Phage-Host Interaction: an Ecological Perspective

Nearly 100 years ago, Felix d'Herelle, the codiscoverer of bacteriophages, used bacteria to control insect pests and used phages against bacterial disease.His approaches reflected ecological insights before this branch of biology became an established scientific discipline.In fact, one might have predicted that phage research would become the springboard for biotechnology and ecology.However, d'Herelle was ahead of his time, and the zeitgeist in the 1930s pushed physicists into the question "What is life?"Phages as the simplest biological systems were the logical choice for this question, and phage research became the cradle of molecular biology.Now many researchers speak of a "new age of phage research."It is now realized that phages play an important role in ecology (e.g., phage impact on the cycling of organic matter in the biosphere at a global level) ( 27), that phages influence the evolution of bacterial genomes (most obviously in the development of bacterial pathogenicity) (7), and that phages might provide potential tools to face the antibiotic resistance crisis in medicine (59).With this new trend, we now see a clear shift from the reductionist approach, focusing on a handful of phages in carefully controlled laboratory conditions, towards the study of many different phages in the complexity of real-life situations.In contrast to the molecular biology-oriented phage research where the interaction of molecules took center stage, ecology focuses on the interactions between organisms and their physical environment.Much of ecology is therefore about the evolution of biological diversity in space and time.In contrast to many branches of biology, ecology attributes a great importance to quantitative relationships and numbers and aims at a mathematical formulation of its observations.It is thus appropriate to start this review with an overview of phage titers encountered in the biosphere.Next, we ask how a parasite targets its host if the latter is scarce or not in an appropriate physiological state.Finally, we report on research that tries to bridge phage ecology and genomics and cell biology approaches.It is concluded that the integration of phages into complex networks of interacting biological systems, and analysis by molecular techniques, could give phage research a model character in biology again.