Chapter 2 Lab Tools Reviewer
Chapter 2 Lab Tools Reviewer
Chapter 2 Lab Tools Reviewer
encourage the multiplication of microbes
Temperatures used in laboratory propagation of
2. TYPES OF MEDIA
microorganisms: 2.1 PHYSICAL STATE
Three Physical States
20 to 45°C
Atmospheric gases such as oxygen or carbon dioxide Liquid
may be required for the growth of certain microbes Semisolid
During the incubation period, the microbe multiplies Solid (Liquifiable)
and produces growth that is observable Solid (Non-liquifiable)
macroscopically
1.3 ISOLATION
Based on the concept that if an individual bacterial cell
is separated from other cells on a nutrient surface, it
will produce a discrete mound of cells called a colony
The “GROWTH” itself
Selective Media
Contains one or more agents that inhibit the growth of
a certain microbe or microbes:
4.2 MAGNIFICATION
Magnification occurs in two phases:
4. MICROSCOPY
4.1 PARTS OF A MICROSPCOPE
Real image: formed by the objective
Importance of Resolution
Dark-Field Microscopy
A bright-field microscope can be adapted as a dark-
field microscope by adding a special disc called a stop
to the condenser
The stop blocks all light from entering the objective
lens, except peripheral light that is reflected off the
sides of the specimen itself
The resulting image is a particularly striking one:
brightly illuminated specimens surrounded by a dark
(black) field
The most effective use is to visualize living cells that
would be distorted by drying or heat or that cannot be
stained with the usual methods
6. STAINS
Unstained cells in a fixed smear are difficult to see,
regardless of magnification and resolving power
Staining
is any procedure that applies colored chemicals (dyes)
to specimens:
Basic dyes have a positive charge
5. SPECIMENS Acidic dyes have a negative charge
5.1 PREPARING SPECIMENS FOR THE
MICROSCOPE Bacteria have numerous negatively charged substances
Specimens are generally prepared by mounting a and attract basic dyes
sample on a glass slide that sits on the stage between
the condenser and the objective Acidic dyes are repelled by cells
6.1 Negative versus Positive Staining
The manner in which a specimen is prepared depends
on:
Positive Stain
The condition of the specimen: living or dead
dye sticks to the specimen and gives it color
The aims of the examiner: observation of overall
structure, identification, or movement Negative Stain
The type of microscopy available: bright-field, dark- does not stick to the specimen but settles some
field, phase-contrast, or fluorescence distance from its outer boundary, forming a silhouette:
5.2 FRESH, LIVING PREPARATIONS Negatively charged cells repel the negatively charged
dye and remain unstained
Placed on wet mounts or in hanging drop mounts to
Smear is not heat fixed so the distortion and shrinkage
observe as close to the natural state as possible of cells is reduced
Cells are suspended in water, broth, or saline to Also used to accentuate a capsule
maintain viability and provide a medium for locomotion Nigrosin and India ink are used
Differential stains
Use two differently colored dyes: the primary dye and
the counterstain
Distinguish cell types or parts
More complex and require additional chemical
reagents to produce the desired reaction Special Stains
Used to emphasize cell parts that are not revealed by
Differential Stains: The Gram Stain
conventional staining methods
Developed in 1884 by Hans Christian Gram
Capsular staining
Consists of sequential applications of:
Used to observe the microbial capsule, an
unstructured protective layer surrounding the cells of
Crystal violet (the primary stain) some bacteria and fungi
Gram’s iodine (the mordant) Negatively stained with India ink
An alcohol rinse (decolorizer)
A contrasting counterstain (for example Safranin) Flagellar staining
Different results in the Gram stain are due to Used to reveal tiny, slender filaments used by bacteria
differences in the structure of the cell wall and how it for locomotion
reacts to the series of reagents applied to the cells Flagella are enlarged by depositing a coating on the
Remains the universal basis for bacterial classification outside of the filament and then staining it
and identification
A practical aid in diagnosing infection and guiding drug
treatment