How resistance develops
Within any bacterial population, there is natural genetic variation. Some bacteria carry mutations in their DNA that happen to give them resistance to a particular antibiotic. When the antibiotic is applied, it kills the non-resistant bacteria. The resistant individuals survive and face less competition for resources.
The resistant bacteria reproduce rapidly, passing the resistance gene to their offspring. Over many generations, the proportion of resistant bacteria in the population increases. This is natural selection: the antibiotic acts as the selection pressure.
Why this is a natural selection question
Students must avoid saying that bacteria "become resistant" as if the drug causes the change. The variation already exists before the antibiotic is applied. The antibiotic does not create resistance; it selects for bacteria that already have it.
The correct sequence for an exam answer is: variation exists in the population; the antibiotic provides a selection pressure; resistant bacteria have a survival advantage; they reproduce and pass on the resistance allele; the frequency of resistance increases in the population over time.
MRSA and superbugs
MRSA (methicillin-resistant Staphylococcus aureus) is a well-known example of an antibiotic-resistant bacterium. It is resistant to several common antibiotics, making infections very difficult to treat. MRSA is particularly dangerous in hospitals where patients may have weakened immune systems and open wounds.
Other resistant strains include multi-drug-resistant tuberculosis (MDR-TB). The emergence of bacteria resistant to multiple antibiotics is sometimes called the "superbug" problem and is considered one of the most serious threats to global health.
Reducing the spread of resistance
Measures to slow antibiotic resistance include: completing the full course of prescribed antibiotics so that all bacteria are killed; not using antibiotics for viral infections; reducing antibiotic use in agriculture; developing new antibiotics; and improving hygiene in hospitals to prevent the spread of resistant strains.
At Supplement level, students should be able to explain why each measure works in terms of natural selection. For example, completing the course removes all bacteria, leaving none with partial resistance to survive and reproduce.