Dynamic interactions between prophages induce lysis in Propionibacterium acnes

Progress in next-generation sequencing technologies has facilitated investigations into microbial dynamics. An important bacterium in the dairy industry is Propionibacterium freudenreichii, which is exploited to manufacture Swiss cheeses. A healthy culture of these bacteria ensures a consistent cheese with formed ‘eyes’ and pleasant flavour profile, and the investigation of prophages and their interactions with these bacteria could assist in the maintenance of the standard of this food product. Two bacteriophages, termed PFR1 and PFR2, were chemically induced using mitomycin C from two different dairy strains of P. freudenreichii. Both phages have identical genomes; however, PFR2 was found to contain an insertion sequence, IS204. Host range characterisation showed that PFR1 was able to form plaques on a wild type Propionibacterium acnes strain, whereas PFR2 could not. The lytic plaques observed on P. acnes were a result of PFR1 inducing the lytic cycle of a pseudolysogenic phage in P. acnes. Further investigation revealed that both PFR1 and PFR2 could infect P. acnes but not replicate. This study demonstrates the dynamic interactions between phages, which may alter their lytic capacity under certain conditions. To our knowledge, this is the first report of two phages interacting to kill their host.