Bacteriophage adherence to mucus mediates preventive protection against pathogenic bacteria

Metazoan mucosal surfaces are major interfaces between the organism and environment. These surfaces have been proposed to host bacteriophages in a symbiotic relationship with metazoans. Considering the so far poorly understood phage-mucus interaction and its role in ecological interactions and for mucosal bacterial infections, empirical evidence and model systems need to be established. Here, using the fish pathogen Flavobacterium columnare and rainbow trout (Oncorhynchus mykiss), we show that phages infecting the pathogen are capable of binding to primary mucus layers and protecting fish from infections. Furthermore, exposure to mucus changes the bacterial phenotype by increasing bacterial virulence and susceptibility to phage infections. Tests using other phage-bacterium pairs suggest that the relevance of mucus for bacteria and phages may be widespread in the biosphere. Therefore, interactions of bacteria and phages inside the mucus environment may be important for disease and evolution, and this phenomenon has significant potential to be exploited for preventive phage therapy approaches.