TEACHER’S NOTES
3.
Investigating metabolism in microorganisms
(a)
Environmental control of metabolism
(i)
Culture conditions
Links to prior/prerequisite knowledge
Students should be familiar with:
 cell structure at a basic level
 cell ultrastructure and function of organelles (Unit 1, Higher)
 the different types of microorganisms
 why microbes are used in laboratory investigation
 the range of products that can be made from microbes .
Matched to CfE outcomes at levels 3 and 4:
 SCN 3-13a
 SCN 3-13b
 SCN 4-13b
 SCN 4-13c
New content areas
 Composition of various growth mediums.
 The effect of environmental conditions on growth .
 Methods of controlling environmental conditions .
 Importance of sterile conditions and sterilisation techniques.
Background information
 Prokaryotes are organisms that lack a nucleus and any cell organelles.
Most are unicellular.
 Eukaryotes have a nucleus, and are more complex than prokaryotes as they
have many cell organelles such as rough endoplasmic reticulum and Golgi
apparatus. These cells often make up multicellular organisms.
 The five types of microorganisms are:
Bacteria: A taxonomic system of classification of bacteria can be found in
Bergey’s Manual of Systematic Bacteriology. Using this classification,
bacteria are divided into four divisions, thre e of which are eubacterial cells,
with the fourth being the archaea. Each division is further divided until the
species level is reached. The members of a bacterial species are similar to
each other but can be distinguished from other species on the basis of several
characteristics. Different types within a species are called strains (groups of
different cells derived from a single cell). Strains may be identified by
numbers, letters or names, for example E. coli 0157 or E. coli 0111. All
METABOLISM IN MICROORGANISMS (H, BIOLOGY)
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TEACHER’S NOTES
bacteria share one characteristic: they are all prokaryotic cells. A range of
tests can be used to distinguish between bacteria: Gram stain, motility tests,
oxygen requirements etc.
Fungi: Fungi are eukaryotic cells and can be divided into yeasts and moulds.
Fungi are useful as decomposers but are a major cause of disease in plants.
They are also responsible for some disease in animals but are most often
known for their role in the fermentation processes that produce wine, beer,
bread and antibiotics. Yeasts are single-celled fungi whereas moulds are
multicellular.
Protozoa: These eukaryotic cells have a variety of shapes but lack a cell wall.
At some point in their lifecycle they are motile. An individual cell is a
complete organism containing all the organelles that are needed for life.
Examples include amoeba and euglena. Many protozoa cause diseases such as
leishmaniasis and toxoplasmosis.
Viruses: Often called obligate intracellular parasites. They must be inside a
living cell to reproduce. They have a protein coat (capsi d) and a nucleic acid
core (which can be DNA or RNA).
Algae: Eukaryotic plant cells that can be either unicellular or multicellular
organisms. They differ in their colour and the photosynthetic pigments that
are present. They live in a variety of habitats . (Student activity 1: Prokaryotes
and eukaryotes)
 Microbes are grown on culture media. This can be a solid or liquid
preparation made to ensure the microbe has all the nutrients it needs for
growth. The media is sterilised before use.
Complex media contain one or more crude sources of nutrients. Often their
exact chemical composition and components are unknown.
Defined media are also called synthetic media. The components of this
media are known and are relatively pure.
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Type of medium
Example
Extra
constituents
Bacteria grown
Complex
Nutrient agar
Meat extracts,
yeast extract
Many bacteria will
grow on this
Defined
M9
Enriched
Blood agar
Blood
Selective
MacConkey
agar
Bile salts and
Gram –ve bacteria
crystal violet dye
Escherichia coli
METABOLISM IN MICROORGANISMS (H, BIOLOGY)
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Streptococcus
pyogenes
TEACHER’S NOTES
 Not all microbes need oxygen, in fact for some microbes exposure to
oxygen can be fatal! A microbe that must have oxygen present in order to
extract energy from foodstuffs is called an obligate aerobe, ie it needs
oxygen for respiration. An organism that would die in the presence of
oxygen is an obligate anaerobe. This is because these organisms lack the
enzymes needed to break down the dangerous chemicals (peroxides) made
in aerobic reactions. Some microbes (especially yeast) can grow in the
presence or absence of oxygen. These are facultative anaerobes. They
grow better in the presence of oxygen. In fermenters, the actual process of
delivering oxygen can be quite tricky. Piping may need to be used or
stirrers may be placed in the fermenter to ensure that the oxygen is
available to all the microbes.
 All microbes have a range of temperatures within which they are able to
grow. However, within this range there will be a temperature at which the
organism grows best: its optimum temperature. Growth occurs due to
chemical reactions and since enzymes are often involved, temperature will
have to be tightly controlled. Some species of microbes live in extremes of
temperature and can survive and grow despite the temperature.
Psychrophiles are found in oceans at the north and south poles. They are
adapted to live in very cold temperatures. Mesophiles are microbes that
live at ‘normal’ temperatures, eg room temperature. They live on the
human body. Thermophiles are microbes that can live in very hot
environments, for examples volcanic vents, compost heaps, hot springs
(Student activity 3, Practical activity 1)
 As with temperature, microbes have an optimum pH. Fungi tend to prefer a
more acidic environment (pH 4–6).
Bacteria like a neutral pH. Vinegar, citric acid and other chemicals can be
used to preserve food because they stop microbe s from growing. Buffers
can be used to keep the pH relatively constant.
 How to prevent contamination.
- Heat sterilisation: This involves using an autoclave. This sealed de vice
allows the entrance of steam under pressure .
The temperature used is 121ºC for 10–15 minutes. It is not the pressure
in the autoclave that causes the death of the microorganisms but the
high temperature achieved when the steam is placed under pressur e.
This procedure also ensures that bacterial spores are eliminated .
- Pasteurisation: The process used to reduce the microbial population in
milk and other heat-sensitive products. Named after Louis Pasteur , who
first used heat for controlling the spoilage of wine. Pasteurisation
involves passing milk through a heat exchanger. The temperature of the
milk is raised to 71°C for 15 seconds. The milk is then rapidly cooled.
- Radiation sterilisation: The number of microbes can be significantly
reduced by using electromagnetic radiation. UV waves, for example,
METABOLISM IN MICROORGANISMS (H, BIOLOGY)
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TEACHER’S NOTES
break down the DNA molecules in the microbe, thus causing death.
Microwaves use thermal effects to kill bacteria .
- Filter sterilisation: Membrane and depth filters can be used. This
method of sterilisation is used for media that are sensitive to heat.
- Using chemical agents: Chlorine is used in air-conditioning systems to
prevent the growth of bacteria such as Legionella.
- Using disinfectants and antiseptics: Disinfectants are chemicals that
kill microorganisms. They should be used on inanimate objects.
Antiseptics are chemical agents that kill or inhibit the growth of
microorganisms but are non-toxic enough to be applied to living tissue.
(Student activities 2, 3 and 4, Culturing Microbes PowerPoint, Yeast
growth practical pdf)
Identification of key concepts
 Factors that affect microbial growth include:
- contamination
- growth media
- availability of oxygen
- temperature
- pH.
Identification of particular areas of difficulty
 Challenging vocabulary to cope with, including psychrophiles, mesophiles,
thermophiles.
Links to websites, animations, PowerPoints, audio or video files etc
Resources made available:
 PowerPoint to introduce/review this section.
 Student activity to complement culturing conditions PowerPoint.
- Two research activities to add notes to the culture conditions
PowerPoint
- Practical: effect of temperature on microbial growth
For this practical students will need to know about aseptic technique s.
For purchase of microbes, try Blades Biological.
- NCBE Practical on yeast growth rates
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METABOLISM IN MICROORGANISMS (H, BIOLOGY)
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TEACHER’S NOTES
Other useful information to stimulate interest
 Useful textbooks:
- Biology by Campbell
- Also, Brock Biology of Microorganisms by Madigan, Martinko and
Parker (8th edition)
- Standard Grade Biology by James Torrance. Aseptic technique work.
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© Learning and Teaching Scotland 2011
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