Coevolution of bacteria and phage: are there endless cycles of bacterial defenses and phage counterdefenses?

The assertion that the coevolution of bacteria and bacteriophage leads to an endless arms race between resistant bacterial mutants and corresponding host-range phage mutants is questioned. In general, structural constraints on the highly site-specific phage adsorption process appear more severe than physiological constraints on resource assimilation by bacteria. Several alternative hypotheses are presented that could account for the persistence of phage, despite this fundamental asymmetry in the coevolutionary potential of bacteria and phage.