Interactions in biofilms of Lactococcus lactis ssp. cremoris and Pseudomonas fluorescens cultured in cold UHT milk

Three Lactococcus lactis ssp. cremoris isolates from refrigerated bulk raw milk were cultured separately and in association with a known psychrotrophic dairy Pseudomonas fluorescens strain, in skim UHT milk for 72 h at 7 degrees C, to determine mutual influences in both the planktonic and biofilm phases. Two levels of inoculum of each culture partner were combined. Protocooperation and commensalism cases were found, all of them in the biofilm phase. Type and intensity of the interactions depended on Lactococcus strain and on the cell density of each partner. Maximum enhancement of attachment was observed to be approximately 100-fold for P. fluorescens and 20,000-fold for one of the L. lactis strains. Confocal scanning laser microscopy images show compact masses of Pseudomonas trapping lactococci cells in cooperative biofilms.