Download Chapter 2. Viewing the Microbial World Chapter 2 Outline

Chapter 2. Viewing the Microbial World
Chapter 2 Outline
• Introduction
• Using the metric system to express the sizes of
microbes
• Microscopes
– Simple microscopes
– Compound microscopes
– Electron microscopes
– Atomic force microscopes
Using the Metric System to Express the Sizes of
Microorganisms
• Metric units are used to express the sizes of
microbes.
• The basic unit of length in the metric system is the
meter (m); it is equivalent to 39.4 inches.
• The sizes of bacteria and protozoa are usually
expressed in terms of micrometers (µm). A
micrometer is one millionth of a meter.
• A typical spherical bacterium (coccus) is
approximately 1 µm in diameter.
• A typical rod-shaped bacterium (bacillus) is
approximately 1 µm wide by 3 µm long.
Representations of Metric Units of Measure and Numbers
Using the Metric System to Express the Sizes of
Microorganisms, cont.
• The sizes of viruses are expressed in terms of
nanometers (nm). A nanometer is equal to one
billionth of a meter.
• Most of the viruses that cause human diseases
range in size from 10 nm to 300 nm.
• One exception is Ebola virus, a cause of viral
hemorrhagic fever. Ebola viruses can be as long as
1,000 nm (1 µm).
• When using a microscope, the sizes of
microorganisms are measured using an ocular
micrometer.
Microscopes
• The human eye, a telescope, a pair of binoculars, a
magnifying glass and a microscope are various
types of optical instruments.
• A 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.
• Each optical instrument has a limit as to what can
be seen using that instrument; this limit is referred
to as the resolving power or resolution of the
instrument.
• The resolving power of the unaided human eye is
approximately 0.2 mm.
Microscope Resolution
Simple Microscopes
•
A simple microscope is one that contains only one
magnifying lens.
• A magnifying glass could be considered a simple
microscope; when using a magnifying glass, images
appear 3-20 times larger than the object’s actual
size.
• Leeuwenhoek’s simple microscopes had a
maximum magnifying power of about X300 (about
300 times).
Compound Microscopes
• A compound microscope contains more than one
magnifying lens.
• Because visible light is the source of illumination, a
compound microscope is also referred to as a
compound light microscope.
• Compound light microscopes usually magnify
objects about 1000 times.
• The resolving power of a compound light
microscope is approximate 0.2 µm (about 1,000
times better than the resolving power of the
unaided human eye).
Compound Microscopes, cont.
• It is the wavelength of visible light (~0.45 µm) that
limits the size of objects that can be seen.
• Objects cannot be seen if they are smaller than
half of the wavelength of visible light.
• Today’s laboratory microscope contains two
magnifying lens systems:
– The eyepiece or ocular lens (usually X10)
– The objective lens (X4, X10, X40, and X100
are the four most commonly used
objective lenses)
Compound Microscopes, cont.
• Total magnification is calculated by multiplying the
magnifying power of the ocular lens by the
magnifying power of the objective lens being used.
– X10 ocular x X4 objective = X40 total mag.
– X10 ocular x X10 objective = X100 total
mag.
– X10 ocular x X40 objective = X400 total
mag.
– X10 ocular x X100 objective = X1000 total
mag.
• Photographs taken through the lens system of the
compound light microscope are called
photomicrographs.
Compound Microscopes, cont.
• Because objects are observed against a bright
background or “bright field,” the compound light
microscope is sometimes referred to as a
brightfield microscope.
•
If the condenser is replaced with what is known as Atomic Force Microscopy:
a darkfield condenser, illuminated objects are seen
Spores Breaking out of bacterium
against a dark background or “dark field;” the
microscope is now called a darkfield microscope.
• Other types of compound microscopes include:
– Phase contrast microscopes
– Fluorescence microscopes
Darkfield Microscopy of
Treponema pallidum (the bacterium that causes syphilis)
Phase Contrast and Fluorescent Microscopes
• Fluorescent microscope contains a built-in
ultraviolet (UV) light source.
– When UV light strikes certain dyes and
pigments these substances emit a longer
wavelength light causing them to glow
against a dark background.
Phase Contrast and Fluorescent Microscopes
• Phase contrast microscopes are used to observe
unstained living microorganisms.
– Organisms are more easily seen because
the light refracted by living cells is different
from the light refracted by the surrounding
medium.
Electron Microscopes
• Electron microscopes enable us to see extremely
small microbes such as rabies and smallpox
viruses.
• Living organisms cannot be observed using an
electron microscope – the processing procedures
kill the organisms.
• An electron beam is used as the source of
illumination and magnets are used to focus the
beam.
• Electron microscopes have a much higher resolving
power than compound light microscopes.
• There are 2 types of electron microscopes transmission and scanning.
Transmission Electron Microscope
• Uses an electron gun to fire a beam of electrons
through an extremely thin specimen (<1 µm thick).
• An image of the specimen is produced on a
phosphor-coated screen.
• Magnification is approx. 1000 times greater than
the compound light microscope.
• Resolving power is approx. 0.2 nm.
Scanning Electron Microscope
Atomic Force Microscopes
• Enable scientists to observe living cells at
extremely high magnification and resolution under
physiological conditions.
• Can observe single live cells in aqueous solutions.
• Provides a true three-dimensional surface profile.