The Lysogenic Cycle
A bacterium infected by a virus should die within the hour. That is the lesson of The Lytic Cycle. But in 1950, André Lwoff watched infected Bacillus megaterium divide peacefully for nineteen generations. The cells appeared virus-free. Then a pulse of ultraviolet light caused every single cell to burst open and release phage. Where had the virus been hiding, and why did it wait? The answer is lysogeny. A bacteriophage genome can integrate into the host chromosome. It can ride along silently through countless rounds of bacterial division. It erupts into lytic replication only when triggered. This is not a minor sideline of virology. It is one of the two fundamental strategies temperate viruses use to persist. It is also the conceptual template for how many animal viruses achieve latency (herpesviruses, HIV as a provirus). Lysogeny matters because it turns a virus's life history into a genetic switch problem. Few decisions in all of biology have been dissected as precisely as the lambda phage lysis/lysogeny switch. It also matters because prophages are not passive stowaways. They carry genes that can change their host's biology outright. This phenomenon is called lysogenic conversion. The toxins behind diphtheria, cholera, and the most severe forms of food poisoning are all phage genes. These genes are expressed from prophages sitting quietly in bacterial chromosomes. Lysogeny is therefore a bridge between pure molecular genetics and real human disease. The big picture is this: a temperate phage injects its DNA into a bacterium. A molecular coin-flip then decides its fate. Heads, the phage commits to the lytic cycle. It replicates explosively and kills the cell. Tails, the phage genome integrates into the host chromosome, or persists as a stable plasmid, as a prophage. It is replicated passively along with host DNA. It is transmitted to every daughter cell — a state called lysogeny. The lysogen can persist indefinitely, but the peace is conditional. DNA damage or other stress can trigger induction. This flips the switch back toward lysis. This article works through the physical mechanism of integration and excision. It covers the genetic circuitry that makes the lysis/lysogeny decision. It explains why that decision depends on how many phages infect a cell. It also depends on how healthy that cell is. The article covers the classical experiments that established every part of this picture. These include Lwoff's induction studies and the deep genetic dissection of bacteriophage lambda. Finally, it covers the medically vital phenomenon of lysogenic conversion. This sets up the next step in the story: how phage genes cause disease.
- Temperate phages (like lambda) can either lyse the host immediately or become a prophage, integrating into the bacterial chromosome and replicating passively with it — this dormant state is lysogeny.
- Integration uses a phage-encoded integrase that catalyzes site-specific recombination between phage attP and bacterial attB sites, requiring no DNA replication.
- The lysis/lysogeny decision is a genetic switch built from mutually repressing regulatory proteins — chiefly CI (repressor) versus Cro in phage lambda — whose balance is tipped by cell physiological state (nutrient availability, multiplicity of infection).
- Induction reverses the switch: DNA damage activates the host SOS response, RecA-stimulated autocleavage destroys CI repressor, and the prophage excises and enters lysis.
- Lwoff's UV-induction experiments (1950) proved prophages exist as latent, inducible genetic elements; decades of lambda genetics (Jacob, Wollman, Ptashne and others) worked out the switch's molecular logic.
- Lysogenic conversion — new host phenotypes from prophage genes — explains toxin production in diphtheria, cholera, botulism, and scarlet fever, linking lysogeny directly to human pathogenesis.
Lysogeny is a genetic switch, not a passive infection: a temperate phage's own regulatory proteins weigh the cell's condition and decide whether to integrate quietly as a prophage or commit to killing the cell outright.
The core idea
A temperate bacteriophage is one capable of two alternative outcomes after infection, in contrast to a strictly virulent phage, which can only lyse. Upon injecting its DNA, a temperate phage genome either enters the lytic cycle — transcription of early and late genes, genome replication, virion assembly, and host lysis, exactly as in a virulent infection — or it enters lysogeny, in which the phage genome is stably maintained in the host without producing new virions. The integrated, dormant form of the phage genome is called a prophage. A bacterium carrying a prophage is a lysogen, and it is said to be immune to superinfection by the same phage, because the prophage itself produces a repressor that blocks a second incoming copy from switching to lysis. The prophage state is genetically stable but not permanent. Induction is the process by which a prophage is triggered to excise from the chromosome and resume the lytic cycle, usually in response to a signal indicating the host cell is in danger — most classically, DNA damage from ultraviolet light or genotoxic chemicals. Crucially, induction affects every cell in a UV-treated lysogenic population essentially simultaneously and completely — this synchronous, all-or-nothing behavior is a signature of a molecular switch rather than a leaky, gradual process, and it was exactly this synchrony that let Lwoff demonstrate the phenomenon so cleanly. The molecular logic underlying the lysogeny decision has been worked out in exquisite detail for bacteriophage lambda infecting Escherichia coli, which stands as one of the best-understood gene regulatory circuits in all of biology. It rests on two phage proteins that repress each other's promoters — a genetic toggle switch — plus a set of accessory proteins that report on the cell's physiological state and multiplicity of infection, biasing which stable configuration of the switch the cell settles into. Related concepts worth keeping straight: the switch itself is downstream of the same early-gene transcriptional machinery used in The Lytic Cycle, and its reversal (induction) reuses the bacterial SOS response that also governs mutagenesis and cell-cycle arrest.
Mechanism
Step 1 — Injection and the decision window. After lambda's DNA enters E. coli, it circularizes (lambda's linear genome has complementary single-stranded "sticky ends," cos sites, that anneal and are sealed by host DNA ligase). Immediately, both possible fates begin transcriptionally: RNA polymerase, aided by the phage's own antiterminator protein N, transcribes the early genes required for both lytic replication and for the repressor that would establish lysogeny. For a window of roughly 10–20 minutes the cell is in a genuinely undecided state.
Step 2 — The molecular switch: CI versus Cro. The decision hinges on a small region of the lambda genome containing two divergent promoters, P_RM (promoter for repressor maintenance) and P_R (promoter rightward), separated by an operator region O_R with three adjacent operator sites (O_R1, O_R2, O_R3). Two proteins compete to bind these operators:
- CI (the lambda repressor), a protein that binds O_R as a dimer with highest affinity at O_R1 and O_R2, blocking P_R (shutting off lytic genes including cro) while simultaneously activating its own promoter P_RM (positive autoregulation) — this is the lysogenic, "peaceful" configuration.
- Cro, which binds the same operators with the opposite preference — highest affinity at O_R3 — repressing P_RM (shutting off further CI synthesis) while leaving P_R active, driving lytic gene expression — this is the lytic configuration.
Because each protein represses the gene for the other's competitor and helps its own persistence, the system behaves as a bistable switch: once CI accumulates and occupies O_R1/O_R2, the lysogenic state is locked in and self-reinforcing; once Cro wins early, the lytic cascade proceeds irreversibly toward cell lysis. Which way the coin falls depends on additional inputs — chiefly the phage protein CII, which is stabilized when host proteases (like HflB) are less active (a proxy for a starved, stressed cell), and CII in turn activates a separate promoter, P_RE, that produces the initial burst of CI needed to tip the switch toward lysogeny. Multiplicity of infection matters too: cells infected by several phage particles at once are statistically biased toward lysogeny, partly because CII accumulates faster when more copies of the phage genome are present.
Step 3 — Integration: site-specific recombination. Once lysogeny is chosen, the phage-encoded integrase (Int), together with the host protein IHF (integration host factor), catalyzes recombination between a specific phage attachment site, attP, and a homologous bacterial attachment site, attB, located (for lambda) between the gal and bio operons on the E. coli chromosome. This is site-specific recombination: Int makes staggered cuts in both attP and attB at a short shared core sequence, exchanges strands, and reseals them — no DNA replication is required, and no free DNA ends are ever released. The result is a single crossover event that inserts the entire circular phage genome into the bacterial chromosome, flanked by two hybrid sites, attL and attR, generated from the recombination.
Step 4 — Maintenance of the prophage state. With the phage genome now a physical part of the bacterial chromosome, it is replicated passively by the host's normal chromosomal replication machinery once per cell cycle and partitioned to daughter cells along with the rest of the genome — no phage-specific replication genes are expressed. CI repressor, continuously produced at a low, self-regulated level from P_RM, keeps P_R (and all downstream lytic genes) shut off, and also blocks expression from any newly infecting lambda genome — this is superinfection immunity.
Step 5 — Induction: flipping the switch back. Environmental insults that damage host DNA (UV radiation, mitomycin C, certain antibiotics) trigger the bacterial SOS response. Single-stranded DNA generated at damaged replication forks activates RecA into its coprotease form (RecA). RecA does not itself cleave CI directly by a normal proteolytic active site; rather, it stimulates CI's latent self-cleavage (autoproteolysis) between its DNA-binding and dimerization domains, in exactly the same molecular mechanism RecA* uses to inactivate the host repressor LexA during a normal SOS response. As CI is destroyed, P_R and P_L are derepressed, Cro and the lytic genes are expressed, and the phage-encoded excisionase (Xis), acting together with Int, catalyzes the reverse recombination reaction — between attL and attR — precisely excising the prophage from the chromosome and regenerating the free circular phage genome and the original, unmodified attB site in the bacterial chromosome. The excised genome then proceeds through the standard lytic cycle: replication, packaging, and lysis.
The evidence
Lwoff's induction experiments (1949–1950) provided the founding proof that lysogeny is a real, physically defined latent state rather than mere phage-free "immunity." André Lwoff, working at the Institut Pasteur, isolated single cells of Bacillus megaterium known to be lysogenic — they never produced detectable free phage in ordinary culture, yet if their culture medium was ever exposed to a lysogenic culture, sensitive indicator bacteria in that medium were killed, implying phage were present but suppressed. Using micromanipulation, Lwoff isolated individual lysogenic bacteria and followed their divisions under the microscope, showing that a single lysogenic cell could divide for many generations — Lwoff documented series through roughly nineteen bacterial generations — without a single spontaneous lysis event, yet every descendant remained lysogenic and capable of producing phage. This ruled out the idea that only a rare "carrier" subpopulation harbored the phage; the potential to make phage was hereditary and universal among the lysogen's progeny. The decisive manipulation was induction by ultraviolet irradiation. When Lwoff exposed lysogenic cultures to a small, carefully calibrated UV dose, the effect was dramatic and synchronous: essentially the entire population, which had shown no signs of infection, began producing phage and lysed within about an hour. This showed conclusively that: 1. every lysogenic cell already carried a complete, latent phage genome (since UV cannot create a virus from nothing), 2. that genome could be switched, in a coordinated and nearly all-or-none fashion, from dormancy to full lytic replication, and 3. the switch was triggered by a defined physical stimulus (UV) rather than occurring at random. Lwoff and colleagues received the Nobel Prize in Physiology or Medicine in 1965 (shared with François Jacob and Jacques Monod) for this and related work establishing the genetic regulation of viral and cellular processes.
Genetic dissection of lambda (Jacob, Wollman, and later Ptashne and colleagues, 1950s–1980s) established the molecular mechanism behind Lwoff's phenomenology. François Jacob and Élie Wollman, using interrupted mating experiments with Hfr E. coli strains, mapped the lambda prophage to a specific, fixed location on the bacterial chromosome (near the gal and bio genes) — direct physical evidence that the prophage is integrated linearly into host DNA rather than existing as a free cytoplasmic particle. They also showed that a lysogen mated with a non-lysogen sometimes triggered massive phage production in the recipient — zygotic induction — because a prophage transferred into a cytoplasm lacking repressor is immediately derepressed; this elegantly demonstrated that lysogenic immunity is due to a diffusible cytoplasmic repressor, not a chromosomal property of the DNA itself. Later, Mark Ptashne's biochemical and structural work (1960s–1980s) purified the CI repressor protein, showed it bound specifically and cooperatively to the O_R operator DNA sequences, demonstrated its dual role in blocking P_R while activating P_RM, and reconstructed the CI/Cro competition in vitro — turning Lwoff's black-box switch into an atomically resolved protein-DNA circuit and making the lambda switch the paradigm case for how a small number of transcription factors can build a bistable genetic memory device.
Worked examples
Example 1 — Reading a spot test for lysogeny and immunity. A microbiology student streaks four E. coli strains — a nonlysogenic sensitive strain (S), a strain lysogenic for phage λ (S(λ)), a strain lysogenic for a different phage P22-like temperate phage (S(P22)), and a strain cured of its prophage by treatment (S-cured) — onto four separate lawns and drops a spot of λ phage lysate onto each.
- S (nonlysogenic): the spot clears completely — a plaque/lysis zone forms, because λ can adsorb, inject, and lyse a cell with no repressor already present.
- S(λ): the spot shows no clearing (or very faint, resistant patches) — this is superinfection immunity. Any incoming λ genome enters a cytoplasm already saturated with CI repressor from the resident prophage, so P_R is immediately shut down and the new phage genome cannot execute lysis.
- S(P22): the spot clears just like S, because immunity is repressor-specific — a P22 prophage's repressor does not recognize λ's O_R operator sequences (different phages have distinct repressor-operator specificity, called immunity groups), so incoming λ is unaffected.
- S-cured: the spot clears like S, because losing the prophage (and its repressor gene) removes the source of CI, restoring full sensitivity.
The logical deduction the pattern supports: resistance to superinfection tracks specifically with the presence of the same prophage's repressor gene, not with any general resistance mechanism (like a lost receptor) — a control every rigorous lysogeny assay must include, since receptor loss would give a false "immune" result on any phage.
Example 2 — Quantifying multiplicity of infection and the lysogeny decision. A researcher infects a culture of 2 × 10⁸ E. coli cells with lambda phage at two different doses: Dose A adds 2 × 10⁷ plaque-forming units (PFU), and Dose B adds 2 × 10⁹ PFU. Multiplicity of infection (MOI) = phage added ÷ cells available.
- Dose A: MOI = (2 × 10⁷) / (2 × 10⁸) = 0.1 — on average 1 phage per 10 cells.
- Dose B: MOI = (2 × 10⁹) / (2 × 10�8) = 10 — on average 10 phage per cell.
Because phage adsorption to a large bacterial population is well approximated by a Poisson process, the fraction of cells receiving k phage particles is P(k) = (MOI^k × e^(–MOI)) / k!. At MOI = 0.1, P(0) = e^(–0.1) ≈ 0.905 (about 90% of cells receive no phage at all), and P(1) ≈ 0.090 — almost all infected cells receive exactly one phage genome. With just one CII/CI signal being generated per cell, lysogeny establishment is comparatively weak, and a substantial fraction of the infected 10% proceed to lysis. At MOI = 10, P(0) = e^(–10) ≈ 0.0000454 — essentially every cell is infected, and most cells receive several phage particles simultaneously (mean 10). Multiple simultaneous copies of the phage genome mean multiple copies of the CII-generating machinery accumulating at once, which biases the switch strongly toward lysogeny; empirically, high-MOI infections by lambda give a substantially higher lysogenization frequency than low-MOI infections. This MOI-dependence is a direct physiological consequence of the CII-based sensing step described in the mechanism above, and it is precisely why laboratory protocols to generate lysogens deliberately use high-MOI infections.
Common misconceptions & traps
| Misconception | Correction |
|---|---|
| "Lysogeny means the phage is destroyed or inactive forever." | The prophage genome is fully intact and functional; it is repressed, not degraded, and remains inducible indefinitely. |
| "A lysogen is immune to all phages." | Immunity is specific to phages sharing the resident prophage's repressor/operator system (its immunity group), not to unrelated phages. |
| "Integration requires phage DNA replication or host recombination enzymes like RecA." | Integration is catalyzed by the phage's own site-specific integrase (Int) plus host IHF; it is a single conservative recombination event, not replication-dependent. |
| "Induction is a rare, leaky event." | Under a strong SOS-inducing signal (e.g., UV), induction is synchronous and near-total across the population — a hallmark of a genetic switch, as Lwoff showed. |
| "The lysis/lysogeny decision is made by the bacterium." | The decision circuitry (CI, Cro, CII, N) is entirely phage-encoded; the bacterium's physiological state only supplies inputs (protease activity, nutrient status) that bias which phage-controlled outcome results. |
| "Lysogenic conversion is rare or trivial." | Many medically important toxins (diphtheria, cholera, botulinum, Shiga toxins) are encoded by prophage genes and expressed only because the host is lysogenized. |
The scientist behind it
Origins · 1 minAndré Michel Lwoff and how this was discovered
Lwoff trained initially as a protozoologist studying growth factors in ciliates before turning, in the late 1940s, to the puzzle of lysogenic bacteria — a phenomenon known since the 1920s (Bordet, then Jules Bronfenbrenner and others had debated whether "lysogeny" was even a real hereditary phenomenon or an artifact of undetected chronic contamination). Lwoff's contribution was methodological rigor: by tracking single bacterial cells through many divisions with a micromanipulator, he settled the argument empirically rather than by debate, proving that the capacity to produce phage was a stable, heritable property transmitted through cell division, and that it could be switched on by a defined external agent (UV light). This reframed lysogeny from a murky bacteriological curiosity into a tractable problem in gene regulation, directly motivating the biochemical dissection of the lambda switch by the next generation of molecular biologists.
Where this matters today
Genetic toxin production and infectious disease. Corynebacterium diphtheriae causes diphtheria only when lysogenized by a beta-phage carrying the tox gene for diphtheria toxin; nontoxigenic strains lacking the prophage are essentially harmless. Vibrio cholerae's cholera toxin is encoded by the filamentous prophage CTXφ. Shiga toxin–producing E. coli (including the O157:H7 strain responsible for major foodborne outbreaks) carry Shiga toxin genes on lambdoid prophages, and — with grim clinical irony — some antibiotics that damage bacterial DNA can trigger SOS-mediated induction of these prophages, increasing toxin release and worsening disease, which is why certain antibiotic classes are avoided in suspected Shiga-toxin E. coli infections. Understanding lysogenic conversion is thus directly clinically actionable, foreshadowing the broader study of Viral Pathogenesis and Host Damage.
Biotechnology and genetic engineering. The lambda Int/att site-specific recombination system, first understood through prophage integration, was engineered into the Gateway cloning system and related recombinase toolkits, letting researchers shuttle DNA fragments between plasmids efficiently without conventional restriction/ligation. Lambda lysogens engineered with temperature-sensitive CI repressor alleles are used as inducible expression systems: bacteria are grown at a low temperature (repressor active, gene silent) then shifted to a higher temperature to inactivate the repressor and switch on downstream genes on demand.
Molecular biology methodology. Lambda phage vectors (exploiting the very att-site biology described here) were workhorses of genomic library construction throughout the 1980s–1990s, because the phage's packaging machinery efficiently accepts foreign DNA inserts within a defined size window.
Microbial ecology and evolution. Prophages are now recognized as a dominant force in horizontal gene transfer across bacterial genomes, contributing not just toxins but antibiotic resistance genes, metabolic genes, and defense systems against other phages — reshaping how microbiologists think about bacterial genome plasticity and the coevolution of phages and their hosts, a theme that recurs throughout modern Bacterial Genetics and clinical microbiology.
Ask the tutor: How does the lambda CI/Cro switch compare mechanistically to eukaryotic viral latency, such as HIV proviral silencing?
What exactly is the difference between a "temperate" phage and a "virulent" phage?
A virulent phage (the subject of The Lytic Cycle) has only one possible outcome after infection: it always replicates lytically and kills the host cell, with no capacity for integration or dormancy. A temperate phage, like lambda, retains a genetic switch that can choose between that same lytic pathway and an alternative, lysogenic pathway in which its genome integrates and persists quietly. The genes needed for both programs (early lytic genes and the CI/Cro decision circuit) are present in a temperate phage's genome from the moment of infection; which program actually runs is determined afterward by the regulatory competition described in the mechanism, not fixed at the moment of infection.
Can a lysogen ever lose its prophage without induction into lysis?
Yes, though it is comparatively rare. Occasionally the prophage can be lost through imprecise excision, spontaneous curing during chromosome replication errors, or deliberate laboratory curing treatments, yielding a "cured" strain that is genetically identical to the original nonlysogen except for the historical presence of the prophage's attB site, which is regenerated exactly upon clean excision. This is distinct from induction, where the excised phage genome goes on to replicate lytically and kill the cell; curing without induction is a much less common outcome and generally requires disrupting the recombination step itself.
Does every temperate phage use CI/Cro-style repressors, and is lambda's switch universal?
No — lambda is the best-understood example, and closely related "lambdoid" phages use homologous machinery, but the molecular details vary substantially across phage families. Some temperate phages use entirely different repressor architectures, different attachment site mechanisms, or maintain themselves as extrachromosomal plasmids rather than integrating at all (a few phages, like P1, are famous for exactly this plasmid-based lysogeny). The general logic — a repressor-based switch weighing environmental and physiological signals, with an inducible trigger tied to host stress — recurs broadly, but the specific proteins and DNA sequences are phage-specific, defining separate "immunity groups" that do not cross-react.
How does lysogenic conversion differ from simple gene transfer by transformation or conjugation?
Lysogenic conversion specifically refers to a phenotypic change in the host bacterium caused by genes carried on an integrated (or otherwise stably maintained) prophage — the new phenotype persists as long as the prophage is retained and is lost if the prophage is cured. Transformation and conjugation are separate horizontal gene transfer mechanisms involving direct uptake of free DNA or plasmid transfer through direct cell contact, and they do not require any phage infection cycle at all. All three mechanisms can move genes like toxin genes or antibiotic resistance genes between bacteria, but only lysogenic conversion couples that gene transfer to the specific molecular biology of a temperate phage's integration and repression machinery.
Why is antibiotic treatment sometimes dangerous in infections by Shiga toxin–producing E. coli?
Certain antibiotics, particularly those that damage bacterial DNA or block DNA replication (such as some fluoroquinolones), activate the bacterial SOS response as an incidental side effect of their intended antibacterial action. If the infecting E. coli strain is lysogenized by a Shiga toxin–encoding prophage, SOS activation triggers RecA-mediated CI cleavage exactly as in laboratory UV induction, causing widespread prophage induction, a burst of lytic replication, and release of large quantities of Shiga toxin as the bacteria lyse. This can worsen clinical outcomes, notably increasing the risk of hemolytic uremic syndrome, which is why clinicians are cautious about antibiotic choice in suspected cases — a direct clinical consequence of the induction mechanism described in this article.
How is the lysogeny/lysis switch actually "read out" as a decision at the level of a single cell, given that gene expression is noisy?
Single-cell studies (using fluorescent reporters) show that early after infection, both CI-promoting and Cro-promoting gene expression fluctuate stochastically in each cell, and the eventual commitment reflects which regulatory pathway crosses a threshold first in that particular cell — meaning genetically identical cells infected under identical bulk conditions can make different decisions. The population-level probability of lysogeny (biased by MOI, host physiological state, and CII protein stability) is really an average over many individual, noisy single-cell "coin flips," each resolved by the CI/Cro bistable switch locking in one stable state or the other, after which reversal (other than by induction) is effectively impossible due to the strong positive-feedback autoregulation.
Is lysogeny unique to bacteriophages, or does it have parallels in animal virology?
The core logic — a viral genome integrating (or persisting episomally) and remaining transcriptionally quiescent within a host cell for extended periods, with the potential to reactivate — recurs prominently in animal virology, though the specific molecules differ entirely. Herpesviruses establish latency in neurons with only a small set of genes expressed, reactivating under stress in a process functionally analogous to induction. Retroviruses such as HIV integrate a DNA copy (via reverse transcriptase and integrase) into the host chromosome as a provirus, which can remain transcriptionally silent for long periods. These are not literally lysogeny (different enzymes, different hosts, different triggers), but the analogy usefully previews concepts explored further under Viral Pathogenesis and Host Damage and viral latency.