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A Brief Journey to the Microbial World

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Title: A Brief Journey to the Microbial World


1
Chapter 2
  • A Brief Journey to the Microbial World

2
Microscopy
  • Microscopes are essential for microbiological
    studies.
  • A microscope is required for the visualization of
    microorganisms
  • Light microscopy is used to observe less detailed
    features of intact cell under low magnification
    than advanced microscopy (i.e. electron and
    laser)
  • Various types of light microscopes exist,
    including bright-field, dark-field, phase
    contrast, and fluorescence microscopes.

3
Magnification vs. Resolution
  • Magnification
  • Increase in apparent size
  • Two sets of lenses form the image
  • Objective lens and ocular lens
  • Total magnification is a product of the
    magnification of the two sets of lenses
  • Objective magnification X ocular magnification
  • Resolution clarity
  • Ability to see 2 nearby objects as distinct
    objects
  • All compound light microscopes optimize image
    resolution by using lenses with high
    light-gathering characteristics (numerical
    aperture).
  • The limit of resolution for a light microscope is
    about 0.2 ?m.

4
Microscopy
Figure 2.1b
5
Simple and/or differential cell staining are used
to increase contrast in bright-field microscopy.
  • Improving contrast results in a better final
    image
  • Staining is an easy way to improve contrast
  • Dyes are organic compounds that have different
    affinities for specific cellular materials
  • Examples of common stains are methylene blue,
    safranin, and crystal violet

6
Staining Cells for Microscopic Observation
Figure 2.3
7
Staining Cells for Microscopic Observation
Figure 2.3
8
Staining Cells for Microscopic Observation
Figure 2.3
9
Differential StainsThe Gram stain
  • The Gram stain is widely used in microbiology
  • On the basis of the Gram stain, bacteria can be
    divided into two major groups gram-positive and
    gram-negative
  • The Gram stain renders different kinds of cells
    different colors
  • Gram-positive bacteria appear purple and
    gram-negative bacteria appear pink to red after
    staining

10
The Gram Stain Steps in the Gram-stain Procedure
Figure 2.4a
11
The Gram Stain Steps in the Gram-stain Procedure
Figure 2.4a
12
The Gram Stain
Figure 2.4b and c
13
Phase Contrast Microscopy
  • Invented in 1936 by Frits Zernike
  • May be used to visualize live samples and avoid
    distortion from cell stains
  • Can see some internal features
  • Resulting image is dark cells on a light
    background

Figure 2.5
14
Electron Microscopy
  • Electron microscopes have far greater resolving
    power than light microscopes, with limits of
    resolution of about 0.2 nm.
  • Electron microscopes use electrons instead of
    photons to image cells and structures
  • Two types
  • Transmission electron microscopy (TEM)
  • For observing internal cell structure down to the
    molecular level
  • Scanning electron microscopy (SEM)
  • For three-dimensional imaging and examining
    surfaces

15
Electron Micrographs
Figure 2.10a
16
Electron Micrographs
Figure 2.10bc
17
All microbial cells share certain basic
structures in common
  • Cytoplasm
  • Cytoplasmic membrane
  • Allows passage of needed molecules (nutrients,
    water, etc.) and barrier to harmful chemicals
  • Ribosomes
  • Site of protein synthesis
  • Cell wall (usually)
  • Cell shape

18
Prokaryote vs Eukaryote
  • Two structural types of cells are recognized
  • Prokaryotic
  • Archaea and bacteria
  • Eukaryotic plants, algae, fungi, protists, and
    animals (variety)
  • Comparing prokaryotic and eukaryotic cells
  • Prokaryote comes from the Greek words for
    prenucleus.
  • Eukaryote comes from the Greek words for true
    nucleus.

19
Prokaryote Eukaryote
  • Simpler internal structure
  • Absence of nucleus
  • One circular chromosome, not in a membrane
  • No histones
  • No membrane enclosed organelles
  • Peptidoglycan cell walls
  • Binary fission for cell division
  • Smaller
  • Contain nucleus
  • Paired chromosomes, in nuclear membrane
  • Histones
  • Membrane enclosed organelles
  • Simple (polysaccharide) cell walls
  • Cell division by mitosis or meiosis
  • Larger


20
Structure of Prokaryotic vs. Eukaryotic cell
Figure 2.11a
Figure 2.11b
21
Viruses
  • Non cellular
  • Obligate intracellular parasites
  • They must live inside another cell to survive
  • Have only one type of nucleic acid
  • DNA or RNA (never both)
  • Single or Double stranded
  • Protein coat (no plasma membrane)
  • Few to no enzymes
  • Takes enzymes and use host cell metabolic
    machinery
  • No metabolic activity
  • They require a host cell to exhibit the
    characteristics of life.
  • Virus diversity
  • Different viruses have different hosts
  • Only some viruses cause disease

22
Size
  • Typical prokaryote ?1 - 5 ?m long
  • Typical Eukaryotic cell ?10 - 100 ?m
  • Typical virus ? 50 - 80 nm

23
Phylogeny
  • The study of the evolutionary relationionships of
    distinct organisms
  • Although species of Bacteria and Archaea share a
    prokaryotic cell structure, they differ
    dramatically in their evolutionary history.
  • Molecular based
  • Compare sequences from common molecules from
    organisms of interest
  • Relationships can be deduced by comparing genetic
    information (nucleic acid or amino acid
    sequences) in the different specimens
  • Carl Woese (1970s)
  • rRNA comparison
  • Ribosomal RNA (rRNA) are excellent molecules for
    determining phylogeny
  • Can visualize relationships on a phylogenetic
    tree

24
Ribosomal RNA (rRNA) Gene Sequencing and Phylogeny
Figure 2.16
25
Phylogeny
  • Comparative ribosomal RNA sequencing has defined
    the three domains of life
  • Bacteria (prokaryotic)
  • Archaea (prokaryotic)
  • Eukarya (eukaryotic)
  • Common ancestor - over 3.8 billion years ago
  • Bacteria and archaea are prokaryotes but archaea
    are more closely related to eukaryotes
  • Eukaryotic microorganisms were the ancestors of
    multicellular organisms
  • Mitochondria and chloroplasts also contain their
    own genomes (circular, like prokaryotes) and
    ribosomes
  • These organelles are ancestors of specific
    lineages of Bacteria
  • Mitochondria and chloroplasts took up residence
    in Eukarya eons ago
  • This arrangement is known as endosymbiosis

26
The Tree of Life Defined by rRNA Sequencing
Figure 2.17
27
2.8 - Physiological Diversity of Microorganisms
  • The phylogenetic diversity we see in microbial
    cells is the product of almost 4 billion years of
    evolution
  • Microorganisms also have a tremendous amount of
    metabolic diversity
  • Microorganisms have exploited every conceivable
    means of making a living consistent with the
    laws of chemistry and physics

28
Physiological Diversity
  • All cells need carbon and energy sources.
  • Different species use different strategies
  • Chemoorganotrophs obtain their energy from the
    oxidation of organic compounds.
  • Chemolithotrophs obtain their energy from the
    oxidation of inorganic compounds.
  • Phototrophs contain pigments that allow them to
    use light as an energy source.
  • Oxygenic versus anoxygenic photosynthesis

29
Metabolic Options for Conserving Energy
Figure 2.18
30
Autotrophs vs. Heterotrophs
  • All cells require carbon as a major nutrient
  • Autotrophs
  • Use carbon dioxide as their carbon source
  • Use sunlight for energy
  • Primary producers
  • Most phototrophs and lithotrophs are autotrophs
  • Heterotrophs
  • Use organic carbon as their carbon source
  • Use products of autotrophs or the autotrophs
    themselves for energy
  • All organotrophs and SOME lithotrophs are
    heterotrophs

31
Extremophiles
  • Thrive under environmental conditions in which
    higher organisms cannot survive.
  • Prokaryotes thrive in habitats that are too cold,
    too hot, too salty, too basic for any eukaryote
  • Many prokaryotes are extremophiles
  • No environment is devoid of prokaryotic life
  • Salt concentrations up to 30 for some
  • halophiles
  • pH 0-12
  • Acidophiles and alkaliphiles
  • Temps below 0 ºC to above 100 ºC
  • Psychrophiles and hyperthermophiles
  • High pressure
  • Barophiles

32
Classes and Examples of Extremophiles
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