Characterization of 1706, a virulent phage from Lactococcus lactis with similarities to prophages from other Firmicutes

The virulent lactococcal phage 1706, isolated in 1995 from a failed cheese production in France, represents a new lactococcal phage species of the Siphoviridae family. This phage has a burst size of 160 and a latent period of 85 min. Its linear double-stranded DNA genome was composed of 55,597 bp with a 33.7% G+C content. Its deduced proteome (76 ORFs) shared limited similarities to other known phage proteins. SDS-PAGE coupled with LC-MS/MS analyses led to the identification of 15 structural proteins. The most striking feature of the 1706 proteome was that 22 ORFs shared similarities with proteins deduced from the genome of either Ruminococcus torques and/or Clostridium leptum. Both are Firmicutes bacteria found in the gut flora of humans. We also identified a four-gene module in phage 1706, most likely involved in host recognition that shared similarities with lactococcal prophages. We propose that the virulent phage 1706 infected another bacterial genus before picking up a lactococcal host recognition module.