Chapter 2

• Metric units


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


  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
• Typical spherical bacterium (coccus)

  • Approximately 1 μm in diameter
• Typical rod-shaped bacterium (bacillus)


  • Approximately 1 μm wide, 3 μm long
• Nanometer (nm)


  One billionth of a meter, used to express the sizes of viruses
• Viruses that cause human diseases

  • Range in size from 10 to 300 nm
• Ebola virus


  • Can be as long as 1,000 nm (1 μm)
• Ocular micrometer


  Used to measure the sizes of microorganisms under a microscope
• Microscope


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


  Limit of what can be seen using an optical instrument
• Resolving power of unaided human eye

  • Approximately 0.2 mm
• Simple microscope


  Contains only one magnifying lens
• Leeuwenhoek's simple microscopes


  • Had a maximum magnifying power of about 300 times
• Compound microscope


  Contains more than one magnifying lens
• Compound light microscopes


  • Usually magnify objects about 1,000 times
• Resolving power of compound light microscope

  • Approximately 0.2 μm (about 1,000 times better than the resolving power of the unaided human eye)
• Wavelength of visible light

  ~0.45 μm, limits the size of objects that can be seen
• Eyepiece or ocular lens


  Usually 10x magnification
• Objective lens


  4x, 10x, 40x, and 100x are the four most commonly used
• Total magnification calculation


  Multiply magnifying power of ocular lens by magnifying power of objective lens
• Photomicrographs


  Photographs taken through the lens system of the compound light microscope
• Brightfield microscope


  Compound light microscope where objects are observed against a bright background
• Darkfield microscope


  Compound light microscope where illuminated objects are seen against a dark background
• Phase-contrast microscope


  Used to observe unstained living microorganisms
• Fluorescence microscope


  Contains a built-in ultraviolet (UV) light source
• Electron microscope


  Enables seeing extremely small microbes such as viruses
• Living organisms cannot be observed using an electron microscope - the processing procedures kill the organisms
• Transmission electron microscope


  Uses an electron beam to fire through an extremely thin specimen and produce an image on a phosphor-coated screen
• Magnification of transmission electron microscope

  • Approximately 1,000 times greater than with the compound light microscope
• Resolving power of transmission electron microscope

  • Approximately 0.2 nm
• Scanning electron microscope


  Bounces electrons off the surface of a specimen to produce an image on a monitor
• Resolving power of scanning electron microscope

  • About 100 times less than that of transmission electron microscope
• Staphylococcus aureus


  • As seen by light microscopy, transmission electron microscopy, and scanning electron microscopy