Abstract

The bacterial physiological state, type of interactions and changes in metabolism ofLactococcus lactis and Propionibacterium freudenreichii strains in co-culture were studied in skimmed milk in response to osmotic [3% (w/v) NaCl] and low temperature (10°C) stress during long-term incubation. Changes in the integrity of cell membrane were examined by LIVE/DEAD^® BacLight™ staining, and culture viability and bacterial interactions studies were performed with the use of the plate count technique. The profiles of volatile organic compounds were assessed by static headspace-gas chromatography. During the stationary growth phase, the number of LIVE cells with intact membranes was reduced in the presence of NaCl in comparison with control conditions, and a longer adaptation phase was observed at 10°C. The viability of starter cultures was high at ~10⁸to 10⁹ CFU/ml at the end of the experiment in all tested conditions. Our results point to the possibility of commensalisms interactions between lactic acid and propionic acid bacteria. Prolonged culture incubation contributed to the accumulation of acetoin, and it enhanced acetic acid and propionic acid synthesis during the stationary growth phase of propionic acid bacteria.

Key words: Lactococcus lactis, Propionibacterium freudenreichii, viability, volatiles compounds, osmotic stress, low temperature stress.

Abbreviation

 LAB, Lactic acid bacteria; PAB, propionic acid bacteria; VBNC, viable but not culturable; PI, propidium iodide; UHT, ultra high temperature; DEFT, direct epifluorescent filter technique; HS, headspace; GC, gas chromatography; FID, flame ionization detector; FISH, fluorescent in situ hybridization.