When is bacteriocin production beneficial for the bacterial phytopathogen Pseudomonas syringae?

Bacteriocins are toxins encoded by bacteria that antagonize closely related competitors. In this work, we investigated the competition between P. syringae pv. syringae (Psy) and P. syringae pv. phaseolicola (Pph) which are both able to cause disease on Phaseolus vulgaris (common bean). Importantly, Psy encodes a bacteriocin that inhibits Pph. A series of co-infiltration treatments were performed in planta with either Psy or a bacteriocin-deficient mutant (Psy Dbac), and Pph. The populations of each strain were measured at day 0, 4, 6, and 12 for all treatments. There was an overall negative effect, up to 100-fold, on the Pph population when co-infiltrated 1:1 with either Psy or Psy Dbac. The effect of bacteriocin production diminished over time between Psy strains. We were interested in the effect of different starting populations on bacteriocin-mediated competition to mimic natural variability of infecting strain observed in nature. While in the minority, Pph exhibited a 100-fold population reduction by Psy and Psy Dbac (compared to single infiltration). Conversely, with a Psy minority there was no benefit from bacteriocin production until day 12. Under the conditions tested, bacteriocin production did promote Psy fitness. These results suggest that bacteriocins are important for long-term persistence within competitive environments.