Antibiotics Quiz

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Which statement best describes horizontal gene transfer in bacteria?

A Exchange of genetic material between bacteria of the same species during reproduction
B Random mutation of chromosomal DNA during bacterial replication
C Transfer of genetic material between bacteria that are not parent and offspring, often via plasmids ✓
D Replication of plasmids and distribution to daughter cells during bacterial cell division
E Acquisition of resistance genes through prolonged exposure to antibiotics

The answer is C: Transfer of genetic material between bacteria that are not parent and offspring, often via plasmids

Explanation

Why C is correct

See Horizontal Gene Transfer. Option C is correct. Horizontal gene transfer means a bacterium acquires genes from another organism that is not its parent, instead of inheriting them when it divides. A useful picture is genes moving sideways between neighbours rather than down a family tree. The genes often travel on plasmids, which are small circles of DNA that sit apart from the main bacterial chromosome and copy themselves independently. Bacteria can pass plasmids to each other by direct contact (conjugation), pick up loose DNA from their surroundings (transformation), or receive DNA carried by a virus that infects bacteria (transduction, where the virus is called a bacteriophage). Because one plasmid can carry several resistance genes together, a single transfer can instantly turn a susceptible bacterium into a multidrug-resistant one. This is why horizontal transfer is such an important route for the spread of antibiotic resistance.

Definition: a bacterium acquires genes from another organism that is not its parent.
Vehicle: genes often travel on plasmids, small circles of DNA separate from the main chromosome.
Routes: conjugation, transformation and transduction.
Consequence: one plasmid can carry several resistance genes, so a single transfer can create a multidrug-resistant bacterium.

Why the other options are incorrect

A. Exchange of genetic material between bacteria of the same species during reproduction: Horizontal transfer is not limited to bacteria of the same species, and it does not require reproduction. Plasmids can pass between different species, which is one reason resistance spreads fast.
B. Random mutation of chromosomal DNA during bacterial replication: Mutation is a random change in a bacterium's own DNA, and it can create a new resistance trait. It is a different route to resistance because no DNA arrives from another organism.
D. Replication of plasmids and distribution to daughter cells during bacterial cell division: This option describes vertical inheritance, where plasmids are passed to daughter cells during division. Horizontal transfer brings in DNA from another bacterium rather than passing it down a family line.
E. Acquisition of resistance genes through prolonged exposure to antibiotics: Antibiotic exposure selects resistant bacteria that already exist; it does not cause bacteria to acquire new genes. This option confuses selection pressure with gene transfer.

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1
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2
Which mechanism best explains how glycopeptide antibiotics exert their antibacterial effect?

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