BSC 310 Lecture Notes - Lecture 14: Freeze-Drying, Petri Dish, Sabouraud Agar
Microbial Cultivation
When microorganisms are cultivated in the laboratory, a growth environment called
a mediumis used. The medium may be purely chemical (a chemically defined medium),
or it may contain organic materials, or it may consist of living organisms such as
fertilized eggs. Microorganisms growing in or on such a medium form a culture. A
culture is considered a pure culture if only one type of organism is present and
a mixed culture if populations of different organisms are present. When first used, the
culture medium should be sterile, meaning that no form of life is present before
inoculation with the microorganism.
General microbial media. For the cultivation of bacteria, a commonly used medium isnutrient
broth, a liquid containing proteins, salts, and growth enhancers that will support many
bacteria. To solidify the medium, an agent such as agar is added. Agar is a polysaccharide
that adds no nutrients to a medium, but merely solidifies it. The medium that results
is nutrient agar.
Many media for microorganisms are complex, reflecting the growth requirements of the
microorganisms. For instance, most fungi require extra carbohydrate and an acidic
environment for optimal growth. The medium employed for these organisms is potato
dextrose agar, also known as Sabouraud dextrose agar. For protozoa, liquid media are
generally required, and for rickettsiae and viruses, living tissue cells must be provided for
best cultivation.
For anaerobic microorganisms, the atmosphere must be oxygen free. To eliminate the
oxygen, the culture media can be placed within containers where carbon dioxide and
hydrogen gas are generated and oxygen is removed from the atmosphere. Commercially
available products achieve these conditions. Anaerobic chambers can also be used within
closed compartments, and technicians can manipulate culture media within these chambers.
To encourage carbon dioxide formation, a candle can be burned to use up oxygen and
replace it with carbon dioxide.
Special microbial media. Certain microorganisms are cultivated in selective media.These
media retard the growth of unwanted organisms while encouraging the growth of the
organisms desired. For example, mannitol salt agar is selective for staphylococci because
most other bacteria cannot grow in its high-salt environment. Another selective medium
is brilliant green agar, a medium that inhibits Gram-positive bacteria while permitting Gram-
negative organisms such as Salmonella species to grow.
Still other culture media are differential media. These media provide environments in which
different bacteria can be distinguished from one another. For instance, violet red bile agar is
used to distinguish coliform bacteria such as Escherichia coli from noncoliform organisms. The
coliform bacteria appear as bright pink colonies in this media, while noncoliforms appear a
light pink or clear.
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Document Summary
When microorganisms are cultivated in the laboratory, a growth environment called a mediumis used. The medium may be purely chemical (a chemically defined medium), or it may contain organic materials, or it may consist of living organisms such as fertilized eggs. Microorganisms growing in or on such a medium form a culture. A culture is considered a pure culture if only one type of organism is present and a mixed culture if populations of different organisms are present. When first used, the culture medium should be sterile, meaning that no form of life is present before inoculation with the microorganism. For the cultivation of bacteria, a commonly used medium isnutrient broth, a liquid containing proteins, salts, and growth enhancers that will support many bacteria. To solidify the medium, an agent such as agar is added. Agar is a polysaccharide that adds no nutrients to a medium, but merely solidifies it.
