Chapter 3 - Methods of Microbiology Flashcards
What microscope type is most commonly used for observing microbes?
A compound light microscope
In the compound microscope, the objective lens forms a magnified _____ _________ inside the tube of the microscope.
Real image
What lens of the compound microscope forms a magnified real image?
The objective lens
The ____________ lens magnifies the real image produced by the objective lens.
Ocular lens
Does the ocular lens produce a image?
No, it produces the illusion of an image, which we call a virtual image
The ultimate magnification of a compound microscope is the product of the individual magnifications produced by which two lenses?
The objective and ocular lenses
Which lens of the compound microscope do you look through?
The ocular lens
Resolution of the light microscope is normally limited to about ____ µm.
0.2 µm
What is resolution?
Resolution is the ability to distinguish between two closely spaced objects; it is the smallest resolvable distance between two objects
What is the limit of resolution (d)?
The distance separating the centers of two dots that can just be barely distinguished as separate
What two characteristics relate to the limit of resolution?
- The wavelength of the light being used (λ)
- The numerical aperture of the objective lens (NA)
What equation demonstrates the relationship between the limit of resolution, the wavelength of light used, and the numerical aperture?
d = 0.5λ/NA
What is the numerical aperature defined as?
The range of angles over which the system can accept or emit light
The numerical aperture is a function of two physical properties of the objective lens: _____________ and __________ __________.
- Aperture
- Working distance
What is the working distance of an objective lens?
How close to a specimen the objective lens must come for the specimen to be in focus
How is the numerical aperture quantitatively determined?
NA = n sinα
In the following equation, what does n represent?
NA = n sinα
The refractive index of the material in between the lens and the specimen, usually air
In the following equation, what does α represent?
NA = n sinα
α is one-half the objective’s opening angle
Because the physical properties of each lens are fixed and cannot be altered, the only way to improve resolution with a given lens is to increase what?
The refractive index of the material between the lens and specimen
A shorter _______ would increase resolution but hurt the eyes.
Wavelength
How does oil help with resolution of a microscope?
Oil has a higher refractive index than air, and thus it increases the resolution of an image
____________________ _____________________ is used for observing stained cells.
Brightfield microscopy
Under compound light microscopy, why does the specimen appear darker than the surroundings?
Because the specimen has absorbed some of the light
When a specimen is very small, like a microbe, it appears nearly translucent. Why is this?
Microbes are so small that they absorb very little light
When is brightfield microscopy used today?
When used with differential staining
__________________ ______________ uses dyes that stain some microbes but not others.
Differential staining
Why is staining not commonly used with microbes?
Staining often kills microbes and prevents the ability to see living microscopes
What type of staining is most commonly used?
The Gram stain
Due to differences in the thickness of a _____________________ layer in the cell membrane between Gram positive and Gram negative bacteria, Gram positive bacteria retain crystal violet stain during the decolorization process, while Gram negative bacteria lose the crystal violet stain and are instead stained by the safranin in the final staining process
Peptidoglycan
Why do Gram-positive bacteria retain the crystal violet stain?
Because they have thicker peptidoglycan layers
Why do Gram-negative cells fail to retain the crystal violet stain and stain later with the safranin?
Because they have slimmer or non-existent peptidoglycan layers
What type of microscopy can resolve very small objects?
Darkfield microscopy
How does darkfield microscopy work?
A condenser is set such that it illuminates the specimen with a cone of light; the specimen scatter somes of the light, which enters the objective lens, thereby illuminating the specimen while keeping the rest of the field dark
Extremely small objects up to ____ nm can be seen utilizing darkfield microscopy.
20 nm
What component of a microscopy limits the light scattering that enters the objective?
The iris diaphragm
Because the iris diaphragm can affect light, a stained sample can be misread using darkfield microscopy. Thus, caution should be used with stained samples, using the “quick and dirty” method. What are the three steps to this method?
- Focus on the stained preparation with iris wide open
- Close the iris until the field just begins to darken
- Repeat for each objective lens
What type of microscopy is routinely used for observing live microbial cells?
Phase-contrast microscopy
Phase-constrast microscopy is achieved by modifying two microscope parts. What are they?
- Condenser
- Objective lens
How is the condenser modified in phase-contrast microscopy?
The addition of an annular diaphragm
How is the objective lens altered in phase-contrast microscopy?
Addition of a phase plate
Like darkfield microscopy, phase-contrast microscopy illuminates a specium with a cone of light because the condenser has a ____________ _____________, a transparent ring in an otherwise opaque plate.
Annular diaphragm
What is the purpose of the annular diaphragm in phase-constrast microscopy?
To bend the light into a hollow cone with the specimen at its apex
What is the purpose of the phase plate in phase-contrast microscopy?
The phase plate has a ring of tinted glass that is sized and positioned such that all of the light in the illuminating cone goes through it, dimming the light by more than 70%
Although most of the light in phase-contrast microscopy passes through the specimen, some of the light is ______________. This light is not dimmed because most misses the tinted phase ring; however, the rest of the phase plate is make of thicker glass, which shifts the phase of the deflected light by one half wavelentgh, reducing the brightness of the object and contributing to contrast.
Deflected
How does phase-contrast microscopy generally work?
The _________________ allows more light to enter an objective.
Condenser
What is the working aperture of a condenser?
The sum of aperture angles of the objective and the condenser
What is a phase shift?
When light enters a dense medium, its velocity decreases; as its velocity decreases, there is simultaneously the formation of destructive interference, decreasing the amplitude and reducing the intensity of light
_________ ___________ _________________ is an optical microscopy technique that converts phase shifts in light passing through a transparent specimen to brightness changes in the image
Phase-shift microscopy
What is the halo effect?
The halo effect describes the appearance of a bright edge for positive phase contrast or a dark edge for negative phase contrast around large objects
____________________ microscopy can locate specific molecules within cells.
Flourescence
What is the purpose of an iris diaphragm?
It limits the light scattering that enters the objective of a microscope, making it easier to see clear samples, like live cells
Since an iris diaphragm affects light, it can cause a ________________ sample to be misread; thus caution should be used with these types of samples.
Stained
What are the three steps to the quick and dirty method for looking at stained methods?
- Focus on the stained preparation with the iris wide open
- Close the iris until the field just begins to darken
- Repeat for each objective lens
What type of microscopy is routinely used for observing live microbial cells?
Phase-constrast
How is phase-contrast microscopy achieved?
Via two modifications to the condenser and the objective lens