Questions: Bacterial Conjugation and Plasmid Transfer

Question 1

A single F+ bacterium is introduced into a culture of 10,000 F− bacteria. Assuming optimal conditions for conjugation, what is expected after several hours?

• A. About 50% of cells become F+, as the donor's resources limit how many transfers it can perform
• B. Nearly all cells become F+, because each newly converted F+ cell can immediately conjugate with remaining F− cells, enabling exponential spread
• C. Only cells that directly contacted the original donor become F+; newly converted cells cannot yet conjugate
• D. The original donor loses its F plasmid after the first transfer and becomes F−

Answer: (B)

The key is that conjugation transfers a COPY of the F plasmid — the donor retains its own copy. This means the donor cell can conjugate repeatedly, and more importantly, every new F+ recipient becomes a donor capable of spreading the plasmid further. The spread is exponential: one becomes two donors, two become four, and so on. Option 3 reflects the misconception that newly converted cells need time before they can conjugate — in fact, they can conjugate immediately once they carry the F plasmid. Option 3 also misunderstands rolling circle replication.

Question 2

After a successful conjugation event, what happens to the F plasmid in the original donor cell?

• A. It is destroyed — the nicking at oriT permanently inactivates the donor's plasmid
• B. It is halved — each cell receives one strand of the original double-stranded plasmid
• C. It is maintained intact — rolling circle replication synthesizes a new complementary strand in the donor as the other strand is transferred
• D. It integrates into the chromosome — the donor becomes an Hfr strain after every transfer

Answer: (C)

Rolling circle replication is the mechanism that makes conjugation so powerful. One strand of the F plasmid is nicked at oriT and fed into the recipient, while the donor simultaneously synthesizes a replacement strand. Both cells end up with a complete double-stranded F plasmid. The donor loses nothing permanently. This is fundamentally different from chromosomal transfer in Hfr strains, and the 'halving' in option 1 confuses strand transfer with molecular splitting.

Question 3

A single conjugation event involving an R (resistance) plasmid can simultaneously confer resistance to multiple antibiotics in the recipient cell.

• T. True
• F. False

Answer: True

R plasmids (resistance plasmids) typically carry multiple resistance determinants organized as resistance cassettes, often within mobile genetic elements called integrons. A single transfer event copies the entire plasmid, including all resistance genes it carries. This is a critical feature of the clinical problem: conjugation can, in a single step, convert a pan-susceptible bacterium into one resistant to several antibiotic classes simultaneously — a dramatic change that would take thousands of independent mutations to achieve through chromosomal evolution.

Question 4

The F pilus forms a permanent hollow tube through which double-stranded DNA is directly threaded from donor to recipient.

• T. True
• F. False

Answer: False

The F pilus does not remain as a permanent channel. It makes initial contact with the F− cell surface and then retracts, pulling the two cells into direct physical contact. A mating bridge (stabilized junction between cell membranes) forms through which DNA transfer occurs. Additionally, the DNA transferred is a single strand — not double-stranded — which is synthesized as a complement by the recipient cell after transfer. The double-stranded plasmid is reconstituted in the recipient, not transferred intact.

Question 5

Why is conjugation clinically more dangerous than chromosomal mutation as a mechanism for spreading antibiotic resistance in bacterial populations?

Model answer: Conjugation is more dangerous for several reasons. First, it is horizontal — it spreads genes between unrelated bacteria (even across species boundaries) rather than only to daughter cells. Second, it is exponential — each newly converted F+ cell becomes a new donor. Third, it transfers entire functional resistance cassettes in a single event rather than accumulating point mutations one at a time. Fourth, R plasmids often carry multiple resistance genes, granting multidrug resistance in a single conjugation. Finally, it occurs in real time within hosts (including hospital settings) without requiring bacterial reproduction, so resistance can spread through a population in hours.

Chromosomal mutation is essentially random and slow — a single bacterium acquires one mutation, and the mutation spreads only through vertical inheritance to daughter cells. Conjugation circumvents all of this: a plasmid can jump horizontally to thousands of cells within hours, bypassing the generational bottleneck entirely. This is why the emergence and spread of resistance plasmids in hospital settings is treated as a public health emergency rather than an ordinary evolutionary phenomenon.