Today we are covering from the specification:

Starter1. What are antibiotics used for?2. Who can obtain antibiotics?3. When shouldn’t antibiotics be given?4. 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

• 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.

• Suggest what your results show (e.g. which is the ‘best’ antibiotic for killing each type of bacteria?).

Questions

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 prevents 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.

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.

• 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 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.

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

Plasmid is replicated and passed to second bacterium

Pilus/conjugation tube

Both bacteria are now resistant

Antibiotic resistance

• Mutations are not only passed on vertically but can also be passed on via horizontal gene transfer.

• 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.

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:

• Read the “Application” section on page 224 and answer questions 1 & 2.

• Read the “Application” section on page 225 and answer question 1.

12 minutes