Vertical Gene Transfer: This is the inheritance of resistance genes from a parent cell to its daughter cells during binary fission. This ensures the trait persists through generations.
Horizontal Gene Transfer (HGT): Bacteria can share genetic material with other individuals, even those of different species. This often occurs via plasmids—small, circular loops of DNA that can be transferred through a process called conjugation.
Mechanisms of Action: Resistance genes may code for proteins that pump the antibiotic out of the cell, enzymes that degrade the drug (e.g., beta-lactamase), or alterations to the drug's target site so it can no longer bind.
| Feature | Normal Bacteria | Resistant Bacteria |
|---|---|---|
| Response to Antibiotics | Growth inhibited or cells killed | Growth continues despite drug presence |
| Genetic Profile | Standard wild-type genome | Contains specific resistance mutations or plasmids |
| Survival Advantage | High in antibiotic-free environments | High in environments where antibiotics are present |
| Clinical Impact | Easily treated with standard medicine | Requires specialized, expensive, or 'last-resort' drugs |
Identify the Selection Pressure: In exam questions, always explicitly state that the antibiotic is the selection pressure that changes the environment.
Sequence the Explanation: Always follow the logical flow: 1. Random mutation exists 2. Antibiotic is applied 3. Non-resistant die 4. Resistant survive and reproduce 5. Resistance gene frequency increases.
Avoid Teleological Language: Never say bacteria 'mutate to survive' or 'try to become resistant.' Mutations are random and occur before the antibiotic is even present; the antibiotic simply selects for those that already have the trait.
Check the Context: If a question mentions 'MRSA,' recognize it as a specific example of a 'superbug'—a strain resistant to multiple common antibiotics.
Misconception: Antibiotics cause mutations: Antibiotics do not create resistance; they only select for pre-existing mutations. The mutation is a random error in DNA replication.
Misconception: Stopping early is fine if symptoms are gone: If a patient stops a course early, the 'weakest' bacteria are dead, but the 'stronger' ones with partial resistance may survive. These survivors then multiply, leading to a fully resistant infection.
Misconception: Humans become resistant: It is a common error to say 'the patient is resistant to penicillin.' Correct terminology is 'the bacterial infection is resistant to penicillin.'
