# Diverse and abundant phages exploit conjugative plasmids

## TL;DR

This study shows that plasmid-dependent phages are common in wastewater and play a role in controlling bacterial gene transfer.

## Contribution

A targeted discovery platform reveals the abundance and diversity of plasmid-dependent phages, especially non-tailed types.

## Key findings

- Plasmid-dependent phages are common and abundant in wastewater.
- Most isolated phages are non-tailed, including tectiviruses, ssDNA, and ssRNA phages.
- A tailed phage dependent on the IncF plasmid shows evolutionary connections to other phage groups.

## Abstract

Phages exert profound evolutionary pressure on bacteria by interacting with receptors on the cell surface to initiate infection. While the majority of phages use chromosomally encoded cell surface structures as receptors, plasmid-dependent phages exploit plasmid-encoded conjugation proteins, making their host range dependent on horizontal transfer of the plasmid. Despite their unique biology and biotechnological significance, only a small number of plasmid-dependent phages have been characterized. Here we systematically search for new plasmid-dependent phages targeting IncP and IncF plasmids using a targeted discovery platform, and find that they are common and abundant in wastewater, and largely unexplored in terms of their genetic diversity. Plasmid-dependent phages are enriched in non-canonical types of phages, and all but one of the 65 phages we isolated were non-tailed, and members of the lipid-containing tectiviruses, ssDNA filamentous phages or ssRNA phages. We show that plasmid-dependent tectiviruses exhibit profound differences in their host range which is associated with variation in the phage holin protein. Despite their relatively high abundance in wastewater, plasmid-dependent tectiviruses are missed by metaviromic analyses, underscoring the continued importance of culture-based phage discovery. Finally, we identify a tailed phage dependent on the IncF plasmid, and find related structural genes in phages that use the orthogonal type 4 pilus as a receptor, highlighting the evolutionarily promiscuous use of these distinct contractile structures by multiple groups of phages. Taken together, these results indicate plasmid-dependent phages play an under-appreciated evolutionary role in constraining horizontal gene transfer via conjugative plasmids.

Some phages use plasmid-encoded conjugation proteins as receptors to infect their bacterial hosts, making their host range dependent on horizontal transfer of the plasmid. Here, the authors present a method for identification of new plasmid-dependent phages, and find that they are common and abundant in wastewater and their genetic diversity is largely unexplored.