To survive in the cold: the evolution of reduced decay rate in a bacteriophage

Viruses typically have high decay rates (mortality rates outside hosts), and applications of phage viruses for combating harmful bacterial in clinical and agricultural contexts would favor slow-decaying phage materials. There is evidence for a trade-off between viral survival and growth rate, which may constrain the evolution of reduced decay rate. Temperature is likely to affect the optimal balance of this trade-off; e.g., faster growth may be more beneficial at warmer temperatures where phages spend less time outside of hosts in waiting for encountering a new host individual. We tested this hypothesis by experimentally evolving a lytic phage that infects the bacterium Pseudomonas fluorescens. Phages evolved at two temperatures for 20 cycles of dilution and propagation, with the ancestral bacteria being supplied every cycle. Phage populations from different temperatures showed different modes of adaptation in growth and decay traits. In particular, phages that evolved at the colder temperature showed a reduction in decay rate, regardless of assay conditions. Our results suggest phage training programs and resource collecting efforts to focus on cold environments for slow-decaying phage materials.