Microbiology: An Introduction, 13e (Tortora et al.)
Chapter 3 Observing Microorganisms Through a Microscope
3.1 Multiple-Choice Questions
1) Which of the following is NOT equal to 1 mm?
A) 0.001 m
B) 1,000,000 nm
C) 0.1 cm
D) 100 μm
E) 1,000.000.000 pm
2) What structure does light pass through after leaving the condenser in a compound light
microscope?
A) ocular lens
B) objective lens
C) specimen
D) illuminator
3) Which of the following pairs is mismatched?
A) nigrosin – negative stain
B) methylene blue – simple stain
C) acidic dye – capsule stain
D) basic dye – negative stain
E) crystal violet – simple stain
4) Which of the following places the steps of the Gram stain in the correct order?
1-Alcohol-acetone
2-Crystal violet
3-Safranin
4-Iodine
A) 1-2-3-4
B) 2-1-4-3
C) 2-4-1-3
D) 4-3-2-1
E) 1-3-2-4
5) Which of the following pairs is mismatched?
A) alcohol-acetone – decolorizer
B) crystal violet – basic dye
C) safranin – acid dye
D) iodine – mordant
E) carbolfuchsin – basic dye
6) Which of the following is NOT true regarding the acid-fast stain?
A) It is used to identify members of the genus Mycobacterium.
B) Acid-fast cells retain the primary dye after treatment with acid-alcohol.
C) If cells are acid-fast, they are gram-negative.
D) Acid-fast cells appear red in a completed acid-fast stain.
E) Non-acid-fast microbes appear blue in a completed acid-fast stain.
7) The purpose of a mordant in the Gram stain is to
A) remove the simple stain.
B) make the bacterial cells larger.
C) make the flagella visible.
D) prevent the crystal violet from leaving the cells.
E) make gram-negative cells visible.
8) Which of the following places the steps in the correct sequence?
1-Staining
2-Making a smear
3-Fixing
A) 1-2-3
B) 3-2-1
C) 2-3-1
D) 1-3-2
E) The order is unimportant.
9) The negative stain is used to
A) visualize endospores.
B) determine Gram reaction.
C) determine flagella arrangement.
D) visualize fimbriae.
E) determine cell size.
10) Simple staining is often necessary to improve contrast in which microscope?
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
11) Which microscope is used to see internal structures of cells in a natural state?
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
12) Which microscope uses visible light?
A) confocal microscope
B) differential interference contrast microscope
C) fluorescence microscope
D) scanning acoustic microscope
E) scanning electron microscope
13) Which microscope achieves the highest magnification and greatest resolution?
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
14) In using this microscope, the observer does NOT look directly at an image through a lens.
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
15) This microscope produces an image of a light cell against a dark background; internal
structures are NOT visible.
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
16) A virus measures 100 nm in length. What is its length in μm?
A) 10 μm
B) 1 μm
C) 0.1 μm
D) 0.01 μm
E) 0.001 μm
17) Which of the following is never useful for observing living cells?
A) phase-contrast microscope
B) darkfield microscope
C) scanning acoustic microscope
D) scanning electron microscope
E) brightfield microscope
18) A microorganism measures 5 μm in length. Its length in mm would be
A) 500 mm.
B) 50 mm.
C) 0.5 mm.
D) 0.05 mm.
E) 0.005 mm.
19) Which of the following correctly traces the path of light through the compound microscope?
A) light source; condenser; specimen; objective lens; ocular lens
B) condenser; light source; specimen; ocular lens; objective lens
C) light source; specimen; condenser; objective lens; ocular lens
D) condenser; light source; specimen; objective lens; ocular lens
E) light source; condenser; objective lens; specimen; ocular lens
20) Figure 3.1
In Figure 3.1, line “c” points to the microscope’s
A) illuminator.
B) condenser.
C) ocular lens.
D) objective lens.
21) Which microscope can be used to visualize DNA or botulinum toxin?
A) compound light microscope
B) phase-contrast microscope
C) scanning tunneling microscope
D) confocal microscope
E) scanning electron microscope
22) Which microscope is used to observe a specimen that emits light when illuminated with an
ultraviolet light?
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
23) Which microscope is most useful for visualizing a biofilm?
A) compound light microscope
B) phase-contrast microscope
C) fluorescence microscope
D) scanning acoustic microscope
E) transmission electron microscope
24) Which microscope takes advantage of differences in the refractive indexes of cell structures?
A) compound light microscope
B) phase-contrast microscope
C) darkfield microscope
D) fluorescence microscope
E) electron microscope
25) You are performing a Gram stain on gram-positive bacteria and you stop after the addition of
the first dye. What is the appearance of the bacteria at this point?
A) purple
B) red
C) colorless
D) brown
26) You are performing a Gram stain on gram-negative bacteria and you stop after the addition
of the mordant. What is the appearance of the bacteria at this point?
A) purple
B) red
C) colorless
D) brown
27) You are performing a Gram stain on gram-negative bacteria and you stop after the
decolorizer step. What is the appearance of the bacteria at this point?
A) purple
B) red
C) colorless
D) brown
28) You are performing a Gram stain on gram-positive bacteria and you stop after the addition of
the counterstain. What is the appearance of the bacteria at this point?
A) purple
B) red
C) colorless
D) brown
29) Which type of stain is most useful in helping clinicians to decide which antibiotic to
prescribe for a bacterial infection?
A) negative stain
B) simple stain
C) Gram stain
D) endospore stain
E) flagella stain
30) Which of the following pairs is mismatched?
A) confocal microscope – produces a three-dimensional image
B) darkfield microscope – uses visible light
C) fluorescence microscope – uses a fluorescent light
D) scanning electron microscope – produces a three-dimensional image
E) scanning tunneling microscope – allows visualization of atoms
31) What is the total magnification of a specimen viewed with a 10x ocular lens and a 45x
objective lens?
A) 4.5x
B) 10x
C) 45x
D) 100x
E) 450x
32) You are studying a cell structure that is approximately 100 nm in size. Which of the
following provides the greatest magnification you can use to see this structure?
A) brightfield microscope
B) darkfield microscope
C) transmission electron microscope
D) phase-contrast microscope
E) scanning electron microscope
33) Which microscope uses two beams of light to produce a three-dimensional color image?
A) fluorescence microscope
B) phase-contrast microscope
C) darkfield microscope
D) DIC microscope
E) electron microscope
34) Which microscope is used to see intracellular detail in a living cell?
A) fluorescence microscope
B) two-photon microscope
C) atomic force microscope
D) transmission electron microscope
E) brightfield microscope
35) Which microscope is used to observe viruses and the internal structure of thinly sectioned
cells?
A) transmission electron microscope
B) darkfield microscope
C) fluorescence microscope
D) brightfield microscope
E) scanning electron microscope
36) Which microscope is used to see detail of a 300-nm virus?
A) fluorescence microscope
B) phase-contrast microscope
C) darkfield microscope
D) DIC microscope
E) electron microscope
37) Assume you stain Bacillus by applying malachite green with heat and then counterstaining
with safranin. Through the microscope, the green structures are
A) cell walls.
B) capsules.
C) endospores.
D) flagella.
E) The answer cannot be determined.
38) Which step in the Gram stain is the critical step in differentiating gram-positive cells from
gram-negative cells?
A) safranin
B) alcohol-acetone
C) iodine
D) crystal violet
39) You find colorless areas in cells in a Gram-stained smear. What should you do next?
A) an acid-fast stain
B) a flagella stain
C) a capsule stain
D) an endospore stain
E) a simple stain
40) Which microscope is best used for observing the surfaces of intact cells and viruses?
A) phase-contrast microscope
B) darkfield microscope
C) fluorescence microscope
D) brightfield microscope
E) scanning electron microscope
41) Bacterial smears are fixed before staining to
A) affix the cells to the slide.
B) make their walls permeable.
C) accept stain.
D) make the cells visible.
42) The resolution of a microscope can be improved by changing the
A) condenser.
B) fine adjustment.
C) wavelength of light.
D) diaphragm.
E) coarse adjustment.
43) A student is looking at a bacterial specimen using the oil immersion lens, but has forgotten to
put immersion oil on the slide. The specimen will appear
A) smaller than it would if immersion oil was used.
B) larger than it would if immersion oil was used.
C) somewhat fuzzy and have poor resolution.
D) the same as it would if the immersion oil was used.
E) to have no color.
44) The purpose of the ocular lens is to
A) improve resolution.
B) magnify the image from the objective lens.
C) decrease the refractive index.
D) increase the light.
E) decrease the light.
45) In microscopy, the term resolution
A) refers to the ability to distinguish fine structure and detail in a specimen.
B) is the same as the total magnification of a specimen.
C) is improved when longer wavelengths of light are employed.
D) is only observed in stained specimens.
E) refers to magnification when using the electron microscope.
46) If a patient had a bacterial infection that was not syphilis but was tested for syphilis using the
FTA-ABS test, what would the technician observe?
A) fluorescent cells on a dark background.
B) no fluorescence in the patient sample.
C) fluorescence of a different color than expected.
D) three-dimensional host tissue cells with organelles clearly visible.
E) brightly colored three-dimensional bacterial cells.
47) An acidic dye
A) has a positively charged chromophore.
B) has a negatively charged chromophore.
C) is used to stain the background in a negative stain.
D) is used to stain nuclei.
E) has a negatively charged chromophore and is used to stain the background in a negative stain.
48) Immersion oil is necessary when using the 100x objective on a compound light microscope
A) to lubricate the objective.
B) to prevent the slide from scratching the lens.
C) otherwise the image will be fuzzy.
D) otherwise too much light is lost to refraction.
E) otherwise too much light is lost to refraction and the image will be fuzzy.
49) Which demonstrates better resolution?
A) measuring two bacteria arranged end to end as individually having a length of 2μm
B) measuring two yeast cells as individually having a diameter of 10μm
C) observing two adjacent bacteria as individually having a width of 0.3μm
D) observing a clump of algae 0.5mm by 0.75 mm
E) measuring a flea as 1.2mm at the longest point
50) Which two staining techniques employ a mordant?
A) Gram stain; Flagella stain.
B) Gram stain; Acid-fast stain.
C) Simple stain; Acid-fast stain.
D) Capsule stain; Flagella stain.
E) Gram stain; Capsule stain.
3.2 True/False Questions
1) The counterstain used in the Gram stain is a basic dye.
2) In a completed Gram stain, gram-negative bacteria are colorless.
3) In a completed Gram stain, gram-positive bacteria are purple.
4) If acid-fast bacteria are stained with the Gram stain, they will stain gram-negative.
5) The limit of resolution of the compound microscope illuminated with visible light is
approximately 0.2 μm.
6) Both phase-contrast microscopy and differential interference contrast microscopy are used to
view the internal structures of cells without staining.
7) Cells viewed in darkfield microscopy appear similar to those stained with the negative stain.
8) The capsules and flagella of bacteria can be observed in gram-stained smears.
9) The greater resolution of the electron microscope compared to the compound microscope is
due to the longer wavelengths of the electrons used to examine specimens.
10) Scanned probe microscopy is used to examine fine detail of molecular complexes, such as
blood clots, or molecules, such as DNA.
3.3 Essay Questions
1) In 1877, Robert Koch thought preparing permanently stained slides would be valuable. Why
was his assessment correct?
2) In 1884, Hans Christian Gram described a method of staining bacterial cells while not staining
surrounding animal tissues. However, he thought that the staining method he developed was
faulty because not all bacteria stained. In a letter to the editor of the journal in which Gram
published his findings, write your response to Gram’s concern.
3) You are working in a clinical laboratory and you need to examine an unstained urine sample
for the presence of bacteria. What type of light microscope should you use to observe this
specimen? Explain your answer.
4) Compare and contrast the Gram stain and the acid-fast stain.
5) How are brightfield, darkfield, phase-contrast and fluorescence microscopy similar?