Before We Start
Two paths, one virus
When a bacteriophage infects a bacterium, it has two fundamentally different paths available: destroy the cell immediately to make more copies of itself, or integrate quietly into the host's chromosome and wait. Which path it takes has real medical consequences, well beyond the fate of that one bacterial cell.
💡 Memory Trick
Lytic: virus destroys cell. Lysogenic: virus hides in chromosome. Stress → lytic switch → toxin release. The dormant, hidden path is the one that turns out to matter most in medicine.
The Key Points
Four stages: lytic, lysogenic, induction, and medical relevance
L
Lytic cycle — active replication, ending in cell destruction
In the lytic cycle, a phage injects its DNA into the host bacterium, hijacks the cell's own replication machinery to produce many copies of itself, and ultimately lyses (bursts) the cell open to release the new viral particles. This is the fast, destructive path — the host cell doesn't survive it.
🦠 A bacteriophage rapidly hijacking a bacterial cell's replication machinery can produce hundreds of new phage particles before bursting the cell open to release them, all within a relatively short window of time.
L2
Lysogenic cycle — dormant integration as a prophage
In the lysogenic cycle, phage DNA instead integrates into the host bacterial chromosome as a prophage, replicating passively along with the host cell every time it divides. A prophage can persist silently this way for many generations, with no active viral replication or cell destruction occurring during this dormant period at all.
🦠 A prophage can remain integrated and dormant within a bacterium's chromosome across many generations of cell division, with no active viral replication occurring during that entire dormant stretch.
I
Induction — stress triggers the switch from lysogenic to lytic
Certain stressors, particularly UV exposure and the SOS DNA-damage response it triggers, can cause a dormant prophage to excise itself from the host chromosome and switch into the active lytic cycle. This is the moment the previously silent viral DNA becomes destructive again.
🦠 UV radiation damaging a bacterium's DNA can trigger the SOS response, which in turn causes a previously dormant prophage to excise itself from the chromosome and enter the lytic cycle.
M
Medical relevance — prophages often carry the toxin genes
Many of the most clinically important bacterial toxin genes are actually encoded within prophages, not within the bacterium's own core genome at all — cholera toxin, diphtheria toxin, Shiga toxin, and Staph toxin superantigens are all examples. This means lysogenic conversion — a bacterium acquiring a prophage — can directly transform an otherwise harmless bacterium into a virulent, toxin-producing one.
🦠 A bacterium that was previously non-toxigenic can become capable of producing diphtheria toxin specifically because it acquired a prophage carrying that toxin gene through lysogenic conversion — the toxin gene didn't evolve within the bacterium itself.
🏥 Applied Scenario
A strain of Corynebacterium that was previously non-toxigenic (unable to produce diphtheria toxin) is found to have suddenly gained this toxin-producing ability.
Step 1
Ask how this new capability could arise: How could a bacterium acquire an entirely new toxin-producing ability like this? Through lysogenic conversion — acquiring a prophage that specifically carries the diphtheria toxin gene, which integrates directly into the bacterium's own genome.
Step 2
Recognize the broader pattern: This illustrates the wider medical relevance of lysogeny: several of the most clinically important bacterial toxins (cholera, diphtheria, Shiga, Staph superantigens) are actually encoded by prophage genes, not by the bacterium's own inherent genetic material.
Step 3
Consider what could reactivate the prophage: If this same lysogenized bacterium were later exposed to a stressor like UV radiation, the SOS response could trigger the dormant prophage to excise itself and enter the lytic cycle, potentially releasing new phage particles capable of infecting and converting other bacteria in turn.
Step 4
Conclusion: The same bacterial species can be either completely harmless or genuinely virulent, depending entirely on whether it happens to carry a toxin-encoding prophage — a distinction that has nothing to do with the bacterium's own core genetic identity.
📌 Exam Application
Exams test whether you can distinguish the lytic cycle (active replication, cell destruction) from the lysogenic cycle (dormant integration as a prophage), and whether you know specific examples of toxin genes carried by prophages (cholera, diphtheria, Shiga, Staph superantigens) — a frequently tested connection between phage biology and bacterial virulence. Expect questions asking why a specific bacterial toxin exists, testing whether you connect it back to prophage acquisition rather than the bacterium's inherent genome.
⚠️ The Trap — Assuming Bacterial Toxin Genes Are Always Part of the Bacterium's Own Genome
The most common trap is assuming bacterial toxin genes are always an inherent part of a bacterium's own core genome, something it evolved on its own. Several of the most clinically significant toxins are specifically encoded by prophages acquired through lysogenic conversion — meaning the same bacterial species can be either harmless or highly virulent depending entirely on whether it has acquired the relevant toxin-carrying prophage. A bacterium's species identity alone doesn't guarantee it produces (or doesn't produce) a given toxin.
✓ Quick Self-Test
Answer before checking:
1. What happens during the lytic cycle?
2. What happens during the lysogenic cycle?
3. What can trigger a prophage to switch from lysogenic to lytic?
4. Name two toxins encoded by prophage genes rather than the bacterium's own core genome.
5. What is lysogenic conversion, and why does it matter clinically?
Answers:
1. The phage hijacks the host cell's machinery to make many copies of itself, then lyses the cell to release the new viral particles.
2. Phage DNA integrates into the host chromosome as a prophage, replicating passively along with the host cell and potentially persisting for many generations.
3. Stressors like UV exposure, which trigger the SOS DNA-damage response.
4. Cholera toxin, diphtheria toxin, Shiga toxin, or Staph toxin superantigens (any two).
5. A bacterium acquiring a prophage that carries a toxin gene, which can directly transform an otherwise harmless bacterium into a virulent one.