Exercise 8:
Growth media
Student Learning Objectives:
Understand the difference between selective & differential media.
Observe & record colony formation from different types of media.
Learn about normal flora.
Collect data which will be useful for identifying unknown bacteria.
Pre-lab questions
1. Name, identify, and describe the ingredients and functions of 3 different examples of growth
media (specific names of media are listed in the introduction and the text).
Mannitol salt agar (MSA) contains 7.5% NaCl which is selective for Staphylococci and inhibits
Streptococci. MSA is also differential for microbes which ferment mannitol, a carbohydrate. MSA
contains the indicator phenol red which turns yellow if the microbe(s) ferments mannitol. Blood
agar is an enriched media which contains 5% sheep blood. The blood also is differential on the
basis of the ability of an organism to hemolyze red blood cells. Alpha hemolysis is the incomplete
lysis or breakdown of the red blood cells and this appears as a green halo around the colony. It is
helpful to hold the plates up the light to see these effects. Beta hemolysis is the complete lysis of
red blood cells which appears as a clear zone around the colonies. MacConkey agar (MAC)
contains crystal violet and bile salts which inhibit the growth of Gram positive bacteria. The
crystal violet also make MAC a differential media because the bacterial colonies appear pink
when there is positive lactose fermentation and colorless when no lactose fermentation occurs.
Thioglycollate (Thio) is a liquid media used to determine the oxygen tolerance of bacteria.
Depending on the supplier, this media contains nutrients to allow bacteria to grow along with
sodium thioglycollate, L-cystine, thioglycollic acid, methylene blue 1 (blue) or resazurin2 (pink) and
0.05% agar. The sodium thiogycollate prevents high concentrations of oxygen from remaining in
the media by changing it to water. The agar serves as a physical barrier preventing the oxygen
from readily diffusing deep into the media. The methylene blue or resazurin are pH indicators. If
oxygen is present is will be blue (methylene blue) or pink (resazurin) near the top of the media.
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, 2. What are the three types of hemolysis and explain each?
Alpha hemolysis is the incomplete lysis or breakdown of the red blood cells. Beta hemolysis is
the complete lysis of red blood cells. Gamma hemolysis is when there is no lysis of the red blood
cells.
3. What is a colony?
A colony is a visible mass of cells grown from a single microbial cell which has divided many
times.
4. What is normal flora and where are they found?
Normal flora includes microbes that colonize our bodies and are necessary for good health.
Normal flora is present on the human body. Two areas it can be found are on the skin and throat.
Introduction
Many different types of growth media are used for growing out microorganisms. Because of the
microscopic nature of these microbes, microbiologists frequently find it easier to identify microbes
by growing them out into macroscopic colonies. A colony is a visible mass of cells grown from a
single microbial cell which has divided many times. Some media is called undefined or complex
media because the exact composition of the media is unknown. This media comes from a variety of
sources like plant and animal tissues, and it is usually less expensive.
Most media which we will use in the microbiology laboratory is called chemically defined media
because all the chemicals used are known and measured and it contains appropriate amounts of
nutrients necessary for microbial growth. The most common growth media for microbes are nutrient
broths and agar plates. There are special types of media used for specific purposes.
Enriched media is media that contains an increased concentration of specific nutrients used to
encourage more growth of specific microbes. For example, Blood agar is an enriched medium which
contains approximately 5% sheep blood.
Differential media is another type of media which distinguishes one type of microbe from another
on the same media through the use of special nutrients, chemicals or indicators which change color
during specific metabolic processes. Examples include eosin methylene blue (EMB) agar which
contains dyes that precipitate to produce a metallic green sheen during lactose and sucrose
fermentation. Another example is mannitol salt agar (MSA) which contains phenol red which turns
yellow during mannitol fermentation. Selective media is used for the growth of specific types of
organisms because it contains chemicals which inhibit the growth of other types of organisms.
MacConkey agar (Mac) is selective for the growth of Gram negative microbes and contains crystal
violet and bile salts which inhibit the growth of Gram positive microbes. In this laboratory exercise
plates which contain many types of media inoculated with bacterial growth will be examined and the
results recorded. It will be important for you to examine and record your observations accurately
and thoroughly as this data will help in the identification of your unknown. Human body samples will
also be used to inoculate several specific types of media. Species of Streptococcus and
This study source was downloaded by 100000898182462 from CourseHero.com on 04-26-2025 09:01:27 GMT -05:00
https://www.coursehero.com/file/24204619/GrowthMediaLab-13-LHdocx/
Growth media
Student Learning Objectives:
Understand the difference between selective & differential media.
Observe & record colony formation from different types of media.
Learn about normal flora.
Collect data which will be useful for identifying unknown bacteria.
Pre-lab questions
1. Name, identify, and describe the ingredients and functions of 3 different examples of growth
media (specific names of media are listed in the introduction and the text).
Mannitol salt agar (MSA) contains 7.5% NaCl which is selective for Staphylococci and inhibits
Streptococci. MSA is also differential for microbes which ferment mannitol, a carbohydrate. MSA
contains the indicator phenol red which turns yellow if the microbe(s) ferments mannitol. Blood
agar is an enriched media which contains 5% sheep blood. The blood also is differential on the
basis of the ability of an organism to hemolyze red blood cells. Alpha hemolysis is the incomplete
lysis or breakdown of the red blood cells and this appears as a green halo around the colony. It is
helpful to hold the plates up the light to see these effects. Beta hemolysis is the complete lysis of
red blood cells which appears as a clear zone around the colonies. MacConkey agar (MAC)
contains crystal violet and bile salts which inhibit the growth of Gram positive bacteria. The
crystal violet also make MAC a differential media because the bacterial colonies appear pink
when there is positive lactose fermentation and colorless when no lactose fermentation occurs.
Thioglycollate (Thio) is a liquid media used to determine the oxygen tolerance of bacteria.
Depending on the supplier, this media contains nutrients to allow bacteria to grow along with
sodium thioglycollate, L-cystine, thioglycollic acid, methylene blue 1 (blue) or resazurin2 (pink) and
0.05% agar. The sodium thiogycollate prevents high concentrations of oxygen from remaining in
the media by changing it to water. The agar serves as a physical barrier preventing the oxygen
from readily diffusing deep into the media. The methylene blue or resazurin are pH indicators. If
oxygen is present is will be blue (methylene blue) or pink (resazurin) near the top of the media.
This study source was downloaded by 100000898182462 from CourseHero.com on 04-26-2025 09:01:27 GMT -05:00
https://www.coursehero.com/file/24204619/GrowthMediaLab-13-LHdocx/
, 2. What are the three types of hemolysis and explain each?
Alpha hemolysis is the incomplete lysis or breakdown of the red blood cells. Beta hemolysis is
the complete lysis of red blood cells. Gamma hemolysis is when there is no lysis of the red blood
cells.
3. What is a colony?
A colony is a visible mass of cells grown from a single microbial cell which has divided many
times.
4. What is normal flora and where are they found?
Normal flora includes microbes that colonize our bodies and are necessary for good health.
Normal flora is present on the human body. Two areas it can be found are on the skin and throat.
Introduction
Many different types of growth media are used for growing out microorganisms. Because of the
microscopic nature of these microbes, microbiologists frequently find it easier to identify microbes
by growing them out into macroscopic colonies. A colony is a visible mass of cells grown from a
single microbial cell which has divided many times. Some media is called undefined or complex
media because the exact composition of the media is unknown. This media comes from a variety of
sources like plant and animal tissues, and it is usually less expensive.
Most media which we will use in the microbiology laboratory is called chemically defined media
because all the chemicals used are known and measured and it contains appropriate amounts of
nutrients necessary for microbial growth. The most common growth media for microbes are nutrient
broths and agar plates. There are special types of media used for specific purposes.
Enriched media is media that contains an increased concentration of specific nutrients used to
encourage more growth of specific microbes. For example, Blood agar is an enriched medium which
contains approximately 5% sheep blood.
Differential media is another type of media which distinguishes one type of microbe from another
on the same media through the use of special nutrients, chemicals or indicators which change color
during specific metabolic processes. Examples include eosin methylene blue (EMB) agar which
contains dyes that precipitate to produce a metallic green sheen during lactose and sucrose
fermentation. Another example is mannitol salt agar (MSA) which contains phenol red which turns
yellow during mannitol fermentation. Selective media is used for the growth of specific types of
organisms because it contains chemicals which inhibit the growth of other types of organisms.
MacConkey agar (Mac) is selective for the growth of Gram negative microbes and contains crystal
violet and bile salts which inhibit the growth of Gram positive microbes. In this laboratory exercise
plates which contain many types of media inoculated with bacterial growth will be examined and the
results recorded. It will be important for you to examine and record your observations accurately
and thoroughly as this data will help in the identification of your unknown. Human body samples will
also be used to inoculate several specific types of media. Species of Streptococcus and
This study source was downloaded by 100000898182462 from CourseHero.com on 04-26-2025 09:01:27 GMT -05:00
https://www.coursehero.com/file/24204619/GrowthMediaLab-13-LHdocx/
