Abstract
Lytic bacteriophages infect and lyse bacteria and, as a by-product, may affect diversity in microbial communities through selective predation on abundant bacterial strains. We used a complex dairy starter named Ur to investigate population dynamics of Lactococcus lactis , Lactococcus cremoris and Leuconostoc mesenteroides strains in terms of constant-diversity and periodic selection models. To mimic the starter Ur, we designed blends of 24 strains representing all eight previously identified genetic lineages in the starter culture. The blends were propagated by daily transfers in milk for over 500 generations in the presence or absence of a cocktail of lytic bacteriophages. The relative abundance of genetic lineages of L. lactis , L. cremoris and Lc. mesenteroides strains present in the complex blend, as well as phage presence, were monitored. Control blends without phage predation showed decreased strain diversity, leading to a stable state due to the domination of the fittest strain(s) of a particular lineage according to periodic selection dynamics. However, in phage-challenged blends, predation caused a large shift in the microbial composition by killing the fittest and sensitive strains. It was demonstrated that phage-challenged blends maintained their diversity at the level of genetic lineages, thus providing experimental support for the constant-diversity dynamics model in a complex microbial community.