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Microscopy, Staining, and Classification
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Microscopy and Staining: Overview
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Units of Measurement Tell Me Why
Why do scientists use metric rather than English units?
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Microscopy Microscopy: the use of light or electrons to magnify objects Science of microscopy begun by Antoni van Leeuwenhoek Various types of light and electron microscopes
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Microscopy General Principles of Microscopy Wavelength of radiation
Magnification Resolution Contrast
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Figure 4.1 The electromagnetic spectrum.
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Light Air Glass Focal point Specimen
Figure 4.2 Light refraction and image magnification by a convex glass lens. Light Air Glass Focal point Specimen Convex lens Inverted, reversed, and enlarged image 8
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Figure 4.3 The limits of resolution (and some representative objects within those ranges) of the human eye and of various types of microscopes. 9
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Microscopy General Principles of Microscopy Contrast
Differences in intensity between two objects or between an object and its background Important in determining resolution Staining increases contrast Use of light that is in phase increases contrast
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Microscopy Light Microscopy Bright-field microscopes Simple
Contain a single magnifying lens Similar to magnifying glass Leeuwenhoek used simple microscope to observe microorganisms
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Microscopy Light Microscopy Bright-field microscopes Compound
Series of lenses for magnification Light passes through specimen into objective lens Oil immersion lens increases resolution Have one or two ocular lenses Total magnification = magnification of objective lens x magnification of ocular lens Most have condenser lens (direct light through specimen)
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Figure 4.4 A bright-field, compound light microscope.
Line of vision Ocular lens Remagnifies the image formed by the objective lens Body Transmits the image from the objective lens to the ocular lens using prisms Ocular lens Path of light Prism Arm Objective lenses Primary lenses that magnify the specimen Body Objective lenses Stage Holds the microscope slide in position Specimen Condenser Focuses light through specimen Condenser lenses Diaphragm Controls the amount of light entering the condenser Illuminator Illuminator Light source Coarse focusing knob Moves the stage up and down to focus the image Fine focusing knob Base 13
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Figure 4.5 The effect of immersion oil on resolution.
Microscope objective Microscope objective Lenses Refracted light rays lost to lens More light enters lens Immersion oil Glass cover slip Glass cover slip Slide Slide Specimen Light source Light source Without immersion oil With immersion oil 14
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Microscopy Light Microscopy Dark-field microscopes
Best for observing pale objects Only light rays scattered by specimen enter objective lens Specimen appears light against dark background Increases contrast and enables observation of more details
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Figure 4.6 The light path in a dark-field microscope.
Objective Light refracted by specimen Light unrefracted by specimen Specimen Condenser Dark-field stop Dark-field stop 16
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Microscopy Light Microscopy Phase microscopes
Used to examine living organisms or specimens that would be damaged/altered by attaching them to slides or staining Light rays in phase produce brighter image, whereas light rays out of phase produce darker image Contrast is created because light waves are out of phase Two types Phase-contrast microscope Differential interference contrast microscope
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Figure 4.7 Principles of phase microscopy.
Rays in phase Rays out of phase Phase plate Bacterium Ray deviated by specimen is 1/4 wavelength out of phase. Deviated ray is now 1/2 wavelength out of phase. 18
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Figure 4.8 Four kinds of light microscopy.
Nucleus Bacterium Bright field Dark field Phase contrast Nomarski 19
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Microscopy Light Microscopy Fluorescent microscopes
Direct UV light source at specimen Specimen radiates energy back as a longer, visible wavelength UV light increases resolution and contrast Some cells are naturally fluorescent; others must be stained Used in immunofluorescence to identify pathogens and to make visible a variety of proteins
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Figure 4.9 Fluorescence microscopy.
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Figure 4.10 Immunofluorescence.
Antibodies Fluorescent dye Antibodies carrying dye Bacterium Cell-surface antigens Bacterial cell with bound antibodies carrying dye 22
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Microscopy Light Microscopy Confocal microscopes
Use UV lasers to illuminate fluorescent chemicals in a single plane Resolution increased because emitted light passes through pinhole aperture Each image is "optical slice" through specimen Computer constructs 3-D image from digitized images
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Light Microscopy
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Microscopy Electron Microscopy
Light microscopes cannot resolve structures closer than 200 nm Electron microscopes have greater resolving power and magnification Magnifies objects 10,000x to 100,000x Detailed views of bacteria, viruses, internal cellular structures, molecules, and large atoms Two types Transmission electron microscopes Scanning electron microscopes
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Figure 4.11 A transmission electron microscope (TEM).
Light microscope (upside down) Column of transmission electron microscope Lamp Electron gun Condenser lens Condenser lens (magnet) Specimen Specimen Objective lens Objective lens (magnet) Eyepiece Projector lens (magnet) Final image seen by eye Final image on fluorescent screen 26
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Figure 4.12 Scanning electron microscope (SEM).
Electron gun Magnetic lenses Beam deflector coil Scanning circuit Primary electrons Secondary electrons Photo- multiplier Specimen Monitor Detector Specimen holder Vacuum system 27
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Figure SEM images. 28
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Electron Microscopy
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Microscopy Probe Microscopy Magnifies more than 100 million times
Two types Scanning tunneling microscopes Atomic force microscopes
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Microscopy Probe Microscopy Scanning tunneling microscopes
Passes metallic probe above specimen surface Measures the electron flow (tunneling current) to and from the probe and the specimen's surface Atomic force microscopes Passes probe lightly on the specimen surface Deflection of laser beam translated into atomic topography
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Figure 4.14 Probe microscopy.
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Microscopy Tell Me Why Why is magnification high but color absent in an unretouched electron micrograph?
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Staining Most microorganisms are difficult to view by bright-field microscopy Coloring specimen with stain increases contrast and resolution Specimens must be prepared for staining
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Figure 4.15 Preparing a specimen for staining.
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Staining Principles of Staining Dyes used as stains are usually salts
Chromophore is the colored portion of the dye Acidic dyes stain alkaline structures Basic dyes stain acidic structures More common because most cells are negatively charged
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Staining Simple stains—composed of single dye
Differential stains—use more than one dye Gram stain Acid-fast stain Endospore stain Histological stains Special stains—reveal specific structures Negative (capsule) stain Flagellar stain
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Figure Simple stains. 40
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Figure 4.17 The Gram staining procedure.
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Figure 4.18 Ziehl-Neelsen acid-fast stain.
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Figure 4.19 Schaeffer-Fulton endospore stain of Bacillus anthracis.
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Staining Differential Stains Histological stains
Two common stains used for histological specimens Gomori methenamine silver (GMS) stain Hematoxylin and eosin (HE) stain
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Figure 4.20 Negative (capsule) stain of Klebsiella pneumoniae.
Bacterium Capsule Background stain 45
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Figure 4.21 Flagellar stain of Proteus vulgaris.
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Staining Staining for Electron Microscopy
Chemicals containing heavy metals are used for transmission electron microscopy Stains may bind molecules in specimens or the background
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Staining
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Microscopy Tell Me Why Why is a Gram-negative bacterium colorless but a Gram-positive bacterium purple after it is rinsed with decolorizer?
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Classification and Identification of Microorganisms
Taxonomy consists of classification, nomenclature, and identification Organize large amounts of information about organisms Make predictions based on knowledge of similar organisms
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Classification and Identification of Microorganisms
Linnaeus and Taxonomic Categories Current taxonomy system began with Carolus Linnaeus His system classified organisms based on characteristics in common Grouped organisms that can successfully interbreed into categories called species Used binomial nomenclature
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Figure 4.22 Levels in a Linnaean taxonomic scheme.
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Classification and Identification of Microorganisms
Linnaeus and Taxonomic Categories Linnaeus proposed only two kingdoms Later taxonomic approach based on five kingdoms Animalia, Plantae, Fungi, Protista, and Prokaryotae Linnaeus's goal was to classify organisms in order to catalog them Modern goal is to understand relationships among organisms Goal of modern taxonomy is to reflect phylogenetic hierarchy Greater emphasis on comparisons of organisms' genetic material led to proposal to add domain
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Classification and Identification of Microorganisms
Domains Carl Woese compared nucleotide sequences of rRNA subunits Proposal of three domains as determined by ribosomal nucleotide sequences Eukarya, Bacteria, and Archaea Cells in the three domains also differ with respect to many other characteristics
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Classification and Identification of Microorganisms
Taxonomic and Identifying Characteristics Physical characteristics Biochemical tests Serological tests Phage typing Analysis of nucleic acids
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Classification and Identification of Microorganisms
Taxonomic and Identifying Characteristics Physical characteristics Can often be used to identify microorganisms Protozoa, fungi, algae, and parasitic worms can often be identified based only on their morphology Some bacterial colonies have distinct appearance used for identification
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Figure 4.23 Two biochemical tests for identifying bacteria.
Gas bubble Inverted tubes to trap gas Hydrogen sulfide produced No hydrogen sulfide Acid with gas Acid with no gas Inert 58
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Figure 4.24 One tool for the rapid identification of bacteria, the automated MicroScan system.
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Classification and Identification of Microorganisms
Taxonomic and Identifying Characteristics Serological tests Serology—study of serum (liquid portion of blood after clotting factors removed) Many microorganisms are antigenic Trigger immune response that produces antibodies Serum is an important source of antibodies Antibodies can be isolated and bind to the antigens that triggered their production
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Figure 4.25 An agglutination test, one type of serological test.
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Classification and Identification of Microorganisms
Taxonomic and Identifying Characteristics Phage typing Bacteriophage (phage)—virus that infects bacteria Phages are specific for the host they infect Phage typing is based on this specificity
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Bacterial lawn Plaques
Figure Phage typing. Bacterial lawn Plaques 63
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Classification and Identification of Microorganisms
Taxonomic and Identifying Characteristics Analysis of nucleic acids Nucleic acid sequence can be used to classify and identify microbes Prokaryotic taxonomy now includes the G + C content of an organism's DNA
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Classification and Identification of Microorganisms
Taxonomic Keys Dichotomous keys Series of paired statements where only one of two "either/or" choices applies to any particular organism Key directs user to another pair of statements or provides name of organism
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Figure 4.27 Use of a dichotomous taxonomic key.
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Dichotomous Keys: Overview
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Classification and Identification of Microorganisms
Tell Me Why Why didn't Linnaeus create taxonomic groups for viruses?
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