Questions: Antibiotic Resistance: Mutations and Gene Regulation

Question 1

A patient takes a fluoroquinolone for a week, and a fluoroquinolone-resistant strain of bacteria is isolated at the end of treatment. What is the most accurate account of how this resistance arose?

• A. The fluoroquinolone induced DNA damage in the bacteria, increasing the mutation rate and generating resistant variants
• B. The bacteria sensed the antibiotic and upregulated their mutation machinery as a stress response
• C. Rare pre-existing mutants with altered DNA gyrase survived while susceptible cells were killed; the antibiotic selected for — not caused — resistance
• D. The antibiotic directly modified the bacterial DNA, inadvertently mutating the gyrase gene in some cells

Answer: (C)

This is the fundamental misconception this topic addresses. Antibiotics do not cause resistance mutations — they select for pre-existing rare mutants. In any large bacterial population, mutations occur spontaneously at low frequency during normal replication. A mutation that alters the fluoroquinolone-binding pocket of DNA gyrase exists at low frequency before treatment begins. Antibiotic treatment kills all susceptible cells, leaving only the rare resistant mutant as the sole survivor, which then replicates to fill the niche. The antibiotic is the selective agent, not the mutagen.

Question 2

A regulatory mutation increases expression of an efflux pump. How does this confer antibiotic resistance without changing the drug target?

• A. The pump degrades the antibiotic into inactive fragments before it can bind its target
• B. The pump expels antibiotics from the cell faster than they enter, keeping intracellular concentrations below the level needed to inhibit the target
• C. The pump modifies the antibiotic chemically, reducing its affinity for the target
• D. Increased pump expression titrates the antibiotic away from the target by binding it in the cytoplasm

Answer: (B)

Efflux pumps are membrane transport proteins that actively expel small molecules from the cell. By increasing pump expression (through a regulatory mutation in a promoter or repressor), the cell removes antibiotics faster than they accumulate, keeping the intracellular concentration below the minimum inhibitory concentration. The drug target itself is unchanged — it is just never reached in sufficient concentration to be inhibited. This is why efflux-mediated resistance often confers broad, low-level resistance to multiple drug classes simultaneously.

Question 3

Antibiotic use increases the rate at which bacteria acquire resistance mutations.

• T. True
• F. False

Answer: False

Antibiotics select for resistance but do not generally increase the spontaneous mutation rate. Resistance mutations arise through ordinary replication errors that occur constantly, regardless of antibiotic exposure. What antibiotics do is dramatically change the fitness landscape: in the absence of the drug, the resistance mutation may be neutral or even slightly costly; in the presence of the drug, it becomes the only surviving genotype. The antibiotic acts as a filter, not a mutagen. (Note: some antibiotics that damage DNA can increase mutation rates indirectly via SOS response induction, but this is not the primary mechanism of resistance evolution.)

Question 4

Using one antibiotic can inadvertently select for resistance to other antibiotics, particularly when resistance genes are co-located on multi-resistance plasmids.

• T. True
• F. False

Answer: True

Resistance genes to multiple antibiotics are often co-located on conjugative plasmids, transposons, or integrons as resistance gene cassettes. When any one antibiotic selects for bacteria carrying such a plasmid, it simultaneously selects for all resistance genes on that plasmid — including genes conferring resistance to antibiotics not currently in use. This co-selection is a major driver of multi-drug resistant bacteria in hospitals and agricultural settings, and is why antibiotic stewardship considers not just which drug to use but how use of one drug affects resistance to others.

Question 5

Why does antibiotic use 'accelerate its own obsolescence,' and what role does horizontal gene transfer play in this process?

Model answer: Every course of antibiotics selects for resistant variants: susceptible cells die, and any rare resistant mutant survives and replicates to dominate the population. Horizontal gene transfer amplifies this beyond a single patient or species — resistance genes arising in one bacterium can be transferred by conjugation to entirely different species via plasmids, transposons, and integrons. A resistance mutation that evolved once can spread globally across many bacterial species within years. This means that antibiotic use in one context (a hospital, a farm) selects for resistance that can subsequently spread far beyond that context, depleting the effectiveness of the drug for everyone.

The key insight is the interplay between selection and horizontal gene transfer. Natural selection alone would favor resistance in individual lineages, but transfer means that resistance genes — once they arise — do not stay within a single lineage. They become a shared resource that any bacterium can acquire. Combined with the speed of bacterial reproduction (a new generation every 20 minutes), this means resistance can sweep through diverse bacterial communities in days to weeks, not generations.