Beyond lysogeny: hidden dynamics in phage lambda infections

What is it about?

Viruses that infect bacteria, called bacteriophages, are often thought to follow one of two simple strategies. They can either reproduce rapidly and burst their host cell open in a process called lysis, or enter a dormant state known as lysogeny, in which the viral DNA integrates into the bacterial chromosome and is copied along with it. In this study, we show that the well-known bacteriophage λ follows a much more flexible and surprising strategy when bacterial host cells become scarce. Rather than always establishing a stable lysogen, many incoming λ genomes remain outside the host chromosome in a previously overlooked prophage-like state that we call pλ. These pλ elements do not integrate into the bacterial genome and are inherited asymmetrically when bacterial cells divide. In contrast, the phage immunity factor known as the CI repressor is distributed symmetrically among daughter cells. This creates a mixed population consisting of stable λ lysogens, cells carrying pλ, and λ-free cells that remain temporarily protected from infection because they have inherited the CI repressor. As these λ-free cells continue to grow and divide, the inherited CI repressor becomes diluted and eventually disappears. Once this temporary protection is lost, the cells become susceptible to infection again, allowing new rounds of phage replication and lysis. This creates an ongoing cycle in which phages persist through a combination of dormancy, reinfection, and continued production of new viral particles, even when hosts are limited. Our findings reveal that phage-host interactions are far more dynamic than previously thought. Instead of making a simple one-time choice between lysis and lysogeny, phage λ can exploit a previously hidden infection strategy that promotes long-term coexistence with its host while maintaining opportunities for continued reproduction. This work challenges a classic model in microbiology and highlights how much remains to be discovered, even in one of the best-studied virus–host systems.

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Photo by CDC on Unsplash

Photo by CDC on Unsplash

Why is it important?

Much of our current understanding of phage biology comes from classical microbiological methods such as agar plating, which primarily reveal stable end-point outcomes. By using high-resolution time-lapse fluorescence microscopy, we were able to follow individual infected bacterial cells and their descendants over time. This allowed us to observe transient infection states and hidden cellular behaviors that would remain invisible in traditional population-level experiments. Our study reveals that stable lysogeny is not always the inevitable outcome when host cells become scarce. Instead, phage λ can maintain itself through a dynamic process involving non-integrated pλ elements, temporary immunity mediated by the CI repressor, and renewed cycles of infection. These hidden dynamics fundamentally change how we think about the balance between phage survival and reproduction. The findings may also have implications for phage therapy, an emerging approach that uses bacteriophages to combat bacterial infections. The existence of transiently protected and later re-sensitized bacterial subpopulations suggests that phage treatments may behave less predictably than classical models assume. Understanding these previously overlooked infection states could help improve how therapeutic phages are selected, combined, and administered, ultimately leading to more reliable treatment strategies.

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