Head to www.savemyexams.

com for more awesome resources

Cambridge (CIE) AS Biology Your notes

Antibiotics
Contents
Penicillin
Antibiotic Resistance
Consequences of Antibiotic Resistance

Page 1 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers
 Head to www.savemyexams.com for more awesome resources

Penicillin
Your notes
Penicillin affects bacteria & not viruses
Antibiotics are drugs that kill or stop the growth of bacteria but that do not harm the cells of the
infected organism
Some antibiotics are derived from living organisms, e.g. penicillin is produced by some fungi in the
genus Penicillium, whilst other are made synthetically in a laboratory
Antibiotics work by interfering with the growth or metabolism of the target bacterium, e.g.
synthesis of bacterial cell walls
activity of proteins in bacterial cell surface membranes
bacterial enzyme action
bacterial DNA synthesis
bacterial protein synthesis

The effect of penicillin on bacteria

When a new bacterial cell is growing it secretes enzymes that create small holes in the bacterial cell
wall
These holes allow the bacterial cell wall to stretch as the cell grows, and new peptidoglycan molecules
join up via formation of cross-link attachments
Penicillin stops the formation of cross-links in the cell wall
However, the enzymes keep creating holes in the bacterial cell wall, making the walls weaker and
weaker
As bacteria live in watery environments and take up water by osmosis, their weakened cell walls
eventually burst as they can no longer withstand the pressure exerted on them from within the cell
This means penicillin is only effective against bacteria that are still growing, as autolysins no longer
create holes and no more cross-links form once the growth of a bacterium is complete

Page 2 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers
 Head to www.savemyexams.com for more awesome resources

Your notes

Penicillin prevents the formation of cross-linkages in bacterial cell walls during cell growth, resulting in a
weakened cell wall

Penicillin & viruses

Antibiotics, such as penicillin, do not affect viruses as they do not have cells and therefore cannot be
targeted in any of the ways that an antibiotic targets a bacterial cell
When a virus replicates, it uses the host cell’s mechanisms for transcription and translation, so not even
these processes can be targeted

Examiner Tips and Tricks

Antibiotics are only effective against bacteria, not viruses. For example, cholera and TB can be
treated with antibiotics, whereas HIV cannot, as it is a viral infection.

Page 3 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers
 Head to www.savemyexams.com for more awesome resources

Antibiotic Resistance
Your notes
Resistance to antibiotics
Genetic variation exists between individuals in a bacterial population; this is the result of random
mutation
A chance mutation might give rise to a new allele that provides resistance to an antibiotic
E.g. some pathogenic bacteria have become resistant to penicillin as they have acquired an allele
that codes for the production of an enzyme which breaks down penicillin
When the bacterial population is treated with this antibiotic any bacteria with the resistance allele do
not die
The antibiotic acts as a selection pressure
The resistant bacteria can continue to reproduce with less competition from the non-resistant
bacteria, which are now dead
The allele for antibiotic resistance is passed on with much greater frequency to the next generation
This can occur especially quickly in bacterial populations, due to:
short generation times
all offspring being clones of their parents
horizontal gene transfer
The antibiotic resistance allele increases in frequency, and a higher proportion of the bacterial
population is resistant to the antibiotic
This is an example of evolution by natural selection

Page 4 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers
 Head to www.savemyexams.com for more awesome resources

Your notes

Bacteria evolve rapidly due to their high mutation rate and short generation times; this means that
antibiotic resistance can arise quickly when a population is exposed to a new antibiotic

Humans have caused an increase in the occurrence of antibiotic resistance; this is because treating
infections with antibiotics provides a selection pressure which drives natural selection
This problem is made worse when antibiotics are used improperly, e.g.
For treatment of non-serious infections
For treatment of viral infections
Routine treatment to animals in agriculture

Page 5 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers
 Head to www.savemyexams.com for more awesome resources

Examiner Tips and Tricks

Be careful when describing the development of antibiotic resistance; there are two major pitfalls to Your notes
look out for:
Antibiotic resistance mutations occur by chance, and not as the result of exposure to
antibiotics; you should never say that antibiotics cause mutation
Mutation gives rise to a new resistance allele, and not a new gene

Page 6 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers
 Head to www.savemyexams.com for more awesome resources

Consequences of Antibiotic Resistance

Your notes
Consequences of antibiotic resistance
Antibiotic-resistant strains are a major problem in human medicine; incidence of resistance is
increasing due to antibiotic use in humans and in livestock farming when they are not needed
By using antibiotics frequently, a selection pressure is exerted on the bacteria, which drives
selection of resistant individuals
Commonly prescribed antibiotics are becoming less effective against some bacterial strains
Some strains of bacteria can be resistant to multiple antibiotics, making diseases very difficult to treat
These bacteria are commonly known as superbugs
The most common example is a strain of Staphylococcus aureus that has developed resistance to
an antibiotic called methicillin, and is now known as MRSA (Methicillin-resistant Staphylococcus
aureus)
Infections caused by resistant strains of bacteria often need to be treated using multiple antibiotics,
resulting in complicated and expensive treatments that may not be available to all

Reducing antibiotic resistance & its impact

Ways to prevent the incidence of antibiotic resistance include:
tighter controls in countries where antibiotics are sold without a prescription
doctors prescribing antibiotics only when needed, e.g. antibiotics not being used for viral
infections
avoiding the blanket use of ‘wide-spectrum’ antibiotic, and instead prescribing specific
antibiotics for different types of infection
tighter control of antibiotics in agriculture
The spread of already-resistant strains can be limited by:
ensuring good hygiene practices, such as handwashing and the use of hand sanitisers, especially
in clinical environments
isolating infected patients to prevent the spread of resistant strains, in particular in surgical wards
where MRSA can infect surgical wounds
Scientists need to find new antibiotics to which bacteria have not yet been exposed, as well as
antibiotic alternatives, but this process is expensive and time-consuming

Page 7 of 7
© 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers