Vertical Gene Transfer: This involves the passing of resistance genes from a parent bacterium to its progeny during binary fission. Spontaneous mutations in the bacterial genome provide the initial source of resistance.
Horizontal Gene Transfer (HGT): This is the primary driver of rapid resistance spread between different bacterial cells, even across different species. It occurs through three main pathways: Conjugation (direct cell-to-cell contact via a pilus), Transformation (uptake of free DNA from the environment), and Transduction (transfer via bacteriophages).
Plasmids: These are small, circular, extra-chromosomal DNA molecules that often carry multiple resistance genes. Because plasmids can be easily shared via conjugation, they allow for the rapid development of multi-drug resistant 'superbugs'.
Innate vs. Acquired: Innate resistance is a natural property of a species (e.g., a drug being too large to enter a specific porin), while acquired resistance results from a new mutation or the gain of foreign DNA.
Bactericidal vs. Bacteriostatic: Bactericidal antibiotics kill bacteria directly, whereas bacteriostatic antibiotics merely inhibit growth, relying on the host's immune system to clear the remaining infection.
| Feature | Conjugation | Transformation | Transduction |
|---|---|---|---|
| Mechanism | Physical bridge (Pilus) | DNA uptake from environment | Viral vector (Phage) |
| Requirement | Living donor and recipient | Competent recipient cell | Bacteriophage infection |
| DNA Type | Usually Plasmids | Short DNA fragments | Bacterial DNA in viral coat |
Identify the Mechanism: When presented with a scenario, look for keywords. If the drug is 'spat out,' it is an efflux pump. If the drug is 'cut' or 'broken,' it is enzymatic degradation.
Check the Transfer Method: If the question mentions a 'pilus' or 'mating,' the answer is always conjugation. If it mentions 'viruses' or 'phages,' it is transduction.
Analyze the Population: Remember that antibiotics do not cause mutations; they select for them. In exam questions, avoid saying the drug 'makes' the bacteria change; instead, say the drug 'eliminates susceptible strains.'
Verify the Scope: Ensure you distinguish between human cells and bacterial cells. Humans do not become 'immune' to antibiotics; the bacteria within them become 'resistant' to the drugs.
Misconception: Humans develop resistance: A common error is stating that a patient has become resistant to a drug. In reality, it is the bacterial population causing the infection that has evolved resistance; the patient's own genetics remain unchanged regarding drug efficacy.
Misconception: Antibiotics treat viruses: Antibiotics target specific bacterial structures (like cell walls or 70S ribosomes) that viruses do not possess. Using antibiotics for viral infections like the flu provides no benefit and only increases selection pressure for resistant bacteria.
Pitfall: Stopping treatment early: Patients often stop taking antibiotics once symptoms improve. This is dangerous because it may leave behind a sub-population of 'partially resistant' bacteria that were not yet killed, allowing them to multiply and cause a fully resistant secondary infection.
