Microbiology Chapter 3
__________ is the quality of the microscope that allows one to distinguish between two points that are very close together.
Resolution
Place these structures of the compound light microscope in the order that light passes through them on the way to the observer's eyes: (1) condenser, (2) ocular lens, (3) light source, (4) specimen, (5) objective lens.
light source → condenser → specimen → objective lens → ocular lens
What does refraction mean?
A measure of the light-bending ability of a medium
Which of the following types of microscopy is most useful for viewing the internal structures of unstained specimens? Phase-contrast Brightfield Confoca Electron Fluorescence
Phase-contrast
Resolution is very good when using an electron microscope because the __________ of the electron beam is much shorter than that of visible light.
wavelength
Acid-fast Mycobacteria differ from non-acid-fast bacteria by the presence of __________.
waxy material in their cell walls Microbiologists use the acid-fast stain to identify all bacteria in the genus Mycobacterium, including the two important pathogens Mycobacterium tuberculosis, the causative agent of tuberculosis, and Mycobacterium leprae, the causative agent of leprosy. This stain is also used to identify the pathogenic strains of the genus Nocardia.
In the Gram stain, what is the purpose of the decolorizer?
To selectively remove stain from cells
The __________ of a compound light microscope focuses light from the light source on the specimen.
condenser It is located right underneath the stage.
The acid-fast stain differs from the Gram stain procedure in the decolorizing agent used. What is the reagent used in an acid-fast stain, and why is it necessary?
Acid-alcohol is used to remove the primary stain from bacteria that do not have waxy cell walls. which removes the red stain from bacteria that are not acid-fast.
**Bacterial cells have an overall negative charge on their surface. If a bacterial smear is stained by a pink, acidic, water-soluble stain and then washed with water, which of the following describes the result that will be observed when viewing the stained slide under a microscope?
Unstained cells with no background Bacteria are slightly negatively charged at pH 7. Acidic dyes are not attracted to most types of bacteria because the dye's negative ions are repelled by the negatively charged bacterial surface, so the stain colors the background instead. However, once you wash the slide with water, you will remove the water-soluble stain causing you to likely observe unstained cells with no background.
In the capsule stain using India ink, capsules are distinguished as __________ surrounding cells.
clear halos Because of their chemical composition, capsules do not accept most biological dyes, such as safranin, and thus appear as clear halos surrounding each stained bacterial cell.
A student is observing an object that displays the internal detail on a virus at great magnification. Which type of microscope is used for this image?
Transmission electron microscope Objects smaller than about 0.2 μm, such as viruses or the internal structures of cells, must be examined with an electron microscope. TEM has a magnification of 10,000-100,000x and a resolving power of 10 pm.
When stained, bacteria in the genus Mycobacterium are __________.
acid fast Mycobacteria differ from non acid-fast bacteria by the presence of waxy material in their cell walls. Microbiologists use the acid-fast stain to identify all bacteria in the genus Mycobacterium.
The __________ charge of a basic dye adheres to the __________ charge of bacterial cell surfaces.
positive; negative Stains are salts composed of a positive and a negative ion, one of which is colored and is known as the chromophore.
Immersion oil is used with the 100 × objective lens of the compound light microscope to __________.
prevent the loss of resolution due to refraction
In the Gram stain, crystal violet is the __________.
primary stain because it is the first dye that is applied during the staining procedure. This purple stain imparts its color to all cells.
Which of the following is NOT visible through a compound light microscope? 0.02-µm ribosome 1.5-mm tick 5-µm nucleus 15-µm white blood cell
0.02-µm ribosome The white light used in a compound light microscope has a relatively long wavelength and cannot resolve structures smaller than about 0.2 μm.
If you use a compound light microscope, a 2-µm bacterial cell is best seen at which magnification?
1000× 1000x would be best to observe a 2-µm bacterial cell. This is commonly the highest magnification at which cells of this size could be observed with a compound light microscope. To increase resolution, immersion oil must be applied.
A microscopist is studying the surface pores on a bacterial specimen. Which microscope provides the best three-dimensional surface structure image? Scanning electron microscope Transmission electron microscope Fluorescence microscope Brightfield microscope
Scanning electron microscope SEM uses a beam of electrons instead of light. The electrons are reflected from the specimen; because of the shorter wavelength of electrons, structures smaller than 0.2 μm can be resolved. The image produced appears three-dimensional.
A student completes a Gram stain on a bacterial smear that has a mix of gram-negative and gram-positive organisms. The student stops the procedure without adding safranin. What would be the outcome of the Gram stain?
The gram-positive organisms would appear purple, and the gram-negative organisms would be colorless.
What is the correct order for the Gram stain process? crystal violet → safranin → decolorizer → iodine Unselected iodine → crystal violet → decolorizer → safranin Unselected malachite green → crystal violet → decolorizer → safranin
crystal violet → iodine → decolorizer → safranin
A(n) __________ is equal to 0.000001 m.
micrometer
__________ is an example of an acidic dye; __________ is an example of a basic dye.
nigrosin; crystal violet Bacteria are slightly negatively charged at pH 7. The colored cation in a basic dye is attracted to the negatively charged bacterial cell, while the negatively charged bacterial cell repels the colored anion in acidic dyes.
You are viewing a sputum smear that has been stained with an acid-fast stain. On this smear you see 5-micrometer-long red cells. You can conclude that __________.
there are acid-fast bacteria in the specimen. Acid-fast organisms would stain pink while non-acid fast organisms would stain blue with the counterstain methylene stain during the acid-fast staining procedure.
When using a 10× ocular lens and a 40× objective, the total magnification achieved is __________.
400
Which type of microscope uses a metal and diamond probe that is gently forced down along the surface of a specimen?
Atomic force
Which type of light microscopy is used to visualize stained specimens?
Brightfield
Through which structure would the light pass before it reaches the specimen affixed to the stage?
Condenser The condenser is located right underneath the stage. It contains lenses that direct the light rays through the specimen.
Before cells are stained, the specimen must be fixed. Fixing a specimen accomplishes all of the following EXCEPT which one? Fixation kills the cells that are being stained for observation. Unselected Fixation attaches cells to the slide so that they will not wash off during the staining process. Unselected Fixation preserves cellular structure in the natural state, with minimum distortion.
Fixation differentiates the various cells being observed. The cells would be differentiated by the application of different colored stains. Heat fixation prevents the stain from being washed off during the staining procedure.
__________ are too small to be seen with the light microscope without adding a mordant and carbolfuchsin. Ribosomes Unselected Nuclei Unselected Mitochondria
Flagella a mordant is used to build up the diameters of flagella until they become visible microscopically when stained with carbolfuchsin.
Which of the following types of microscopy can be used to specifically identify pathogens (such as Mycobacterium tuberculosis or rabies virus) in specimens? Fluorescence Brightfield Phase-contrast Darkfield Transmission electron
Fluorescence Fluorescent antibodies are added to a microscope slide containing an unknown bacterium. If the antigens are found on this unknown bacterium, the fluorescent antibodies bind to them, causing it to fluoresc
__________ is important in medical microbiology because the results help physicians select appropriate antibiotics for treatment. Simple staining Unselected The observation of morphology of bacteria Unselected The observation of the size of bacteria
Gram staining is one of the most useful staining procedures because it classifies bacteria into two large groups: gram-positive and gram-negative bacteria. This separation is important in medical microbiology because the results help physicians select appropriate antibiotics for treatment.
Your lab partner tells you the bacteria are moving in his Gram-stained smear. You can conclude that __________. the bacteria are gram-negative Unselected the bacteria have capsules Unselected he did a flagella stain, not a Gram stain Unselected the bacteria are acid-fast
He didn't properly fix the smear. Heat fixation not only kills cells, it also ensures that the cells are properly adhered to the slide to prevent the cells from being washed off during the staining procedure.
Which of the following steps is common to both the Gram stain and the acid-fast stain? Steam exposure for stain penetration Unselected Acid-alcohol for decolorizing Unselected The application of iodine as a mordant Unselected The use of methylene blue as a counterstain
Heat fixation of the smear before staining In addition to major differences in their procedures, Gram staining is one of the most useful staining procedures because it classifies bacteria into two large groups; gram-positive and gram-negative. Microbiologists use the acid fast stain to identify all bacteria in the genus Mycobacterium and the pathogenic strains of the genus Nocardia.
__________ acts to decrease refraction of light rays and thus increase resolution.
Immersion oil
A student is observing a Gram stain with the 100 × (oil immersion) lens. The image appears cloudy and dark. Which of the following actions will NOT increase the clarity of the image? -Lowering the stage to increase the distance between the object and the objective lens -Increasing the light using the diaphragm -Using lens paper to clean any dirt or old oil off the lenses -Adjusting the condenser to better focus light through the specimen
Lowering the stage to increase the distance between the object and the objective lens In order to increase clarity: the student could open the diaphragm to allow more light to enter the specimen; the student could also try and clean the objectives or adjust the height of the condenser.
Which of the following is the most convenient and appropriate unit for expressing the size of an average bacterial cell? Micrometer Millimeter Centimeter Nanometer
Micrometer Most bacteria are in the 1 - 10 micrometer size range. Microorganisms are measured in both micrometer and nanometer
What metric unit would be most appropriate for expressing the size of most viruses?
Nanometer Most viruses commonly range in size from 20 to 250 nm.
Which of the following kinds of microscopy would be most appropriate for viewing the shape and arrangement of pili or fimbriae on the surface of a bacterial cell?
Scanning electron microscopy
What is a diaphragm?
The diaphragm allows direct light to pass through the condenser, focusing light on the specimen and a diffraction plate in the objective lens.
In the Gram stain, what is the purpose of the counterstain?
To make gram-negative cells visible
How do you calculate total magnification?
We calculate the total magnification of a specimen by multiplying the objective lens magnification (power) by the ocular lens magnification (power).
The __________ is the part of the microscope that controls the amount of light entering the condenser.
diaphragm
In the Gram stain, crystal violet remains in gram-positive cells after treatment with alcohol because crystal violet--iodine (CV--I) complexes are trapped __________.
in thick layers of peptidoglycan
In the Gram stain, if the decolorizing step is deleted, gram-negative cells will appear __________ at the completion of the staining procedure.
purple
In a negative staining procedure, the bacterial cells would appear __________ when viewed under a microscope.
unstained in a colored background
Which of the following statements is NOT true for both TEM and SEM? -The specimen must be sectioned before viewing. -The illuminating source is an electron beam. -Both can be used to view specimens smaller than 0.2 micrometers. -Black-and-white images are produced. -The microscope is focused using electromagnetic lenses.
-The specimen must be sectioned before viewing.
Which of the following is NOT a type of a compound microscope? Electron microscope Brightfield Darkfield Fluorescence
Electron microscope A modern compound light microscope has a series of lenses and uses visible light as its source of illumination; with electron microscopy, a beam of electrons is used instead of light.
Which of the following staining procedures is correctly matched with the principal use for that particular stain? cid-fast stain for microbes with capsules Unselected Negative stain used to differentiate cell wall components Unselected Flagella stain used to detect capsules
Endospore stain to distinguish dormant structures formed during adverse environmental conditions. During this staining procedure, malachite green is applied to a heat-fixed smear of bacterial cells. The stain penetrates the endospores and stains them green. When safranin (red) is then applied, it stains the remainder of the cells red or pink.
Which of the following types of microscopy is used to study the structure of the slimy accumulation of bacteria on an IV catheter tip? Scanning acoustic Atomic force Scanning electron Phase-contrast Confocal
Scanning acoustic This type of microscopy consists of interpreting the action of a sound wave that was sent through a specimen.
In the Gram-stain procedure, a clear oval in the center of a cell could indicate __________.
the presence of an endospore
