Bacteriophages (phages) are viruses of bacteria. The complex host-parasite interaction and the constitutive arms race between them and their hosts have been under investigation for many years. In this work, we found evidence for a possible communication process that occurs during phage infection. When Bacillus subtilis 168 is being infected with its temperate phage Phi3T, most cells are lysed. But if the bacteria are grown in a conditioned medium taken from a previous infection by the same phage, cell survival is significantly increased. This effect indicates that some kind of a molecule is released to the medium during the infection process, that protects naïve bacteria from further infection, or that alters the life cycle of the phage. This effect was found to be specific for a subset of temperate phages from the SP-Beta like genus, whereas infection by phages from other families did not produce such an effect. We further found that the active molecule responsible for this effect is a short peptide, consistent with peptides being signaling molecules in most quorum sensing systems found in gram positive bacteria. The exact effect of the short peptide on the infection course was not established yet, but we present two possibilities regarding the peptide’s mode of action: (i) this is a defensive system where bacteria sense the signal and respond by changing their properties to avoid being infected; (ii) the signal is generated by the phage in order to signal to phages in downstream cycles to alter their lifestyles and become lysogenic.
