Bacterial gene transfer and antibiotic resistance

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Use transfer controls to distinguish transformation, conjugation, and transduction, then separate acquisition of resistance from selection for resistant cells.

About 17 minutes, including the transfer experiment and checks.

Give each transfer route a distinguishing requirement

Horizontal gene transfer changes bacterial genotype without requiring inheritance from a parent cell. In transformation, a competent bacterium takes up extracellular DNA. In conjugation, DNA transfer depends on direct contact between donor and recipient cells through a transfer apparatus. In transduction, bacteriophages carry bacterial DNA from one cell to another.²

The mechanisms can produce the same final phenotype—such as antibiotic resistance—so the phenotype alone does not identify the route. The experiment must test what each route uniquely requires.

Concept: DNA can move without cell division

A recipient bacterium can acquire a resistance gene through transfer and then pass that gene to descendants by ordinary DNA replication and cell division. That makes horizontal acquisition and vertical inheritance different steps. The original transfer event explains how the genotype enters the lineage; later reproduction explains how the genotype expands through the population.

Plasmids can carry resistance genes and can sometimes be conjugative. Chromosomal fragments can be acquired by transformation or transduction and may require recombination for stable inheritance. A passage should state enough about the DNA element to determine whether it can replicate independently or must integrate into a chromosome.

Worked application: a DNase, barrier, and phage test

An original donor strain carries resistance gene r. A marked recipient strain is initially sensitive. Investigators count resistant recipient colonies after several treatments. A validated membrane prevents bacterial cells from crossing but allows small phage particles and soluble DNA through. DNase destroys exposed extracellular DNA but does not enter intact phage particles. A separate validated phage-depletion treatment removes infectious phage while leaving soluble DNA detectable.

Transfer persists when donor and recipient cells cannot contact one another, arguing against conjugation as the major route. Transfer also persists after DNase, arguing against exposed naked DNA as the major route. But transfer disappears when infectious phage is removed. Under the stated controls, the strongest inference is phage-mediated transduction.

The experiment does not prove that transformation or conjugation can never occur in this species. It identifies the mechanism responsible for the transfer measured under these conditions.

Pattern: transposons move genes but are not a cell-to-cell route by themselves

Transposons are mobile DNA elements that can move within or between DNA molecules. A resistance gene on a transposon can move from chromosome to plasmid or between replicons, making later horizontal spread easier. But transposition itself is not the same as transformation, conjugation, or transduction.

Think of the two questions separately: Where within DNA did the element move? and How did DNA move between cells? A transposon can solve the first problem while a conjugative plasmid or phage solves the second.

Rule: antibiotics select resistance; they do not create needed alleles on demand

When antibiotic exposure kills sensitive cells and resistant cells survive and reproduce, the population becomes enriched for resistance. That is selection. The resistance allele may have existed before exposure, arisen by mutation, or arrived by horizontal transfer. The antibiotic does not need to generate a specific useful mutation because the bacteria “need” it.

Some stresses can alter mutation rates, but an increased mutation rate is not the same as directed production of exactly the beneficial allele. The evolutionary inference still requires distinguishing the origin of variation from selection acting on that variation.

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Continue learning

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• Experimental Controls
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Sources

1. AAMC. Content Category 2B.
2. NCBI Bookshelf. Medical Microbiology: Genetics.