Bacterial viruses known as phages influence many aspects of microbial processes including the population dynamics, diversity and evolution of their hosts. To survive and propagate, phages take over host metabolic processes while the host initiates a number of defence mechanisms to overcome infection. Albeit central for diversity and evolution, the physiological interactions between hosts and phages during infection remain largely unknown. To address this, scientists on the EU-funded PIMCYV (Physiological interactions between marine cyanobacteria and their viruses) project used marine cyanobacteria as a model system and studied cyanophages that infect them. Using an experimental evolution approach, they selected for cyanobacteria resistant to infection by T7-like podoviruses. Via whole-genome sequencing, they identified the mutations that conferred resistance. The mutations were localised in single hypervariable genomic islands primarily in non-conserved, horizontally transferred genes. As a result, viruses displayed a diminished capacity to attach to the mutant cyanobacteria, most likely due to altered cell surface properties. Interestingly, these mutations caused bacteria to grow at lower rates or present with a hypersensitivity to other viruses, thereby imposing a fitness cost to the host. Nonetheless, over time, many of the resistant strains improved their growth rate and narrowed the resistance range. Researchers observed that phages rarely presented with counter-mutations that allowed them to re-infect resistant hosts. Instead, they seemed to sustain long-term coexistence in hosts through passive host-switching. Taken together, the findings of the PIMCYV study indicated that phages in cyanobacteria serve as a selective pressure that enhances the diversity of cell-surface genes. In turn, this influences the effective host population size that can be infected by phages, leading to a dynamic network of interactions.
Phage, cyanobacteria, ecology, evolution, mutation, fitness