Questions: Lysogenic Conversion and Phage-Encoded Virulence

Question 1

A researcher discovers a strain of Vibrio cholerae that lacks the CTXφ prophage. What can be predicted about this strain?

• A. It cannot survive in the human intestine at all, since the prophage provides essential metabolic genes
• B. It can colonize the intestinal mucosa but cannot produce cholera toxin and therefore cannot cause classical cholera
• C. It will be identical in virulence to toxin-producing strains because virulence factors are always encoded in the core genome
• D. It will spontaneously acquire the CTXφ prophage through mutation within the host

Answer: (B)

Cholera toxin is encoded by the CTXφ prophage, not the core V. cholerae chromosome. Strains lacking this prophage retain the ability to colonize — they have the colonization factors encoded in their own genome — but they cannot produce cholera toxin and therefore cannot cause the profuse watery diarrhea that defines cholera. This distinction is clinically and epidemiologically important: it means that 'V. cholerae' encompasses both virulent and avirulent strains depending solely on prophage carriage.

Question 2

Why would natural selection favor a bacteriophage that carries and expresses a virulence factor gene benefiting its bacterial host?

• A. Phages compete with the bacterium for host resources, so virulence genes reduce competition
• B. A phage that makes its bacterial host more successful at colonizing and spreading to new hosts creates more copies of the phage genome, since the phage replicates every time the bacterium divides
• C. Virulence genes are selectively neutral for the phage and accumulate by genetic drift alone
• D. Bacteriophages always carry genes that benefit bacteria because they evolved from ancestral bacterial plasmids

Answer: (B)

This is evolutionary mutualism. When a prophage's host bacterium colonizes more hosts, spreads more efficiently, or evades immune clearance better, the prophage genome replicates more frequently — it travels inside the bacterium to every new host. Cholera toxin causes massive intestinal fluid secretion that flushes enormous numbers of V. cholerae into the environment, directly enhancing transmission. More transmission = more bacterial hosts carrying the prophage = more copies of the phage genome. The toxin gene benefits the phage indirectly by benefiting the bacterium it depends on.

Question 3

The genes encoding the most dangerous bacterial toxins — cholera toxin, Shiga toxin, diphtheria toxin — are part of the core bacterial chromosome, conserved through millions of years of bacterial evolution.

• T. True
• F. False

Answer: False

These toxins are encoded by prophages, not the core bacterial chromosome. Cholera toxin is encoded by CTXφ prophage, Shiga toxin by lambdoid prophages in E. coli O157:H7, and diphtheria toxin by the β-prophage in Corynebacterium diphtheriae. This means virulence was acquired *horizontally* — bacteria gained these toxin genes by incorporating phage DNA, not by ancestral vertical inheritance. A bacterium can therefore become virulent suddenly (by acquiring a phage) or lose virulence (by losing a prophage), rather than evolving these capabilities slowly over long timescales.

Question 4

Two strains of the same bacterial species can differ dramatically in their ability to cause disease based solely on whether they carry a particular prophage.

• T. True
• F. False

Answer: True

Lysogenic conversion means prophage-encoded genes can fundamentally change a bacterium's phenotype. An avirulent strain that acquires the relevant prophage can become a dangerous pathogen overnight. The examples are concrete: E. coli O157:H7 is pathogenic because it carries Shiga toxin prophages; many E. coli strains without these prophages are harmless gut commensals. This has major implications for clinical microbiology — species identification is insufficient for virulence prediction; you must also characterize prophage content.

Question 5

Why does the phage-encoded origin of major bacterial toxins matter for understanding how new pathogens emerge, and what does it imply about the pace of pathogen evolution?

Model answer: Because virulence via lysogenic conversion can be acquired rapidly through horizontal gene transfer — a single infection event — rather than through the slow accumulation of mutations over generations. A harmless bacterium can become a dangerous pathogen when it acquires a phage carrying a toxin gene. This means new virulent strains can emerge far more quickly than traditional evolutionary models predict, and pathogen emergence does not require long-term co-evolution with hosts. It also means that epidemiological tracking must consider phage movement between bacterial populations, not just bacterial evolution alone.

Traditional models of pathogen evolution imagined virulence arising slowly through selection on bacterial mutations. Lysogenic conversion overturns this: the critical genetic event is phage integration, which can happen in a single bacterial generation. This helps explain why dangerous pathogens sometimes appear suddenly in populations — they may be old bacteria newly armed with phage-derived weapons. It also explains why the same species can be pathogenic in one region and harmless in another, depending on local phage populations.