Port Said University
Faculty of Nursing
Microbiology Exam of First Term 2013- 2014
Answer the following:
1- Mention types chemical agents used for sterilization (5 marks)
2- Mention phases of bacterial growth (5 marks)
3- Mention difference between dry heat and moist heat in sterilization
(5 marks)
4- Write on pathogenicity of Streptococci (5 marks)
5- Mention types of haemolysis (5 marks)
6- Mention diseases that may encountered with the gastrointestinal
tract (5 marks)
7- Write on EPEC, ETEC and EIEC (5 marks)
8- Write on pathogenicity of Neisseria sp (5 marks)
---------------------------------------------------------------------------
Home
Laboratory
Studies
Recordkeeping,
Writing,
& Data Analysis
Overview
Overview
Microscope studies Keeping a lab
Flagella
notebook
experiment
Writing research
Laboratory math
papers
Blood fractionation Dimensions & units
Gel electrophoresis Using figures (graphs)
Protein gel
Examples of graphs
analysis
Experimental error
Mitochondria
Representing error
Concepts/ theory Applying statistics








overview;
types; bright
field
microscopy
dark field
optics
phase contrast
oil immersion
differential
interference
contrast
measuring
using a
counting
chamber
wet (Vaseline)
mount
Laboratory
Methods
Overview
Principles of
microscopy
Solutions & dilutions
Protein assays
Spectrophotometry
Fractionation &
centrifugation
Radioisotopes and
detection
Dark Field Viewing
Dark field optics are a low cost alternative to phase contrast optics. The
contrast and resolution obtained with inexpensive dark field equipment
may be superior to what you have with student grade phase contrast
equipment. It is surprising that few manufacturers and vendors promote
the use of dark field optics.
A dilute suspension of yeast cells makes a good practice specimen for
dark field optics, particularly when cultured with living Paramecium.
Principle
To view a specimen in dark field, an opaque disc is placed underneath
the condenser lens, so that only light that is scattered by objects on the
slide can reach the eye (figure 2). Instead of coming up through the
specimen, the light is reflected by particles on the slide. Everything is
visible regardless of color, usually bright white against a dark
background. Pigmented objects are often seen in "false colors," that is,
the reflected light is of a color different than the color of the object.
Better resolution can be obtained using dark field as opposed to bright
field viewing.
You don't need sophisticated equipment to get a dark field effect,
although the effect is most dramatic when the occulting disk is built into
the condenser itself. You do need a higher intensity light, since you are
seeing only reflected light. At low magnification (up to 100x) any decent
optical instrument can be set up so that light is reflected toward the
viewer rather than passing through the object directly toward the viewer.
To set up a dissecting microscope for "dark field" viewing, the specimen
should be placed over an opening so that light reflects only from surfaces
between cover slip and slide, not from a surface beneath the slide. You
may need to make a stand to hold the slide. The surface beneath the
opening should be a flat black. Turn off any built-in illuminator. Aim a
high-intensity light source toward the specimen at an angle, from the top
or side through a glass dish or jar.
With a compound microscope, dark field is obtained by placing an
occulting disk in the light path between source and condenser. A cheap
set of occulting disks can be prepared by cutting circular pieces of black
electrical tape ranging from dime-size up to a diameter that equals the
width of the slide, and sticking them to the slide in a row. The circles
should be spaced well apart. A specimen is placed on the microscope
stage as usual, and the illumination should be made as uniform as
possible. If there is an aperture diaphragm in the condenser (contrast
lever), it should be opened up wide. After focusing at low power, the
slide with occulting disks is placed in the light path between source and
condenser, bringing it as close to the bottom of the condenser as it will
go.
I would start with the largest disk, sliding it around until it is directly in
the center of the light path. Increasing the illumination should then
produce a good dark field effect. To optimize, first try stopping down the
field diaphragm to get the best contrast between background and
specimen. Try to match the size of the occulting disk to the field
diameter, so that the edge of the disk is just outside the field of veiw smaller disks are appropriate for higher power objectives. Vertically, the
disk should be a close to the condenser as possible, to make the contrast
the greatest. On microscopes with built-in dark field equipment, the view
is so impressive because the occulting disk is built into the condenser very close and focused. After testing the set-up this way, a stand might
be rigged to fit under the microscope, so the slide can be placed in
position without holding it. Something that 'grabs' the condenser and
supports the occulting disks would be ideal. The less the students have to
mess with, the better.
I set this up on the crummiest little piece of garbage microscope I could
find, and it looked very good. A relatively new student-model
microscope should give a much better effect.
Suspensions of cells and samples of pond water look spectacular in dark
field. While specimens may look washed out and lack detail in bright
field, protists, metazoans, cell suspensions, algae, and other microscopic
organisms are clearly distinguished and their details show up well. At
100x you can readily see bacteria, even distinguish some structure (rods,
curved rods, spirals, or cocci) and movement. Non-motile bacteria look
like vibrating bright dots against a dark background. Motile bacteria can
be seen moving in a definite direction, sometimes remarkably fast. In
pond water samples you may find Spirillum volutans, a very large (up to
0.5 mm) motile spiral bacterium.
When to use dark field illumination
Dark field illumination is most readily set up at low magnifications (up
to 100x), although it can be used with any dry objective lens. Any time
you wish to view everything in a liquid sample, debris and all, dark field
is best. Even tiny dust particles are obvious. Dark field is especially
useful for finding cells in suspension. Dark field makes it easy to obtain
the correct focal plane at low magnification for small, low contrast
specimens. Use dark field for




Initial examination of suspensions of cells such as yeast, bacteria,
small protists, or cell and tissue fractions including cheek
epithelial cells, chloroplasts, mitochondria, even blood cells
(small diameter of pigmented cells makes it tricky to find them
sometimes despite the color).
Initial survey and observation at low powers of pond water
samples, hay or soil infusions, purchased protist or metazoan
cultures.
Examination of lightly stained prepared slides. ? Initial location
of any specimen of very small size for later viewing at higher
power.
Determination of motility in cultures
Copyright and Intended Use
Visitors: to ensure that your message is not mistaken for SPAM, please include the acronym "Bios211" in
the subject line of e-mail communications
Created by David R. Caprette (caprette@rice.edu), Rice University 11 MAY 00
Updated 10 Aug 12