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).
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.
