How Science Works and A level microbiology
Students who have chosen to study biology in post-16 courses should have substantial knowledge and
understanding of microbiology from their KS4 studies.
At A level there is very little new content specifically relating to microbes but students will study the
structure and functioning of animal and plant eukaryotic cells in detail. This enhanced knowledge can
be easily applied to eukaryotic and prokaryotic microbial cells. Students will also have learnt about the
use of statistics for the objective determination of validity of data.
Microbiologically relevant content of A level courses

Ultrastructure of prokaryotic and eukaryotic cells.

Molecular structure and chemistry of carbohydrates, proteins, lipids and nucleic acids.

The main biochemical pathways, anaerobic and aerobic respiration and photosynthesis.

Mitotic and meiotic cell division in eukaryotic cells.

The genetic control of cellular processes, in particular protein synthesis.

The control of gene expression.

The effects of mutation on the cell and organism.
Microbes involved in such aspects as such as ecological cycles, will also be important. Students are also
expected to be able apply their knowledge and understanding widely across biology.
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Some A level non-aseptic microbiology investigations
Investigation
Factors
affecting the
rate of
anaerobic
respiration of
yeast cells
Exploration of the unknown
- Students can use different
sugars, establishing
whether the yeast initially
has the enzymes to respire
each substrate, or whether
the enzymes are produced
after a period of time due
to gene initiation.
- Students can add salts
such as sodium chloride to
the yeast and sugar
mixture to establish
whether this has an effect
on rate of respiration.
- If the salt has an effect,
they can then alter the ions
used, to establish whether
the effect is specific to an
ion, or whether it is a more
general osmotic effect.
HSW delivery approach
Primary data
Opportunities for class
collection
team work
Students will
Groups can make repeat
need to make
records on:
precise readings
Different sugars.
of carbon dioxide
output, at various
Different
intervals after
concentrations of
mixing yeast with
sugars.
the substrate.
Different salts.
Students will
need to make
sufficient repeat
readings for their
data to be
analysed
statistically.
-
Different alcohols.
-
Different
concentrations of
ethanol / other
alcohols.
-
Different species of
Saccharomyces.
- If the effect is suspected to
be due to osmosis,
osmotically-active
chemicals such as urea
can be compared with
those that have both an
osmotic effect, and can be
absorbed through the
membrane.
Students can add ethanol
or other relevant alcohols,
An effect on respiration
would indicate a possible
end-product inhibitory
effect.
Yoghurt cultures affected by
the addition of various
chemicals such as antibiotics
(as in e.g. TL012e) could be
sampled and the bacteria
stained and viewed under
high-resolution microscopy.
The effects of the chemicals
on the pH changes of the
yoghurt culture can be
related to alterations in
bacterial population density
and structure.
Students will
need to make
careful counts of
the bacteria from
several samples.
If students made
these counts in a
‘double-blind’
manner, this
would allow
greater validity of
the findings.
Alternatively
automated
counting could
be carried out.
-
-
-
-
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Students will
need to use
knowledge and
understanding of
enzymes,
respiration,
genetics and cell
division to
formulate
explanations for
results. Their
findings can be
applied to the
baking and
brewing
industries, and
also have
significance in
gene technology
as yeast cells are
common
eukaryotic host
cells.
In the event of
unexpected
findings, students
could explore
whether animal,
plant and fungal
cells operate
using the same
mechanisms.
-
Factors
affecting the
bacteria
causing
yoghurt to set
(extension of
work in
TL012e, to
include
changes in
cellular
structure)
Applications
A range of chemicals
could be tested by the
different groups.
Students could
investigate different
species of yoghurt
forming bacteria.
Different
concentrations of each
chemical could be
investigated by the
groups.
Samples could be
taken at different
stages of the yoghurt
production process, to
establish whether the
oxygen reduction
phase, or the lactate
formation phase was
more affected.
Bacteria in yoghurt
formed from nonsterile pasteurised
milk could be
compared with those
in sterile milk, to
investigate the effects
of competition.
The findings of
these
investigations
can be related to
cell biology, intra
and interspecific
competition, and
aspects of
disease control.
© CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
A typical non-aseptic post-16 investigation
To compare the effects of penicillin and streptomycin on the bacteria in
yoghurt cultures

Yoghurt cultures were made using skimmed UHT milk with added antibiotic discs, as described in
TL012e.

The yoghurt cultures containing 0.9 g per 100 cm penicillin or 125 g per 100 cm streptomycin
failed to set, and demonstrated little change in pH during formation.

Samples of non-setting yoghurt cultures containing antibiotics were Gram stained (GL95), and
compared with a yoghurt culture that had no added antibiotics.
Appearance of the yoghurt culture bacteria after Gram staining
Culture
40 objective lens
100 objective lens
(scale bar = 500 m)
(scale bar = 100 m)
Comment
There are numerous bacteria
Yoghurt culture
without antibiotics
evident. These seem to associate
in chains of 3 - 8 bacteria.
There seem to be a similar
number of bacteria to the control
culture, but these are associated
Yoghurt cultured
with streptomycin
in longer chains, up to 12
bacteria. There appear to be very
few individual bacteria; The 40
photo shows large clumps of
what appear to be tangled
bacterial chains.
There are much lower numbers
of bacteria present.
Yoghurt cultured
The bacteria seem to be single or
with penicillin
in a very short chains of 2 - 3
bacteria.
There are few clumps present.
Conclusions
The results demonstrate that penicillin and streptomycin both affect the bacteria in the yoghurt and
provide an explanation for why yoghurt manufacturers insist on using only antibiotic-free milk.
TL12f 09/11
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© CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
Some aseptic A level microbiological investigations
Investigation
Factors
controlling the
lac-operon gene
complex
Exploration of
the unknown
Students could
investigate a
range of
chemicals that
activate the
gene complex in
E.coli and other
bacterial cells.
Factors
affecting the
reproduction of
bacterial viruses
(phages)
As phages have
a very simple
structure
containing
largely DNA and
a protein
envelope,
students can
investigate the
direct effects of
chemical and
physical factors
on the viral
genes. This is
very close to
investigating the
intracellular
control of genes.
Comparing the
effects of
antimicrobial
chemicals on
prokaryotic and
eukaryotic
microbes
Students make
plates that
contain
antimicrobial
chemicals in a
gutter.
A range of
different
microbes are
streaked across
the gutter, so
that sensitivity
can be
estimated.
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HSW delivery approach
Significant primary data Opportunities for class
collection
team work
Different groups could
Novice technique:
investigate:
Students use aseptically
inoculated bacterial broth - Different sugars,
comparing
cultures, adding a range
effectiveness with that
of chemicals to the
of lactose, those
bacteria. The ONPG
which have stearic
coloration in each broth
similarities to lactose
culture could be
would be particularly
measured using a
interesting.
colorimeter.
- Different
A number of repeats
concentrations of
should be carried out,
lactose.
and statistical
significance determined if - Different strains of
E.coli.
possible.
- Other Gram negative
bacteria in
Competent technique:
comparison with
Students who have full
E.coli.
competence will often
- Testing cultures that
produce more reliable
have been incubated
results as their cultures
with lactose for
will not be contaminated.
different periods.
- Glucose and
galactose effects on
lactase production.
Groups of students could
Novice technique:
investigate pure phage
Students measure the
cultures using:
reduction in turbidity of
- Different bacterial
broth cultures containing
species / strains, for
phages and their host
susceptibility to a
bacteria. The cultures
phage species.
can be exposed to
- Different chemical
different conditions
treatments added to
and/or chemicals.
the phage solution
before inoculation.
Competent technique:
- Different temperature
Students make overlay
or pH regimes that
plates, as described in
the phages could be
GL 97, and count
exposed to before
plaques to determine
inoculation into the
phage populations
bacterial culture.
accurately.
- The above
Several repeats of each
investigations could
culture need to be made
be carried out after
for reliable data.
the phage and
bacteria are mixed, to
determine the factors
affecting invasion of
the bacteria by the
phage.
The method for making
Different groups could
gutter plates is described investigate:
in GL 96. Several repeat
- Different species of
plates need to be
microbes.
produced, for reliability.
- Gutters containing.
The distance between
different chemicals or
the microbial growth and
different combinations
the edge of the gutter
of chemicals.
needs to be measured as - Different potential
accurately as possible.
sources of antimicrobial chemicals,
such as plant
extracts.
Applications
Students will need
to apply their
understanding of
gene initiation and
suppression
mechanisms to
explain their
results.
Students could
also relate their
findings to the
control of
oncogenes, and
therefore cancer
treatment.
The students can
apply their results
to their knowledge
and understanding
of the spread of
viral diseases. This
can be related to
the specificity of
the virus and to
human diseases,
for which some
populations seem
to have increased
susceptibility. They
can also gain an
understanding of
the effectiveness of
heat treatment and
chemicals in
destroying viruses.
Students should
streak both
prokaryotic and
eukaryotic
microbes on the
same plate, and
relate their findings
to the different
structures and
biochemistry of the
microbes.
© CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
A typical A level investigation requiring aseptic
technique
Investigating the susceptibility of different strains of E.coli to the T4(B) phage
For the following plates, the T4 (B) phage was mixed with E.coli cultures of different strains, and plated
on top of nutrient agar on one side of a compartmented plate. In the other side of the compartmented
plate, the overlay agar contained only the host bacterium. The plates were then incubated at 25 °C for
a week. The pictures below show example plates after incubation, the compartment with phages is
shown on the top. The plaques (where the bacteria have been destroyed by the phage) can be seen as
‘holes’ in the bacterial lawn. Some examples are marked by arrows.
Phage plate with E.coli (B)
Phage plate with E.coli (K12)
There are large numbers of plaques on the upper
plate, indicating that E.coli (B) is very susceptible
to the phage.
There are only six plaques on the upper
compartment of the plate, indicating that E.coli
(K12) is not very susceptible to the phage.
Conclusions
These results indicate that E.coli (B) bacteria is more susceptible to attack from the phage than is
E.coli (K12). It might be interesting for the students to consider the potential mechanisms for this
difference.
TL12f 09/11
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© CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk