Chapter 2: A Brief Journey to the Microbial World Flashcards
Bright-field scope
Specimens are visualized
because of differences in
contrast (density) between
specimen and surroundings
Objective lens and ocular lens calculation
• Total magnification = objective
magnification x ocular
magnification
• Maximum magnification is
~2,000x
Resolution:
– the ability to distinguish two adjacent
objects as separate and distinct
– Resolution is determined by the wavelength of light
used and numerical aperture of lens
– Limit of resolution for light microscope is about
0.2 u m
True or false: Improving contrast results in a better final image
True
Staining improves contrast
• Dyes are organic compounds that bind to specific cellular
materials
• Examples of common stains are methylene blue, safranin,
and crystal violet (basic dyes)
Bacteria can be divided into two
major groups:
gram-positive and
gram-negative
Differential stains: the Gram stain
Differential stains separate bacteria
into groups
Phase-Contrast Microscopy
• Invented in 1936 by Frits Zernike
• Phase ring amplifies differences in
the refractive index of cell and
surroundings
• Improves the contrast of a sample
without the use of a stain
• Allows for the visualization of live
samples
• Resulting image is dark cells on a
light background (Figure 2.5b)
Dark-Field Microscopy
• Light reaches the specimen from
the sides
• Light reaching the lens has been
scattered by specimen
• Image appears light on a dark
background (Figure 2.5c)
• Excellent for observing motility
Fluorescence Microscopy
• Used to visualize specimens that
fluoresce
• Emit light of one color when
illuminated with another color of light
• Cells fluoresce naturally
(autofluorescence) or after they
have been stained with a
fluorescent dye like DAPI
• Widely used in clinical diagnostic
microbiology, microbial ecology for
enumerating bacteria in natural
samples, or in cell suspension
Differential Interference Contrast
(DIC) Microscopy
• Uses a polarizer to create two
distinct beams of polarized light
• Gives structures such as
endospores, vacuoles, and granules
a three-dimensional appearance
(Figure 2.7a)
• Structures not visible using brightfield microscopy are sometimes
visible using DIC without staining
Atomic Force Microscopy (AFM)
• A tiny stylus is placed close to a
specimen
• The stylus measures weak repulsive
forces between it and the specimen
• A computer generates an image
based on the data received from
the stylus
Confocal Scanning Laser
Microscopy (CSLM)
• Uses a computerized microscope
coupled with a laser source to
generate a three-dimensional
image (Figure 2.8)
• Computer can focus the laser on
single layers of the specimen
• Different layers can then be
compiled for a three-dimensional
image
• Resolution is 0.1 m for CSLM
Electron Microscopy
• Electron microscopes use electrons
instead of photons to image cells and
structures
• Two types of electron microscopes:
• Transmission electron microscopes (TEM)
• Scanning electron microscopes (SEM)
Transmission Electron Microscopy
(TEM)
• Electromagnets function as lenses
• System operates in a vacuum
• High magnification and resolution
(0.2 nm)
• Enables visualization of structures
at the molecular level (Figure 2.10a
and b)
• Specimen must be very thin (20–60
nm) and be stained
Scanning Electron Microscopy
(SEM)
• Specimen is coated with a thin film
of heavy metal (e.g., gold)
• An electron beam scans the object
• Scattered electrons are collected by
a detector and an image is
produced (Figure 2.10c)
• Even very large specimens can be
observed
• Magnification range of 15–
100,000 but only the surface of an
object is visualized
All cells have the following in common:
• Cytoplasmic membrane
• Cytoplasm
• Ribosomes
Elements of Microbial Structure: Eukaryotes
• DNA enclosed in a membrane bound nucleus
• Cells are generally larger and more complex
• Contain organelles
Elements of Microbial Structure: Prokaryotes
No membrane-enclosed organelles, no nucleus
Generally smaller than eukaryotic cells
Viruses
• Not considered cells
• No metabolic abilities of their own
• Rely completely on biosynthetic
machinery of infected cell
• Infect all types of cells
• Smallest virus is 10 nm in diameter
Genome
A cell’s full complement of genes
Prokaryotic cells generally have a
single, circular DNA molecule called
chromosome
Plasmids
Prokaryotes also may have small
amounts of extra-chromosomal DNA
called plasmids that confer special
properties (e.g., antibiotic resistance)
Eukaryotic DNA is linear and found within the nucleus
• Associated with proteins that help in folding of the DNA
• Usually more than one chromosome
• Typically two copies of each chromosome
• During cell division, nucleus divides by mitosis
• During sexual reproduction, the genome is halved by
meiosis
Arrangement of DNA in Microbial Cells: Escherichia coli Genome
• 4.64 million base pairs
• 4,300 genes
• 1,900 different kinds of protein
• 2.4 million protein molecules
Arrangement of DNA in Microbial Cells: Human Cell
• 1,000x more DNA per cell than E. coli
• 7x more genes than E. coli
Evolution
The process of change over time that results in new varieties and species of organisms
Phylogeny
• Evolutionary relationships between organisms
• Relationships can be deduced by comparing genetic information in the different specimens
• Ribosomal RNA (rRNA) is excellent for determining phylogeny
• Relationships visualized on a phylogenetic tree
Comparative rRNA sequencing has defined three distinct lineages of cells called domains:
• Bacteria (prokaryotic)
• Archaea (prokaryotic)
• Eukarya (eukaryotic)
Endosymbiosis
a term used to describe two organisms living together with one inside the other
ancestors of multicellular organisms
eukaryotic microorganisms
Metabolic Diversity
• The diversity in microbial cells is the product of
almost 4 billion years of evolution
• Microorganisms differ in size, shape, motility,
physiology, pathogenicity, etc.
• Microorganisms have exploited every conceivable
means of obtaining energy from the environment
Chemoorganotrophs
• Obtain their energy from the
oxidation of organic molecules
• Aerobes use oxygen to obtain energy
• Anaerobes obtain energy in the
absence of oxygen
Chemolithotrophs
• Obtain their energy from the
oxidation of inorganic molecules
• Process found only in prokaryotes
Phototrophs
• Contain pigments that allow them to
use light as an energy source
• Oxygenic photosynthesis produces
oxygen
• Anoxygenic photosynthesis does not
produce oxygen
All cells require ________ as a major nutrient
carbon
Autotrophs use this as their carbon source
Carbon dioxide
Sometimes referred to as primary producers
Autotrophs
Heterotrophs
• Require one or more organic molecules for their carbon source
• Feed directly on autotrophs or live off products produced by
autotrophs
extremophiles
organisms that inhabit extreme environments are
called
Habitats of extremophiles
Habitats include boiling hot springs, glaciers, extremely salty bodies of water, and high-pH environments
All known pathogenic
prokaryotes are
Bacteria
Make up the largest phylum of Bacteria
Proteobacteria
Gram-negative bacteria examples
E. coli, pseudomonas, salmonella
Photosynthetic bacterias
Green sulfur and green nonsulfur bacteria
Bacteria extremely resistant to radioactivity
Deinococcus
Obligate intracellular parasites
Chlamydia
Two Phyla of the Domain
Archaea
Euryarchaeota and crenarchaeota
Euryarchaeota
• Methanogens: degrade organic
matter anaerobically, produce
methane (natural gas)
• Extreme halophiles: require high
salt concentrations for
metabolism and reproduction
• Thermoacidophiles: grow in
moderately high temperatures
and low-pH environments
Crenarchaeota
• Vast majority of cultured Crenarchaeota are hyperthermophiles
• Some live in marine, freshwater,
and soil systems
Ribosomal RNA are uses to infer evolutionary relationships between
organisms. Woese recognized genes encoding rRNA are:
(1) Universally distributed
(2) Functionally constant
(3) Highly conserved (slowly changing)
(4) Adequate length to provide a deep view of evolutionary relationships
Eukaryotic microorganisms include
algae, fungi, protozoa, and slime
molds
True or false: algae and protozoa are phototrophic
False
Algae and fungi have _________,
whereas protozoa and slime molds do
not
cell walls
mutualistic relationship
between two groups of protists
lichens
Examples of lichens
• Fungi and cyanobacteria
• Fungi and algae
