Cambridge O Level Biology · Syllabus 5090 · Disease and Immunity
Antibiotic
What is Antibiotic?
A medicine that acts against bacteria. Different antibiotics act on structures or processes that bacteria possess, such as the bacterial cell wall or bacterial ribosomes, and which the cells of the person taking them do not share in the same form; they have no effect on viruses.
This definition is part of the Disease and Immunity chapter in Cambridge O Level Biology.
Antibiotic in context
Antibiotics treat bacterial infections. They have no effect on viruses, because viruses do not have the bacterial structures or carry out the independent processes that antibiotics act on. Taking an antibiotic for a viral illness does not help the illness and does add selection pressure for resistant bacteria.
Common mistakes with Antibiotic
- "Antibiotics kill viruses." Antibiotics act on bacterial structures and processes. Viruses have none of them. Write Antibiotics act against bacteria only. Viruses lack the bacterial structures and independent processes that antibiotics target.
- "The person becomes resistant to the antibiotic." People do not become antibiotic-resistant. The bacteria do. Write The population of bacteria becomes resistant, so the antibiotic is less effective against infections caused by that strain.
- "The antibiotic made the bacteria mutate." Mutation is random and happens whether or not the antibiotic is present. The antibiotic selects; it does not cause. Write Random mutation had already produced resistant bacteria; the antibiotic then killed the bacteria that were not resistant.
- "Individual bacteria adapt to survive the antibiotic." An individual bacterium does not change during its lifetime in response to a need. The population changes because different individuals survive at different rates. Write Resistant bacteria survive and reproduce, passing resistance alleles to their offspring, so the resistant strain becomes more common.
Questions students ask about Antibiotic
Is a virus a living organism, and does that matter for the definition of a pathogen?
For O Level purposes, treat a virus as a pathogen. The syllabus definition of a pathogen is "a disease-causing organism", and viruses are named as pathogens throughout the topic. The debate about whether viruses are alive is not examined; what is examined is that a virus is genetic material in a protein coat that can only reproduce inside a host cell, which is why antibiotics do not affect it.
How can antibiotics kill bacteria inside me without harming my own cells?
Because they act on things bacteria have and human cells either do not have or do not have in the same form — a bacterial cell wall, for example, or bacterial ribosomes. Human cells have no cell wall, and their ribosomes differ from bacterial ones. That difference is what makes a selective medicine possible.
Do bacteria become resistant because we take antibiotics wrongly, or would it happen anyway?
It would happen anyway, because mutation is random and continual. What unnecessary or incomplete antibiotic use changes is the rate: each course applies a selection pressure that favours resistant bacteria, so the more often antibiotics are used without need, the faster resistant strains become common.