Mucosal surfaces are a main entry point for pathogens and the principal sites of defense against infection. Both bacteria and phage are associated with this mucus. We show that phage-to-bacteria ratios were increased, relative to the adjacent environment, on all mucosal surfaces sampled ranging from cnidarians to humans. In vitro studies of tissue culture cells with and without surface mucus demonstrated that this mucus-dependent increase in phage abundance protects the underlying epithelium from bacterial infection. Enrichment of phage in mucus occurs via interactions between mucin glycoproteins and immunoglobulin-like protein domains exposed on phage capsids. This interaction was found to be a result of phage immunoglobulin-like domains binding variable glycan residues that coat the mucin glycoprotein component of mucus. Metagenomic analysis found these immunoglobulin-like proteins present in many environments, particularly those adjacent to mucosal surfaces. Based on these observations, we present the Bacteriophage Adherence to Mucus (BAM) model, providing a ubiquitous but non-host-derived immune system applicable to mucosal surfaces. This model suggests that metazoan mucosal surfaces and phage have coevolved so that phage stick to them, thus benefiting the metazoan host through increased killing of mucosal bacteria, and the phage through increased interactions with bacterial hosts.

The adaptability of phage-derived immunoglobulin-like domains provides a protein scaffold that enables phage to continually adapt to the host's ever-changing patterns of mucin glycosylation, thus maintaining phage in the mucus surface. Incorporating protein crystallization, multiple-particle tracking, sequencing and functional glycan-binding profiles, we present novel insights into the strength and efficacy of these phage immunoglobulin-glycan binding interactions. Through In silico modeling we postulate that BAM provides a 14-fold increased chance of a bacterium encountering a phage within a mucosal surface compared to the surrounding environment. So far, the relationships presented here open a new arena for immunological study, introduce the first phage-metazoan symbiosis, and recognize the key role of the world's most abundant entities in the metazoan immune system.