Bacteriocin presence enhances phage-related risks in dairy fermentations

Bacteriophages infecting dairy starter cultures are a serious threat in the dairy industry leading to failed or low-quality milk fermentations. The aim of this work was to assess if the presence of bacteriocins, either intentionally added or produced in situ during milk fermentation, increase the risk of fermentation failure in the event of phage contamination. Bacteriocin-phage interactions (BaPIs) were determined in vitro for nisin, a bacteriocin commonly used as a food biopreservative. Nisin increased the plaque size for the lactococcal phages P2 and bIL170. However, bIL170 burst size and latent period remained unaltered. Additionally, it was observed that plaque size enlargement caused by nisin and lactococcin 972 (Lcn972) was abolished when the phage sk1 was plated on a bacteriocin-resistant host. The impact in milk fermentations was monitored by following milk acidification and calculating the percentage of inhibition (PI_A) in single and combined treatments. When combining nisin at 0.125 μg/mL and bIL170 at 10 pfu/mL, positive BaPI was recorded, as the PI_A of the combination was greater than the sum of single treatments. Finally, to test the impact of bacteriocin production in situ, acidification of phage sk1-contaminated milk by co-cultures of strains displaying high or low resistance to Lcn972 along with a Lcn972 producer were carried out. Compared to co-cultures with non-Lcn972 producers, acidification was significantly affected only when a Lcn972-sensitive strain was present. Overall, the results reinforce the notion that lactococcal-driven milk fermentations may be more prone to phage infection when bacteriocins are present, whether intentionally added or produced in situ.