Microbiology Chapter 2 reading quiz

Which of the following is an advantage of electron microscopy?

Electron microscopy affords high resolution because the wavelength of electrons is small. However, the electron microscope is an expensive, sophisticated piece of equipment. Samples are nonliving and artifacts may arise because samples must be stained with an electron-dense material and sectioned.

Empty magnification occurs when

Empty magnification refers to an increase in an image size with no increase in resolution.

Fixation and stained preparation of specimens

Fixing and staining will kill a specimen but improves contrast and resolution. causes cells to stay in one place and improves contrast as well as usually imparts color, improving detection.

For a molecule to fluoresce, it must first ________ light.

For a molecule to fluoresce (emit light), it must first have an electron boosted to a higher energy level by absorption of a photon.

Which of the following bacterial shapes is believed to have evolved only once?

Spirochete Cocci (in any arrangement) and bacilli are common to both bacteria and archaea. Spirochetes are found in only one taxon of bacteria.

The path of light through a compound microscope is

The condenser consists of one or more lenses that collect a beam of rays from the light source onto a small area of the slide containing the specimen. The objective and ocular lenses collect light that has passed through the specimen.

The nosepiece of a compound microscope contains

The nosepiece contains one or more objective lenses of various magnifications.

Bacterial cultures appear cloudy because of the _______________ of light.

Small particles, like bacteria, scatter light and cause suspensions with more than 1 million cells per ml to look cloudy.

A physician informs a patient that the pathogen that has been causing their infection is a bacillus. When they look at a magnified image of cells from the infection, they see cells of many different shapes and sizes. What shape is the pathogen?

A bacillus is a rod-shaped bacterium.

A nanometer, nm, is equal to _____ meters.

A nanometer is equal to 10-9 meters.

Which technique does NOT rely on a beam of electrons or light interacting with the specimen?

Atomic force microscopy is a type of scanning probe microscopy (SPM). SPM methods differ from light and electron microscopy, in which the sample interacts with a beam of light or electrons. Instead, SPM methods measure a physical interaction, such as the "atomic force" between the sample and a sharp tip.

Eagles can see smaller objects than humans can because eagle eyes

Higher resolution allows the detection of smaller objects.

In bright-field microscopy

In bright-field microscopy, an object such as a bacterial cell is perceived as a dark silhouette blocking the passage of light. Details of the dark object are defined by the points of light surrounding its edge.

The characteristic of light that allows magnification by a microscope is

Magnification requires refraction, the bending of light as it passes through an object.

Viruses cannot be observed using a light microscope because viruses

Most viruses are smaller than 200 nm and are, therefore, unable to be resolved by light with wavelengths in the visible range (400-750 nm).

In microscopy, the term "resolution" refers to

Resolution is the smallest distance by which two objects can be separated and still be distinguished.

In a proper Gram stain, Gram-positive cells appear _______________ in color.

The primary stain in the Gram stain, crystal violet, is retained by Gram-positive cells.

The resolution of the human retina is

The resolution of the human eye is about 150 μm because that is how closely our photoreceptor cells are spaced.

Distortion when using a light microscope at the highest levels of magnification is decreased by

The use of immersion oil increases the amount of light collected by the objective lens. This increases the numeric aperture of the system and improves resolution.

Bright-field microscopy depends on the balance between light and contrast as well as the quality of the lenses used. A(n) __________

The use of multiple lenses in the compound microscope allows magnification but aberrations are minimized as the image passes through multiple lenses.

Wet mount preparation of specimens

Wet mounts allow for viewing cells that are still alive and in a "natural" state, but have low contrast as most cells are transparent and therefore have limited resolution. Water also absorbs light and converts it to heat.

Chemical imaging microscopy

visualizes the distribution of chemicals in a sample using mass spectrometry Fluorescence microscopy is coupled with nanoscale secondary ion mass spectrometry to map the distribution of chemicals with cells.

The three-dimensional structure of a virus particle would best be visualized using __________ microscopy.

cryo-electron By combining multiple images taken from different angles using a high-power electron beam, a three-dimensional image can be generated for symmetrical objects like virus particles.

Which technique requires prior crystallization of the specimen?

X-ray diffraction analysis, also known as X-ray crystallography, uses X-ray diffraction (interference patterns) from crystallized macromolecules to determine structure at atomic resolution.

To clearly observe a single bacterium's subcellular structure, the best tool is

a transmission electron microscope. Bacteria are too small for their subcellular structure to be seen clearly with the unaided human eye or even a light microscope. X-ray crystallography is best for deciphering the structure of proteins. Transmission electron microscopy is the best tool listed for resolving subcellular bacterial structure.

Simple stains

add dark color to all cells specifically, improving contrast Simple stains affect all cells the same, and increase contrast. Spore staining will affect the endospore coat only. Acid-fast staining affects mycolic acids. Gram-staining differentiates the types of cell wall.

A disadvantage of dark-field microscopy is that

any objects in the specimen will scatter light, leading to increased background noise.

Green fluorescent protein can be fused to other proteins to visualize them via fluorescence microscopy. If the emitted light is green, what color light may have been used as the excitation wavelength?

blue The emitted light is always of a longer wavelength (lower energy) than the absorbed light (excitation wavelength). Of the colors listed, only blue has a shorter wavelength than green.

Crystal violet

complexes with iodide ions, producing a positively charged molecule that stays more strongly within the peptidoglycan cell wall. The Gram stain procedure begins with crystal violet, which can stain any cell and penetrates the cell wall readily if it is exposed. It can be fixed within the layers of peptidoglycan using iodine, which will complex as an ion with the dye. When decolorizer is used, it removes only loosely bound crystal violet, which would include any on the outer membrane.

In order to resolve spherical bacteria 1 μm in diameter,

light of a wavelength less than 1 μm must be used. Objects can only be resolved by wavelengths of light smaller than the size of the object to be resolved.

Visualization techniques that rely on interference effects are

phase-contrast microscopy and X-ray crystallography. Both phase-contrast microscopy and X-ray crystallography produce images based on interference patterns generated by light interacting with the specimen. Bright-field microscopy relies on light absorption and dark-field microscopy relies on light scattered by the specimen. Scanning electron microscopy relies on electrons reflecting from the specimen.

Differential interference contrast microscopy uses

polarized light to produce multiple images that are superimposed and produce interference, highlighting differences in the refractive index of different parts of the cell, producing a false 3D effect. Unlike phase contrast microscopy, which uses refracted light and transmitted light to destructively interfere to make patterns of light and dark, differential interference contrast microscopy uses polarized light to produce an image of refracted light that is then superimposed on another beam of light where interference will occur. Even small differences in the refractive index are enough to produce patterns of light and dark, giving a false 3D effect.

The best microscope technique for observing the arrangement of different proteins of the surface of bacteria is

scanning electron microscopy. In scanning electron microscopy, electrons reflect off a specimen to reveal the contours of its three-dimensional surface. Transmission electron microscopy is best for observing the inside of a bacterium, not the surface. Phase-contrast microscopy, a form of light microscopy, does not have enough magnifying power to observe proteins, and X-ray crystallography is best for observing crystals of pure protein, not a mixture of proteins on a cell surface.

Dark-field microscopy depends on which characteristic of light?

scattering In dark-field microscopy, the scattering of light allows objects smaller than the wavelength of light to be viewed as bright images on a dark field.