Week 2

• Metric units


  Used to express the sizes of microbes
• Meter (m)


  The basic unit of length in the metric system, equivalent to 39.4 inches
• Micrometer (μm)


  One millionth of a meter, used to express the sizes of bacteria and protozoa
• A typical spherical bacterium (coccus) is approximately 1 μm in diameter
• A typical rod-shaped bacterium (bacillus) is approximately 1 μm wide x 3 μm long
• Metric units


  • Centimeters
  • Millimeters
  • Micrometers
  • Nanometers
• Staphylococcus bacteria are 7 μm in size
• Microscope


  An optical instrument used to observe tiny objects that cannot be seen with the unaided human eye
• Resolving power


  The limit as to what can be seen using an optical instrument
• The resolving power of the unaided human eye is approximately 0.2 mm
• Simple microscope


  • Contains only one magnifying lens
  • Magnifies objects 3 to 20 times larger than actual size
  • Leeuwenhoek's simple microscopes had a maximum magnifying power of about x300
• Compound microscope


  • Contains more than one magnifying lens
  • Usually magnifies objects about 1,000 times
  • Resolving power of approximately 0.2 μm
  • Wavelength of visible light (~0.45 μm) limits the size of objects that can be seen
• Components of a compound microscope

  1. Eyepiece or ocular lens (usually x10)

  2. Objective lens (x4, x10, x40, and x100 are the four most commonly used)

  3. Total magnification is calculated by multiplying the magnifying power of the ocular lens by the magnifying power of the objective lens being used
• Photographs taken through the lens system of the compound light microscope are called photomicrographs
• Brightfield microscope


  Objects are observed against a bright background
• Darkfield microscope


  Illuminated objects are seen against a dark background
• Phase-contrast microscope


  • 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
• Fluorescence microscope


  • Contains a built-in ultraviolet (UV) light source
  • Substances emit a longer-wavelength light, causing them to glow against a dark background
• Electron microscope


  • Enables us to see extremely small microbes such as rabies and smallpox viruses
  • Living organisms cannot be observed as the processing procedures kill them
  • Uses an electron beam as the source of illumination and magnets to focus the beam
  • Has a much higher resolving power than compound light microscopes
• Transmission electron microscope


  • Uses an electron gun to fire beam of electrons through an extremely thin specimen (<1 μm thick)
  • Magnification is approximately 1,000 times greater than with the compound light microscope
  • Resolving power is approximately 0.2 nm
• Scanning electron microscope


  • Electrons are bounded off the surface of a specimen and the image appears on a monitor
  • Used to observe the outer surfaces of specimens
  • Resolving power is about 100 times less than that of transmission electron microscope
• Transmission and scanning electron micrographs are black-and-white images