Microbiology: What is a microbe?

A microbe or microorganism is a member of a large, extremely divers, group of organisms that are lumped together on the basis of one property. The fact that, normally, they are so small that they cannot be seen without the use of a microscope. Microbe Range of size Nature

Viruses 0.01-0.25 µm Cellular Bacteria 0.1-10 µm Prokaryote

Fungi 2 µm- 1 meter Eukaryote

protozoa 2- 1000 µm Eukaryote

Prokaryotes and Eukaryote

Prokaryotes Eukaryotes

DNA free in cytoplasm. DNA is contained with a membrane bound nucleus

Only one chromosome. Diploid two copies of each

chromosome. Energy metabolism associated

with cytoplasmic membrane. Mitochondria

70 s ribosome. 80 s ribosome.

Peptidoglycan cell walls. Polysaccharide cell walls (cellulose or chitin).

The importance of microbiology:

1-The environment: microbes are responsible for the cycling of carbon, nitrogen and phosphorus.

2- Medicine: production of antibiotics.

3- Food: wine and bread making, cheese and milk products production.

4- Biotechnology: Some microorganisms have been used to synthesize many chemicals such as acetone and acetic acid.

5- Research: microbes have been used as modal organisms for the biochemical and medical investigations.

Microscopy:A microscope is the most important piece of equipment used in clinical microbiology. Microscopy forms 70-90% of the work.

Working principle of a microscope: A microscope is a magnifying instrument. The magnified image of the object (specimen) is first produced by a lens close to the object called the objective lens. A second lens near the eye called the eyepiece enlarges the primary image, converting it into one that can enter the pupil of the eye.

Total magnification = objective magnification* eyepiece

magnification (4, 10, 40, 100 X) * ( 10 X)

Resolution of Microscope: It is the smallest distance that separate two sources of light points reflected

from two particles close together on object.

Resolution = light wavelength / 2

Example: yellow light of a wavelength of 0.4 μm give a resolution of 0.2

μm.

Types of microscopy:1- The light microscope: (simple and compound).

A- The student microscope

B- Phase contrast microscope.

C- Darkfield illumination microscope.

2- The Fluorescence microscope.

3- The Electron microscope.

A- The light microscope: Brightfield illumination microscope is mainly used to examine stained

smear

B-Phase contrast microscope: This is needed to visualize transparent microorganisms suspended in a fluid.

The phase microscope takes advantage of the fact that light waves passing through transparent slid or objects (cells) emerge in different phases (different times).

A special optical system coverts

difference in phase into difference

in intensity. So that some

structure appear darker

than others.

Micrcoccus species examined by phase contrast microscope ( left) and Leptospira examined by Darkfield microscope (Right):

C-Dark field illumination microscope:

This method is used for visualizing organisms suspended in fluid and can

not be stained by Gram’s stain.

Both the structure and the motility of the organisms can be seen.

In this method the light enters the special condenser

which has a central blacked-out area, so that light can not pass

directly through it to inter the objective.

Instead the light is reflected to pass

through the outer rim of the

condenser at a wide angle which

illuminates the microorganism

by a ring of light surrounding them.

2- The fluorescence microscope:

In this method, microorganisms must be stained with a fluorescent dye such

as rhodamine.

A fluorescent lamp emits visible light which is filtered off using

optical filters. (Ultraviolet (UV) wavelengths will not be filtered off).

A fluorescent dye will absorb this UV-light and change it to visible (yellow or orange) light.

3-The electron microscope: Electron microscope is an instrument that magnify very small objects which

can not be seen by light microscopy. Resolution of this microscope is higher than the light one.

The energy source of this kind is the electron beam while light waves are used in the light microscope.

Electromagnetic lenses are used instead of the glass lenses of light microscope.

Visible light of 500 nm wavelength give a resolution of 250 nm.

Electron beam of 0.001 nm wavelength give a resolution of 0.0005 nm.

Viruses with a diameter of 0.01-0.2 micrometer can be easily seen.

aa

Light waves and Electron beam:

a