Today we are covering from the specification:
Starter
1.
2.
3.
4.
What are antibiotics used for?
Who can obtain antibiotics?
When shouldn’t antibiotics be given?
What does “resistant” mean?
What are antibiotics?
• Antibiotics are substances which can inhibit
growth or destroy bacteria.
• They are produced by other living organisms,
e.g. fungi.
Experiment to assess the efficiency of
antibiotics in killing bacteria
- 3 different bacterial cultures in liquid culture
medium (you will use one)
- 2 different antibiotics, one of which has 2 different
strengths
As you complete the practical, make a note of the
method you use.
Watch the demonstration carefully!
Homework
Make a note of the method we
used today.
Bring it to the lesson on Monday.
Results – Lab Book
• Draw a labelled diagram of your agar plate.
• Measure the diameter of each clear area (the
zone of inhibition).
• Collect results from groups that have tested
the other species of bacteria.
Questions
1. Describe how you used aseptic technique to
ensure your agar plate didn’t become
contaminated with other microorganisms.
2. Suggest what your results show (e.g. which is
the ‘best’ antibiotic for killing each type of
bacteria?).
Discovery of Antibiotics
How do antibiotics work?
There are two types of antibiotic:
• Bacteriostatic – which inhibit growth of
bacterial cells.
• Bactericidal - which directly destroy bacterial
cells.
Broad and narrow spectrum antibiotics
Some antibiotics are able to be used against a wide range
of pathogens – “broad spectrum”. Others are very specific
in their targets and are said to be “narrow spectrum”.
Broad spectrum
e.g. tetracycline
Enter the bacterial cell and
interfere with the metabolism
e.g. protein synthesis.
This will prevent the cell
from functioning and growing
or reproducing.
Narrow spectrum
e.g. penicillin
Will act on specific
processes only found in
some bacteria e.g. cell wall
formation.
Bacterial cell wall synthesis
• Bacterial cells, like
plant cells,
constantly have an
influx of water via
osmosis.
• Due to the rigid cell
wall of bacterial cells
they are able to
withstand expansion
and prevent further
entry of water.
Bacterial cell walls
& bactericidal antibiotics
• Penicillin is a bactericidal antibiotic.
• Inhibits an enzyme involved in cell wall
formation.
• No protection against excess water entering
by osmosis.
• Leads to osmotic lysis, killing the bacterium.
• Only effective against bacterial cells which are
growing and use a specific pathway for cell
wall formation.
Why don’t antibiotics work against
viruses?
• Penicillin and other antibiotics work to inhibit
cell wall formation or to disrupt other cell
processes.
• Viruses are non-living and do not have the
same structure as bacteria; these processes
don’t take place and therefore can’t be
disrupted.
Testing the effectiveness of antibiotics
Antibiotic-sensitivity testing. Petri dishes were spread-inoculated
with Staphylococcus albus (white growth) or Micrococcus luteus
(yellow growth) before antibiotic assay "rings" were placed on the
agar surface. The coloured disks at the end of each spoke of the
rungs are impregnated with different antibiotics. Clockwise from the
top (arrow) these are: Novobiocin, Penicillin G, Streptomycin (white
disk), Tetracycline, Chloramphenicol, Erythromycin, Fusidic acid
(green disk) and Methicillin. Clear zones of suppression of
bacterial growth around the individual antibiotic disks are evidence
of sensitivity to these antibiotics.
To do:
• Read through the information on pages 188191 of the textbook.
• Answer the questions in the yellow boxes on
pages 192-193.
Today we are covering from the specification:
Antibiotic resistance
• Some strains of bacteria are resistant to
antibiotics (e.g MRSA – methicillin-resistant
Staphylococcus aureus)
• This means that use of antibiotics against that
strain will be ineffective.
Antibiotic resistance
• Antibiotic resistance can arise from a mutation
in the bacterial DNA.
• Most often occurs in a plasmid.
• The mutation may cause the bacteria to
produce an enzyme that blocks the action of
the antibiotic, or breaks it down.
Example:
• A mutation occurrs in a Streptococcus
pneumoniae bacterium.
• This mutation means that the bacterial cell is
able to synthesise a new protein.
• The new protein is an enzyme which is able to
break down the penicillin before it takes
effect.
• The enzyme is called penicillinase.
• The S. pneumoniae bacterium is now resistant
to penicillin.
How do bacteria reproduce?
• Asexually, via binary fission.
• This does not lead to an increase in variation.
• Asexual reproduction also means that the
DNA is not altered as it is via sexual
reproduction.
• Therefore bacterial cells have another way of
passing genetic information between one
another.
This means that bacteria can pass on genes for
antibiotic resistance to their offspring.
This is vertical transmission.
Bacterial
DNA
Plasmid
containing
resistance
gene
Antibiotic resistance
• Mutations are not only passed on vertically but
can also be passed on via horizontal gene
transmission.
Plasmid
containing
resistance
gene
Bacterial
DNA
Bacterium with no antibiotic
resistance genes
Antibiotic resistance
Plasmid is replicated and
passed to second bacterium
Pilus/
conjugation
tube
Antibiotic resistance
Both bacteria now contain a copy of the
plasmid which contains an antibiotic
resistance gene.
Antibiotic resistance
• Conjugation between bacterial species also
means that antibiotic resistance is not
contained to one species.
• This also means that bacterial cells can now
also be resistance to a range of different
antibiotics.
The role of natural selection
What is natural selection?
-
Mutation
Variation
Competition
Best adapted organisms survive
They are able to pass on their beneficial alleles.
How does this apply to antibiotic resistance in
bacteria?
Natural Selection and Antibiotic
Resistance
To Do:
• “MRSA” Worksheet
• Read the two passages
first.
• Answer all of the
questions in as much
detail as you can.
• You may write on the
sheet.
17 minutes
Resistance in TB
• Mycobacterium tuberculosis is a causative
agent of TB.
• Antibiotic treatment of TB is challenging as a
course of treatment can last 6-9 months.
• Initially, weak strains of the M.tuberculosis
are killed off.
• Some patients then feel better and cease
their antibiotics.
Resistance in TB
• This means that stronger and more resistant
strains of M.tuberculosis are able to reproduce
with less competition.
• These strains are then more likely to be spread
to others.
• They are also likely to take part in conjugation
and become resistant to multiple antibiotics.
• To overcome this problem, sufferers of TB are
given a cocktail of 4 different antibiotics.
To Do:
Check that you understand the topic we have
just covered. Use the textbook to help if
necessary. Ask if you still don’t understand!
Answer the questions about multi-drug resistant
TB on the sheet.
Bring your work to hand in next lesson.