Double the phage, double the diversity

Review written by: Megan Mauriello

Temperate phages can switch between lytic and lysogenic reproductive styles. Some temperate phages are known to sense bacterial autoinducers, the signal molecules bacteria use for quorum sensing. These phages are therefore able to switch from a lysogenic to lytic replication mode at high cell density, when more hosts are theoretically available. However, potential hosts that are already lysogens (already have an integrated phage genome), those host cells could be a dead end due to mechanisms like superinfection exclusion or homoimmunity. Superinfection exclusion is a mechanism by which infection of a lysogen by another phage is prevented. Homoimmunity is a mechanism by which a resident lysogenic phage suppresses transcription of a newly infecting phage.

Figure. Phage VP882 engages with both host quorum-sensing systems.

Vibrio parahaemolyticus has two quorum sensing systems. The autoinducer DPO binds to VqmA, a transcription factor, to control expression of vqmR, encoding a small RNA which promotes expression of genes controlling group behaviors. In the other system, three autoinducers control activity of the LuxO transcription factor. When LuxO is phosphorylated at low cell density, it controls individual behaviors. When LuxO is dephosphorylated at high cell density, it controls group behaviors. Phage VP882 is able to respond to both of these quorum sensing systems. Wild-type V. parahaemolyticus strain 882 has mutations inactivating both the vqmA/vqmR quorum sensing system and the luxO system, with both behaving as if the cell density was always low.

Phage VP882 was unable to adsorb to or form plaques on mutants with transposon insertions in the K-antigen/capsule locus.  Only O3:K6 serotype V. parahaemolyticus isolates were susceptible to VP882 infection. Based on genome sequencing, O3:K6 isolates appear to have distinct K-antigens. Repair of the K-antigen/capsule locus epimerase in V. parahaemolyticus strain VP81, a O3:K6 serotype-strain, restored the ability of VP882 to adsorb and plaque. These results suggest that specific K-antigens are the receptor for VP882.

Restoration of the vqmA/vqmR system did not affect VP882 plaquing, but restoration of the luxO system reduced plaque number and size. When the V. parahaemolyticus 882 luxO system was in-tact, less phage VP882 absorption occurred. RNA-seq data of V. parahaemolyticus 882 with different luxO alleles revealed no differences in K-antigen transcript levels. Deletion of an operon predicted to encode homologs of Wzabc proteins restored VP882 adsorption to and lysogenization of the host. This operon is under control by LuxO. Adsorption by VP882 to all strains tested required the K-antigen, even when this operon was absent. These results suggest that LuxO-controlled extracellular polysaccharide transport can protect the K-antigen from VP882.

VP882 did not form plaques on V. parahaemolyticus 882 which had already been lysogenized by VP882. However, VP882 could adsorb to and inject its genome into these lysogens, meaning the phage does not have a superinfection exclusion mechanism against other VP882. Injection of a second VP882 genome formed what the authors term “superlysogens”. Lysogens of V. parahaemolyticus 882 with a nonfunctional luxO system converted to superlysogens more frequently than those with a functional luxO system, confirming again that luxO functionality affects phage entry. A dual antibiotic selection assay revealed that, in superlysogens, recombination could occur between the original VP882 and the newly infecting VP882 phages, and given enough time, only one VP882 genotype will be maintained.

The authors propose that superinfection and recombination, enabled by phage monitoring of host quorum sensing, increases VP882 genomic diversity. The specificity of VP882 for O3:K6-serotype V. parahaemolyticus strongly restricts its host range, making it much more likely that lysogens the newly infecting VP882 encounters harbor a phage genotype with which it can recombine. This makes superinfection beneficial, rather than detrimental as it is in many other phage systems. If true, this model has interesting implications for phage evolutionary trajectories.

Host-phage and phage-phage dynamics are important to understand in order to design and optimize biotechnological tools involving phage, such as phage therapy, and to better combat threats such as antibacterial resistance. Shielding of the K-antigen by the host, and doing so via a quorum sensing-controlled mechanism, is yet another example of how bacteria can respond to phage pressure.

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READ MORE:

Sargen MR, Bassler BL. Vibrio parahaemolyticus quorum sensing controls phage VP882 transmission. mBio. 2026 Jun 10;17(6):e0073726. doi: 10.1128/mbio.00737-26. Epub 2026 May 19. PMID: 42153673; PMCID: PMC13251360.