Chapter 2: A Brief Journey to the Microbial World Flashcards
(56 cards)
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
