Host-parasite coexistence: The role of spatial refuges in stabilizing bacteria-phage interactions

We manipulated a bacteria-phage model system to investigate empirical and theoretical conditions allowing for the coexistence of an Escherichia coli host with each of two virulent phage species, a T1-like phage (T1X) and lambda vir. In minimal medium in the laboratory, bacteria coexisted with each phage species in continuous (chemostat) culture; however, in serial culture, T1X rapidly became extinct in all cases and lambda vir became extinct in most cases. When we refined a previously developed mechanistic model of bacteria-phage interactions in continuous culture, this model failed to predict our laboratory observations of long-term stability of bacteria and phage in chemostats. A serial transfer version of this model, however, came much closer to predicting bacteria-phage dynamics. To investigate why models of continuous culture failed, we tested hypotheses for phage persistence by manipulating experimental culture conditions. We found that wall populations of bacteria strongly influenced the stability of both phage species. When wall populations were not allowed to develop in chemostats, both phage species became extinct rapidly; conversely, when wall populations were allowed to develop in serial transfer, both phage species showed evidence of long-term persistence. The final percentage of sensitive bacteria was significantly higher in walls than in liquid populations, suggesting that glass surfaces did indeed act as a spatial refuge for sensitive bacteria. In addition, serial transfer of bacteria and phage on agar surfaces produced more stable interactions than in liquid. We discuss these results in light of previous observations of bacteria-phage interactions and in the broader context of parasite-host and predator-prey coexistence conditions.