Communication between phages alters infection spread in bacterial populations

A growing amount of research evidences how co-infections can alter infection likelihood in a wide range of organisms. Most of this work has focused on how weakened host defences can facilitate new infections, hence contributing to the spread of directly-transmitted parasites in a population. This situation can lead to vicious circles and trigger disease outbreaks (reviewed in Beldomenico & Begon 2010). The work by Erez et al. (2017) presents new perspectives on how infections can disease spread in host populations.

This work focuses on phages infecting bacteria (Bacillus subtilis). These pathogens can enter lytic or lysogenic life-cycle when invading the host. While the lytic cycle allows high replication from the phage, its strong virulence also leads to the death of the host. Phage lysogeny implies the phage to integrate the host DNA, providing it resistance against further infection from the same pathogen, and replicates with the host cellular division without killing the host. Lytic life-cycle is thought to be advantageous when hosts are abundant in the environment while lysogeny is seen as an advantage when the probability of infecting new host cells is low (see Chibani-Chennoufi et al. 2004). However, the mechanisms inducing lytic over lysogenic transmission are still poorly known (Davidson 2017). 

source for picture: http://www.nature.com/nature/journal/vaop/ncurrent/images_article/nature21118-f1.jpg

The study by Erez et al. evidences that communication between phages can influence life-cycle determination in phages. By infecting B. Subtilis bacteria with four different phages, the authors found one of the phages, phi3T, to protect hosts from infections by the same pathogen by promoting phage lysogeny. The authors suggested that the phage protein AimP triggers the release of a peptid fragment they called Arbitrium from infected cells. When this compound is taken up by neighbouring cells, high concentrations would favour lysogeny while low concentrations would favour lytic infections by phages. This study is the first to evidence such a quorum sensing mechanism in phage and could change our perception of life-cycle determination.