Matrix-trapped viruses can protect bacterial biofilms from invasion by colonizing cells

Bacteria often live in the context of spatially restricted groups held together by a self-secreted, adhesive extracellular matrix. These groups, termed biofilms, are likely where many phage-bacteria encounters occur. A number of recent studies have documented that phages can be trapped in the outer matrix layers of biofilms, such that the bacteria inside are protected from exposure. It is not known, however, what might happen after this: are the trapped phages still viable on the biofilm exterior? If so, do they pose a threat to newly arriving cells that might otherwise colonize the existing biofilm? Here we set out to address these questions using a biofilm-producing strain of Escherichia coli and its lytic phage T7. Prior work has demonstrated that T7 phages are trapped in the outermost layers of curli polymers within the E. coli matrix. We show that these phages do remain viable and kill incoming colonizing cells so long as they are T7-susceptible. If colonizing cells arrive to the outside of a resident biofilm before phages do, they can still be killed by phage exposure if it occurs soon thereafter. However, if colonizing cells are present on the biofilm long enough before phage exposure, they gain phage protection via envelopment within curli-producing clusters of the resident biofilm cells. This work establishes that phages trapped in the outer matrix layers of a resident biofilm can be incidentally weaponized as a mode of protection from competition by newly arriving cells that might otherwise colonize the biofilm exterior.