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Bacteria and antibiotics
Page refs for this section = Textbook p218-225
BACTERIA
Good ref = Textbook p66 labels, p65 functions of the structures.Unit 2 Handbook (part 3) p22-24.
Name the features A to I below
A H
C
G
D
I
E
B
F
Bacterial DNA
• One large circular chromosome• May also contain a small circle of DNA called a
plasmid• Not associated with histones (attached
proteins)
Variation in bacterial DNA
• Occurs by mutation• Change in the organic base sequence in the DNA.• Leads to an alteration in the protein produced and
changes a characteristic/or a reaction by the bacterial cell
• This may allow the bacterial cell to become resistant to an antibiotic.
Antibiotics
Are ........• Substances produced by living organisms that
can destroy or inhibit the growth of microorganisms
Osmotic lysis
• The antibiotic prevents the formation of bacterial cell walls.
• Water can then enter the cell, causing it to swell and burst.
• This is osmotic lysis. The bacterium dies.
Read section on top of HB p23 about MRSA (Methicillin Resistant Staphylococcus aureus)
C NH CH CH C
O
O C N CH
CH3
CH3
COOH
S
The mutation allows the bacterium to make a new protein enzyme called penicillinase which breaks down the β lactam ring structure of the penicillin. This inactivates the penicillin antibiotic.
THE RESULT OF A CHANCE DNA MUTATION LEAD TO SOME POPULATIONS OF BACTERIA BECOMING RESISTANT TO
THE ANTIBIOTIC PENICILLIN The Structure of Penicillin Antibiotic:
HOW ANTIBIOTIC RESISTANCE CAN BE PASSED ON TO MORE BACTERIA
Vertical Gene Transmission – by Bacterial Cell Division ie Asexual reproduction/binary fission
Vertical Transmission
Bacterial reproduction
• Bacteria reproduce by a method called binary fission, which is asexual.
• The first stage involves DNA replication. • Then the cell splits into 2, so that each new
daughter cell produced contains a genetically identical copy of the main circular chromosome and the plasmids.
Horizontal Gene Transmission – by Bacterial Conjugation
Second bacterium receives a copy of the plasmid from the other bacterium.
Conjugation
• A sex pilus forms a temporary link between the donor and recipient bacterial cells.
• A plasmid with a resistance gene(s) is transferred directly to the recipient cell.
• Both bacterial cells now contain a copy of the plasmid and will both now be resistant to a certain antibiotic.
Conjugation in Bacteria with a Plasmid
bacterial DNA plasmid
The plasmid with a resistance gene (R) is in the donor cell initially
R
RR
The plasmid begins to replicate
RR The plasmid is sent
through to the recipient bacterial cell
R R
The cells separate – both species now contain a copy of the plasmid with a gene on it for antibiotic resistance.
A pilus links the 2 bacterial cellsR
Species A Species B
1. In a large enough population of microorganisms a small number are naturally resistant through random genetic variation i.e. via a spontaneous mutation
2. Exposure to antibiotic eliminates the susceptible majority.3. The small number of survivors multiply rapidly in the absence of competition
Evolution of Antibiotic Resistance in Bacteria
4. The population is now more or less 100% antibiotic-resistant
Some bacteria will be resistant to a particular antibiotic Resistant bacteria have a selective advantage in the presence of
the antibiotic
In the presence of the antibiotic
Non-resistant bacteria are inhibited or destroyed by
the antibiotic
Resistant bacteria flourish in the presence of the
antibiotic, because there is little competition for
nutrients. (selected population)
How does resistance develop in a population of bacteria?There will be variation in the population as a result of:
Mutation or Conjugation
TASK: Check your missing words task top of HB p24
ANSWER WORDS in the correct order for the answers:
AlleleKilledAdvantageReproduce
Development of antibiotic
resistance to TB
Why is TB so difficult to treat:1 Treatment for TB involves taking antibiotics for 6-9
months non-stop.2 As patients start to feel better they tend to stop
taking their antibiotics. However, at this point even though the most susceptible TB bacteria have been killed, a few TB bacteria that are more resistant will remain alive in the body.
3 Resistant strains of TB Mycobacterium survive, multiply and can even pass on their alleles for resistance to other strains/species of bacteria by conjugation.
Multiple-antibiotic resistant strains of TB have developed.
Superbugs!
• Bacteria which are resistant to one antibiotic may pass the plasmid containing the resistance gene to another bacterium which is already resistant to a second antibiotic.
• This then multiplies leading to resistance to both antibiotics etc.
How bacteria can become resistant to more than one antibiotic
Bacteria treated with antibiotic A Bacteria treated with antibiotic B
resistant bacterium
resistant bacterium
most bacteria are killed most bacteria are killed
during conjugation the plasmid with the gene for resistance to antibiotic A is copied into the bacterium already resistant to antibiotic Bbacterium resistant to
antibiotics A and B
resistant bacterium multiplies to form a new strain resistant to antibiotics A and B
MRSA• MRSA (methicillin-resistant Staphylococcus aureus)
infections are more difficult to treat due to the antibiotic-resistance of the bacteria
• Some strains are resistant to nearly all antibiotics• It causes a range of illnesses from skin infections to
life-threatening meningitis or septicaemia• It can take time to determine which drugs to use to
treat it, so patients can become very ill
MRSA (cont)
• People in hospitals tend to be weaker and more vulnerable to infection
• Careful attention has to be paid to not transfer infection between patients
A few pictures of MRSA sufferers
Do the application exercise from p222 NT textbook on
If you have a Biology AS revision guide, there is a REALLY EXCELLENT double page spread ON P84-85 to study about the use of antibiotics, the ethical considerations connected with this and antibiotic resistance by bacteria on p84-85. It would be valuable for you to read this!
