electrons have much shorter wavelengths than visible light.
143
What is transmission electron microscopy (TEM) used for?
Viewing internal structures of cells at very high resolution.
144
What is scanning electron microscopy (SEM) used for?
Viewing surface details and three-dimensional images of specimens.
145
How does SEM differ from TEM?
SEM shows surface structure, while TEM shows internal structure.
146
What are scanned-probe microscopes used for?
Imaging surfaces at the atomic or molecular level.
147
What does staining mean in microbiology?
Coloring microorganisms with a dye that emphasizes certain structures.
148
Why must microorganisms be fixed before staining?
Fixing attaches microorganisms to the slide, kills them, preserves their natural structure, and prevents them from washing off during staining.
149
What is a smear?
A thin film of material containing microorganisms spread over the surface of a microscope slide.
150
How is a smear prepared before staining?
The smear is spread on the slide and allowed to air dry.
151
How are smears fixed in most staining procedures?
By passing the slide through a Bunsen burner flame several times (smear side up) or by covering the slide with methanol for 1 minute.
152
What happens if a smear is not fixed before staining?
The stain may wash the microorganisms off the slide.
153
What is done after the stain is applied?
The stain is washed off with water, and the slide is blotted with absorbent paper.
154
What are stains chemically composed of?
Salts composed of a positive ion and a negative ion, one of which is colored (the chromophore).
155
Where is the chromophore located in basic dyes?
In the cation (positive ion).
156
Where is the chromophore located in acidic dyes?
In the anion (negative ion).
157
Why do basic dyes stain bacterial cells?
Bacteria are slightly negatively charged, so the positively charged cation of a basic dye is attracted to the bacterial cell.
158
Why are acidic dyes generally not attracted to bacterial cells?
Their negatively charged ions are repelled by the negatively charged bacterial surface.
159
What is negative staining?
A staining technique where the background is stained, leaving the bacteria colorless.
160
Why does negative staining color the background instead of the cells?
Acidic dyes are repelled by the negatively charged bacterial surface.
161
What is negative staining useful for observing?
Overall cell shape, size, and capsules.
162
Why does negative staining minimize distortion of cells?
Fixing is not required and the cells do not absorb the stain.
163
What are examples of acidic dyes used in negative staining?
Eosin, acid fuchsin, and nigrosin.
164
What are the three main staining techniques used in microbiology?
Simple staining, differential staining, and special staining.
165
What is a simple stain?
An aqueous or alcohol solution of a single basic dye.
166
What is the primary purpose of a simple stain?
To highlight the entire microorganism so that cellular shapes and basic structures are visible.
167
How is a simple stain applied?
It is applied to a fixed smear for a set time, washed off, dried, and examined.
168
What is a mordant?
A chemical added to intensify staining.
169
What are two functions of a mordant?
To increase the affinity of a stain for a specimen and to coat structures (such as flagella) to make them thicker and easier to see.
170
What are common simple stains used in the laboratory?
Methylene blue, carbolfuchsin, crystal violet, and safranin.
171
Why doesn’t a negative stain color a cell?
Because the acidic dye’s negatively charged ions are repelled by the negatively charged bacterial surface, so the dye stains the background instead of the cell.
172
Why is fixing necessary for most staining procedures?
Fixing attaches microorganisms to the slide, kills them, preserves their natural structure with minimal distortion, and prevents them from being washed off during staining.
173
What is staining?
Coloring microorganisms with a dye that emphasizes certain structures so they can be observed under a microscope.
174
What is a smear?
A thin film of material containing microorganisms that is spread over the surface of a microscope slide and allowed to air dry before staining.
175
What is a chromophore?
The colored ion of a dye; it is the part of the stain responsible for its color.
176
What is an acidic dye?
A dye in which the chromophore is in the negatively charged ion (anion); it is repelled by negatively charged bacterial cells and stains the background.
177
What is a basic dye?
A dye in which the chromophore is in the positively charged ion (cation); it is attracted to the negatively charged bacterial cell and stains the cell.
178
What is negative staining?
A staining technique in which acidic dyes stain the background while the cells remain colorless, allowing cell shape, size, and capsules to be observed with minimal distortion.
179
What is a differential stain?
A staining technique that reacts differently with different kinds of bacteria, allowing them to be distinguished from one another.
180
Which two differential stains are most frequently used for bacteria?
The Gram stain and the acid-fast stain.
181
What is the purpose of the Gram stain?
To classify bacteria into two major groups: gram-positive and gram-negative.
182
Who developed the Gram stain and when?
Hans Christian Gram in 1884.
183
What is the primary stain used in the Gram stain, and what does it do?
Crystal violet; it stains all cells purple initially.
184
What is iodine’s role in the Gram stain?
Iodine acts as a mordant and combines with crystal violet to form the crystal violet–iodine (CV-I) complex.
185
What is the decolorizing agent in the Gram stain?
Alcohol or an alcohol-acetone solution.
186
What does the decolorizing step do?
It removes the purple stain from some bacteria but not others.
187
What is the counterstain used in the Gram stain, and what color is it?
Safranin; it is a basic red dye.
188
What color are gram-positive bacteria after Gram staining?
Purple.
189
What color are gram-negative bacteria after Gram staining?
Pink.
190
Why do gram-positive bacteria remain purple after alcohol washing?
They retain the crystal violet–iodine (CV-I) complex in their thick peptidoglycan cell wall.
191
Why do gram-negative bacteria lose the purple stain during decolorization?
Alcohol disrupts their outer lipopolysaccharide layer, allowing the CV-I complex to wash out of the thin peptidoglycan layer.
192
How does the cell wall of gram-positive bacteria differ from gram-negative bacteria?
Gram-positive bacteria have a thicker peptidoglycan layer and no outer lipopolysaccharide layer.
193
What unique cell wall feature do gram-negative bacteria have?
An outer layer containing lipopolysaccharides.
194
Why don’t crystal violet or iodine alone remain in the cell wall?
Each is water-soluble on its own and can drain out of the cell.
195
Why does the CV-I complex remain in gram-positive bacteria?
The CV-I complex is insoluble in water and becomes trapped in the thick peptidoglycan layer.
196
Why are Gram stain results not universally applicable?
Some bacteria stain poorly or not at all.
197
When is the Gram reaction most reliable?
When used on young, actively growing bacteria.
198
Why might Gram staining overestimate gram-negative bacteria in the intestine?
Because some gram-positive bacteria stain poorly, making gram-negative bacteria appear more numerous.
199
How does the Gram reaction help guide antibiotic treatment?
It provides information about bacterial cell wall structure, which influences antibiotic effectiveness.
200
Why are gram-positive bacteria generally more susceptible to penicillins and cephalosporins?
These antibiotics target the peptidoglycan cell wall, which is thicker in gram-positive bacteria.
201
Why are gram-negative bacteria generally more resistant to many antibiotics?
Their outer lipopolysaccharide layer prevents antibiotics from penetrating the cell.
202
What is the acid-fast stain used for?
To identify bacteria with waxy cell walls.
203
Which bacterial genus is identified using the acid-fast stain?
Mycobacterium.
204
Which diseases are associated with acid-fast bacteria?
Tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae).
205
What other genus can be identified using the acid-fast stain?
Nocardia.
206
What is the primary stain used in the acid-fast stain?
Carbolfuchsin (a red dye).
207
Why is heat applied during acid-fast staining?
Heat enhances penetration and retention of the dye in waxy cell walls.
208
What is the decolorizer used in acid-fast staining?
Acid-alcohol.
209
What counterstain is used in the acid-fast stain?
Methylene blue.
210
What color are acid-fast bacteria after staining?
Pink or red.
211
What color are non-acid-fast bacteria after staining?
Blue.
212
Why do acid-fast bacteria retain the red dye after acid-alcohol treatment?
Carbolfuchsin is more soluble in the lipid-rich, waxy cell walls than in acid-alcohol.
213
Why do non-acid-fast bacteria lose the red dye during decolorization?
Their cell walls lack lipid components, so the dye is rapidly removed.
214
Why is Mycobacterium tuberculosis easily identified by the acid-fast stain?
Because it has a waxy, lipid-rich cell wall that strongly retains carbolfuchsin even after acid-alcohol decolorization.
215
Why is the Gram stain important?
Because it classifies bacteria into two major groups—gram-positive and gram-negative—based on differences in their cell walls, and this information helps guide antibiotic treatment.
216
Which stain would be used to identify microbes in the genera Mycobacterium and Nocardia?
The acid-fast stain.
217
What is the purpose of the Gram stain?
To classify bacteria into gram-positive and gram-negative groups based on differences in their cell walls.
218
What is the primary stain used in the Gram stain, and what color is it?
Crystal violet, a purple dye.
219
Why is crystal violet called the primary stain?
Because it initially stains all bacterial cells purple.
220
What reagent is applied after crystal violet, and what is its function?
Iodine, which acts as a mordant to strengthen the crystal violet stain.
221
What color are both gram-positive and gram-negative bacteria after iodine is applied?
Dark violet or purple.
222
What is the decolorizing agent in the Gram stain?
Alcohol or alcohol-acetone.
223
What does the decolorizing step do?
It removes the purple stain from gram-negative cells but not from gram-positive cells.
224
What stain is applied after decolorization, and what is its role?
Safranin, a basic red dye that acts as a counterstain.
225
What color are gram-positive bacteria at the end of the Gram stain?
Purple.
226
What color are gram-negative bacteria at the end of the Gram stain?
Pink.
227
Why are gram-negative bacteria pink after Gram staining?
Because they lose the crystal violet during decolorization and are then stained by safranin.
228
Why are gram-positive bacteria not affected by the safranin counterstain?
Because they retain the original purple crystal violet stain.
229
In Figure 3.14, which cells are gram-positive and which are gram-negative?
The cocci (purple) are gram-positive, and the rods (pink) are gram-negative.
230
List the steps of the Gram stain in correct order.
Crystal violet (primary stain)
Iodine (mordant)
Alcohol or alcohol-acetone (decolorizer)
231
How can the Gram reaction be useful in prescribing antibiotic treatment?
Because gram-positive and gram-negative bacteria differ in their cell wall structure, they respond differently to antibiotics; gram-positive bacteria are generally more susceptible to penicillins and cephalosporins, while gram-negative bacteria are more resistant due to their outer lipopolysaccharide layer.
232
What are all the steps of the Gram stain, in order, including the role of each reagent?
1. Crystal violet – primary stain; stains all cells purple
2. Iodine – mordant; binds with crystal violet to strengthen the stain
3. Alcohol or alcohol-acetone – decolorizer; removes purple stain from gram-negative cells
4. Safranin – counterstain; stains gram-negative cells pink while gram-positive remain purple
233
What is the purpose of the Gram stain?
The Gram stain is a differential stain that classifies bacteria into gram-positive and gram-negative groups based on differences in their cell wall structure.
234
List all steps of the Gram stain in order (one card, full process).
Crystal violet (primary stain) is applied — stains all cells purple
Iodine (mordant) is applied — forms the crystal violet–iodine (CV-I) complex
Alcohol or alcohol-acetone (decolorizer) is applied — removes CV-I from some cells
Safranin (counterstain) is applied — stains decolorized cells pink
235
What is the role of crystal violet in the Gram stain?
Crystal violet is the primary stain that initially colors all bacterial cells purple.
236
What is the role of iodine in the Gram stain?
Iodine acts as a mordant, combining with crystal violet to form the CV-I complex, which is less soluble in water.
237
What is the role of alcohol (or alcohol-acetone)?
Alcohol is the decolorizing agent that removes the CV-I complex from gram-negative cells but not from gram-positive cells.
238
What is the role of safranin?
Safranin is the counterstain that colors gram-negative bacteria pink after they have been decolorized.
239
Why do gram-positive bacteria remain purple?
Gram-positive bacteria have a thick peptidoglycan layer that traps the CV-I complex, preventing it from being removed by alcohol.
240
Why do gram-negative bacteria lose the purple color?
Gram-negative bacteria have a thin peptidoglycan layer and an outer lipopolysaccharide layer that is disrupted by alcohol, allowing the CV-I complex to wash out.
241
What color are gram-positive and gram-negative cells after Gram staining?
Gram-positive: purple
Gram-negative: pink
242
Why is the Gram stain most consistent on young, growing bacteria?
Older cells may stain poorly or inconsistently because their cell walls degrade, making them easier to decolorize.
243
What does “gram-variable” mean?
Gram-variable cells are gram-positive bacteria that decolorize easily, causing Gram staining to overestimate gram-negative bacteria.
244
How can the Gram reaction help guide antibiotic treatment?
Gram-positive bacteria are usually more susceptible to penicillins and cephalosporins, while gram-negative bacteria are more resistant because antibiotics cannot easily penetrate their lipopolysaccharide layer.
245
Why is the Gram stain so useful? (3-9)
Because it quickly classifies bacteria into gram-positive or gram-negative groups, which provides valuable information for diagnosis and antibiotic selection.
246
Which stain identifies Mycobacterium and Nocardia? (3-10)
The acid-fast stain, because these bacteria have waxy cell walls that strongly retain the stain.
247
What is the purpose of the acid-fast stain?
To identify bacteria with waxy cell walls, especially Mycobacterium and Nocardia.
248
What dye is used in acid-fast staining?
Carbolfuchsin, a red dye.
249
Why is heat applied during acid-fast staining?
Heating enhances penetration and retention of carbolfuchsin into waxy cell walls.
250
What happens during decolorization in acid-fast staining?
Acid-alcohol removes the stain from non-acid-fast bacteria, but acid-fast bacteria retain the red dye.
251
What counterstain is used in acid-fast staining?
Methylene blue, which stains non-acid-fast cells blue.
252
What colors result from acid-fast staining?
Acid-fast bacteria: red or pink
Non-acid-fast bacteria: blue
253
Why is Mycobacterium tuberculosis easily identified by acid-fast staining?
Because it has a waxy cell wall rich in lipids, allowing it to retain carbolfuchsin even after acid-alcohol decolorization.
254
What are special stains used for?
To stain specific structures such as endospores, flagella, or capsules.
255
What is a capsule?
A gelatinous covering surrounding some bacteria.
256
Why are capsules important in medical microbiology?
Because the presence of a capsule is associated with virulence, or the ability to cause disease.
257
Why is capsule staining difficult?
Capsules are water-soluble and can be removed during rigorous washing.
258
How is capsule staining performed?
Bacteria are mixed with a negative stain (india ink or nigrosin) to stain the background, then the cells are stained with a simple stain like safranin.
259
Why do capsules appear as clear halos?
Because capsules do not accept most dyes, leaving an unstained area surrounding the stained cell.
260
Why is Mycobacterium tuberculosis easily identified by the acid-fast stain?
Mycobacterium tuberculosis has a waxy, lipid-rich cell wall that strongly retains the red dye carbolfuchsin, even after treatment with acid-alcohol. As a result, it remains red or pink, while non–acid-fast bacteria are decolorized and later stained blue.
261
Why is Mycobacterium tuberculosis easily identified by the acid-fast stain?
Because its cell wall contains high lipid content (mycolic acids) that retain carbolfuchsin even after acid-alcohol decolorization, so the cells remain pink/red.
262
What happens to non–acid-fast bacteria during acid-fast staining?
They lose carbolfuchsin during decolorization, become colorless, and then take up the methylene blue counterstain, appearing blue.
263
How is the Gram reaction useful in prescribing antibiotic treatment?
It rapidly distinguishes Gram-positive vs Gram-negative bacteria, which helps clinicians choose effective antibiotics based on differences in cell wall structure.
264
What color do Gram-positive bacteria appear and why?
Purple, because they retain crystal violet–iodine complexes due to their thick peptidoglycan cell wall.
265
What color do Gram-negative bacteria appear and why?
Pink, because they lose crystal violet during alcohol decolorization and are then stained by safranin.
266
List the steps of the Gram stain in order and the purpose of each.
Crystal violet – primary stain (stains all cells purple)
Iodine – mordant (forms crystal violet–iodine complex)
Alcohol – decolorizer (removes stain from Gram-negative cells)
Safranin – counterstain (stains Gram-negative cells pink)
267
What is the purpose of special stains?
To color and isolate specific structures, such as capsules, endospores, and flagella.
268
What is a bacterial capsule?
A gelatinous covering surrounding some bacteria.
269
Why is capsule staining clinically important?
Because the presence of a capsule is associated with virulence (ability to cause disease).
270
Why do capsules appear as clear halos in negative staining?
Capsules do not accept most dyes, so they remain unstained, appearing as halos against a dark background.
271
What dyes are commonly used in capsule (negative) staining?
India ink or nigrosin for the background + a simple stain (e.g., safranin) for the cell.
272
What is an endospore?
A highly resistant, dormant structure formed inside certain bacteria to survive adverse environmental conditions.
273
Why can’t endospores be stained with simple or Gram stains?
Because dyes cannot penetrate the endospore wall.
274
Which stain is used for endospores?
Schaeffer–Fulton endospore stain.
275
What is the primary stain used in endospore staining and why is heat applied?
Malachite green; heat helps the stain penetrate the endospore wall.
276
What is the counterstain in endospore staining?
Safranin.
277
How do stained endospores and vegetative cells appear?
Endospores: green
Vegetative cells: red or pink.
278
How do unstained endospores appear? How do stained endospores appear?
Unstained: clear, highly refractive structures
Stained: green endospores inside red/pink cells.
279
Why is flagella staining necessary?
Flagella are too thin to be seen with a light microscope without staining.
280
How does flagella staining work?
A mordant builds up layers of stain, increasing the diameter of flagella so they become visible.
281
Why are flagella useful diagnostically?
The number and arrangement of flagella help identify bacterial species.
282
What is a simple stain used for?
To determine cell shape and arrangement using a single basic dye.
283
What is a differential stain?
A stain used to distinguish between different kinds of bacteria.
284
What stain differentiates Gram-positive and Gram-negative bacteria?
Gram stain.
285
What stain is used to identify Mycobacterium and some Nocardia species?
Acid-fast stain.
286
What structures are identified using special stains?
Capsules, endospores, and flagella.
287
Of what value are capsules, endospores, and flagella to bacteria?
Capsules
Provide protection and increase virulence by helping bacteria avoid host immune defenses (e.g., resisting phagocytosis).
Endospores
Allow bacteria to survive harsh environmental conditions (heat, desiccation, chemicals) by entering a dormant, highly resistant state.
Flagella
Enable motility, allowing bacteria to move toward favorable environments and away from harmful ones.
288
How do unstained endospores appear?
Unstained endospores appear as clear, highly refractile (shiny) structures within bacterial cells.
289
How do stained endospores appear?
Stained endospores (Schaeffer–Fulton stain) appear green, while the vegetative cells appear red or pink.
290
What is a simple stain used for?
A simple stain is used to highlight microorganisms to determine cell shape and arrangement. It uses an aqueous or alcohol solution of a single basic dye (such as methylene blue, crystal violet, safranin, or carbolfuchsin). A mordant may sometimes be added to intensify staining.
291
What is a differential stain used for?
A differential stain is used to distinguish different kinds of bacteria based on how they react to multiple dyes.
292
What does the Gram stain classify and how do cells appear?
The Gram stain classifies bacteria into gram-positive and gram-negative groups. Gram-positive bacteria retain crystal violet and appear purple. Gram-negative bacteria lose crystal violet, are counterstained with safranin, and appear pink.
293
What is the acid-fast stain used for and what are the results?
The acid-fast stain is used to identify Mycobacterium species and some Nocardia. Acid-fast bacteria retain carbolfuchsin and appear pink or red after acid-alcohol treatment. Non–acid-fast bacteria lose carbolfuchsin and take up methylene blue, appearing blue.
294
What are special stains used for?
Special stains are used to color and isolate specific bacterial structures, such as capsules, endospores, and flagella, and are sometimes used as diagnostic aids.
295
What is a negative stain used to demonstrate?
A negative stain is used to demonstrate capsules. Because capsules do not accept most stains, they appear as unstained halos surrounding bacterial cells against a contrasting background.
296
How does the endospore stain work and what do results look like?
Malachite green is applied to a heat-fixed smear and penetrates endospores, staining them green. Safranin is then applied, staining the rest of the bacterial cell red or pink.
297
What is the purpose of a flagella stain?
A flagella stain is used to demonstrate the presence of flagella. A mordant builds up the diameter of the flagella until they are visible under the light microscope when stained with carbolfuchsin.
298
What does capsule staining demonstrate?
It demonstrates the presence of a capsule, which appears as a clear or light halo surrounding the stained bacterial cell against a dark or contrasting background.
299
Why do capsules appear unstained in capsule staining?
Capsules do not accept most biological dyes, so they remain unstained while the background and cell are stained.
300
What organism is shown in Figure 3.16a and what feature is visible?
Klebsiella pneumoniae is shown, and its capsule is visible as a light halo around the cell.
301
Why is capsule staining clinically important?
Capsules are associated with virulence, helping bacteria evade host defenses.
302
What does endospore staining detect?
It detects the presence of endospores, which are resistant, dormant structures formed by some bacteria.
303
What organism is shown in Figure 3.16b?
Bacillus anthracis.
304
What color are endospores and vegetative cells after Schaeffer–Fulton staining?
Endospores appear green; Vegetative cells appear red or pink.
305
Why is heat used during the endospore staining procedure?
Heat helps malachite green penetrate the tough endospore wall.
306
Why can’t endospores be stained with simple or Gram stains?
Because dyes cannot penetrate the endospore’s thick protective wall.
307
What does flagella staining demonstrate?
It demonstrates the presence and arrangement of flagella, which are too thin to be seen without staining.
308
What organism is shown in Figure 3.16c?
Bacillus pumilus.
309
Why do flagella appear thicker than normal in stained preparations?
A mordant builds up layers of stain on the flagella, increasing their diameter so they become visible.
310
Why are flagella important to bacteria?
Flagella provide motility, allowing bacteria to move toward nutrients or away from harmful environments.
311
What is the overall purpose of special stains shown in Figure 3.16?
To visualize specific bacterial structures (capsules, endospores, flagella) that are not easily seen with simple or Gram staining.
312
Of what value are capsules, endospores, and flagella to bacteria?
Capsules: protection and increased virulence; Endospores: survival in harsh environmental conditions; Flagella: movement and environmental adaptation.
313
What is a stain in microbiology?
A stain is a colored chemical (dye) that binds to cellular components to increase contrast and make microorganisms easier to see under a microscope.
314
What is the chromophore of a stain?
The chromophore is the colored, ionizable portion of a dye that gives the stain its color and allows it to bind to cellular structures.
315
What is the difference between an acidic dye and a basic dye?
Acidic dyes have a negatively charged chromophore and stain the background (repelled by the negatively charged cell surface). Basic dyes have a positively charged chromophore and stain the cell itself by binding to negatively charged cell components.
316
What is staining the background instead of the cell called?
Negative staining.
317
Why does negative staining leave the cell unstained?
Because the dye is repelled by the negatively charged cell surface, so the background is stained while the cell remains clear.
318
What is a simple stain?
A simple stain uses a single basic dye to color microorganisms so their size, shape, and arrangement can be observed under a light microscope.
319
What type of dye is used in a simple stain?
A basic dye is used in a simple stain.
320
Why are basic dyes used in simple staining?
Because basic dyes have a positively charged chromophore that binds to the negatively charged bacterial cell surface, allowing the cell to be stained.
321
What is a mordant?
A mordant is a chemical added during staining to increase the affinity of a dye for a specimen or to coat a structure, making it thicker and easier to see.
322
Why is a mordant used in simple staining?
A mordant may be used to hold the stain in place or to coat and enlarge the specimen, improving visibility under the microscope.
323
What does a simple stain allow you to determine?
It allows determination of cell shape, cell size, and cellular arrangement (such as chains or clusters).
324
What are differential stains?
Differential stains are staining techniques used to distinguish between different types of bacteria based on differences in their cell structure or chemical composition.
325
What is the purpose of differential stains?
To separate bacteria into distinct groups by causing them to stain differently.
326
Which stains are considered differential stains?
Gram stain
Acid-fast stain
327
How does the Gram stain distinguish between bacteria?
It differentiates bacteria into Gram-positive and Gram-negative based on differences in their cell wall structure, specifically the thickness of the peptidoglycan layer.
328
What does the acid-fast stain distinguish?
It distinguishes acid-fast bacteria from non-acid-fast bacteria based on the presence of waxy lipid-rich cell walls that retain the primary stain.
329
Why are differential stains important in microbiology and medicine?
Because they provide rapid identification of bacterial groups, which helps guide diagnosis and antibiotic selection.
330
What is the Gram stain?
The Gram stain is a differential staining technique that classifies bacteria into Gram-positive or Gram-negative groups.
331
What two groups does the Gram stain classify bacteria into?
Gram-positive bacteria
Gram-negative bacteria
332
Why is the Gram stain a differential stain?
Because it distinguishes between different kinds of bacteria by causing them to stain different colors based on cell wall structure.
333
What determines a bacterium’s Gram reaction?
Differences in the structure and composition of the bacterial cell wall, especially the thickness of the peptidoglycan layer.
334
Why is the Gram stain important?
It provides rapid classification of bacteria, which is useful for identification, diagnosis, and treatment decisions.
335
What is the key exam takeaway from the Gram stain slide?
It classifies bacteria
It is differential
It is a multi-step staining procedure
336
What is the primary stain in the Gram stain, and what color does it give cells?
The primary stain is crystal violet, and it stains both Gram-positive and Gram-negative cells purple.
337
What is iodine used for in the Gram stain?
Iodine acts as a mordant, forming a crystal violet–iodine complex that helps the dye bind to the cell wall.
338
After iodine is added, what color are Gram-positive and Gram-negative cells?
Both Gram-positive and Gram-negative cells are purple.
339
What is the decolorizing agent used in the Gram stain?
Alcohol-acetone.
340
What color are Gram-positive cells after alcohol-acetone is applied?
They remain purple.
341
What color are Gram-negative cells after alcohol-acetone is applied?
They become colorless.
342
What is the counterstain used in the Gram stain?
Safranin.
343
After safranin is added, what color are Gram-positive cells?
Purple (they retain crystal violet).
344
After safranin is added, what color are Gram-negative cells?
Red or pink.
345
At which step do Gram-positive and Gram-negative cells first differ in color?
During the decolorization step.
346
Why don’t Gram-positive cells turn pink after the counterstain?
Because they retain the crystal violet–iodine complex, which masks the safranin.
347
What is the acid-fast (Ziehl-Neelsen) stain used to detect?
It is used to detect tuberculosis- and leprosy-causing organisms of the Mycobacterium species.
348
What genus of bacteria is identified using the acid-fast stain?
Mycobacterium.
349
Why is a unique staining procedure required for acid-fast bacteria?
Because Mycobacterium species have a unique cell wall structure that does not stain well with standard staining methods.
350
What is the primary stain used in the acid-fast stain?
Carbolfuchsin
351
What color are acid-fast cells after the primary stain (carbolfuchsin)?
Red
352
What color are non–acid-fast cells after the primary stain (carbolfuchsin)?
Red
353
What is the decolorizing agent used in the acid-fast stain?
Acid-alcohol
354
What color are acid-fast cells after decolorization with acid-alcohol?
Red
355
What color are non–acid-fast cells after decolorization with acid-alcohol?
Colorless
356
What is the counterstain used in the acid-fast stain?
Methylene blue
357
What color are acid-fast cells after the counterstain is applied?
Red
358
What color are non–acid-fast cells after the counterstain is applied?
Blue
359
Acid-fast staining of a patient’s sputum is a rapid, reliable, and inexpensive method to diagnose tuberculosis. What color would bacterial cells appear if the patient has tuberculosis?
Red (pink/red rods)
360
What is a capsule stain?
A special stain used to demonstrate the presence of a capsule surrounding bacterial cells by staining the background, leaving the capsule as a clear halo.
361
What is an endospore stain?
A special stain used to detect endospores in bacteria by staining the endospores a different color than the vegetative cell.
362
What is a flagella stain?
A special stain used to demonstrate the presence of flagella by thickening them with a mordant so they become visible under a light microscope.
363
What is the primary stain used in the endospore (Schaeffer–Fulton) stain?
Malachite green, usually applied with heat.
364
Why is heat used with malachite green in the endospore stain?
Heat helps malachite green penetrate the tough endospore wall.
365
What is used to decolorize cells in the endospore stain?
Water.
366
What does water remove during the endospore stain?
Malachite green from vegetative cells but not from endospores.
367
What is the counterstain used in the endospore stain?
Safranin.
368
After the full endospore stain, what color are endospores and vegetative cells?
Endospores are green; vegetative cells are red or pink.
369
What stain is used in flagella staining?
Carbolfuchsin.
370
Why is a mordant used in flagella staining?
To build up layers of stain on the flagella, increasing their diameter so they become visible under a light microscope.
371
Why do flagella require a special staining technique?
Flagella are too thin to be seen with a light microscope unless their diameter is increased using a mordant and stain.
372
What structure is being visualized in flagella staining?
Bacterial flagella.
373
What is the main purpose of flagella staining?
To demonstrate the presence and arrangement of flagella on bacterial cells.
Microbiology
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