metric unit in which the sizes of bacteria and protozoa are usually expressed
micrometer
is one millionth of a meter
1 µm in diameter
A typical spherical bacterium (coccus) is approximately
1 µm x wide 3 µm long
A typical rod-shaped bacterium (bacillus) is approximately
nanometers
The sizes of viruses are expressed in terms of
nanometer
is equal to one billionth of a meter
10 to 300 nm
Most of the viruses that cause human diseases range in size from
Ebola virus
a cause of viral hemorrhagic fever
Ebola viruses
can be as long as 1,000 nm (1 µm)
ocular micrometer
When using a microscope, the sizes of microorganisms are measured using this
microscope
is an optical instrument that is used to observe tiny objects, objects so small that they cannot be seen with the unaided human eye
the resolving power or resolution
Each optical instrument has a limit as to what can be seen using that instrument
0.2 mm
The resolving power of the unaided human eye is approximately
simple microscope
a microscope that contains only one magnifying lens
magnifying glass
could be considered a simple microscope as images appear 3 to 20 times larger than the object’s actual size when using this
Leeuwenhoek’s simple microscopes
had a maximum magnifying power of about x300 (about 300 times)
compound microscope
a microscope that contains more than one magnifying lens
compound light microscope
Because visible light is the source of illumination, a compound microscope is also referred to as
Compound light microscopes
usually magnify objects about 1,000 times
0.2 µm
The resolving power of a compound light microscope is approximately
wavelength of visible light
It limits the size of objects that can be seen.
The eyepiece or ocular lens and objective lens
the two magnifying lens systems in today's laboratory microscope
ocular lens
with x10 magnification
scanning, low power, high power, and oil immersion
four commonly used objective lenses
scanning
with 4x magnification
low power
with 10x magnification
high power
with x40 magnification
oil immersion
with x100 magnification
Total magnification
is calculated by multiplying the magnifying power of the ocular lens by the magnifying power of the objective lens being used
photomicrographs
are photographs taken through the lens system of the compound light microscope
brightfield microscope
Because objects are observed against a bright background or “bright field,” the compound light microscope is sometimes referred to as a
darkfield microscope
If the condenser is replaced with what is known as a darkfield condenser, illuminated objects are seen against a dark background or “dark field”; the microscope is then called
Phase-contrast microscopes and Fluorescence microscopes
Other types of compound microscopes include
Phase-contrast microscopes
are used to observe unstained living microorganisms
Fluorescence microscopes
contain a built-in ultraviolet (UV) light source
Electron microscopes
enable us to see extremely small microbes such as rabies and smallpox viruses