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Kefir is a dairy product obtained by fermentation of milk with kefir grains. Kefir grains are clusters of micro-organisms that include primarily lactic acid bacteria (lactobacilli, lactococci, leuconostoc), yeasts and acetic acid bacteria held together in a matrix of polysaccharides and proteins (Angulo et al. 1993; Garrote et al. 2001). Several health-promoting properties are associated to kefir consumption; indeed, in vitro and animal trials showed that kefir and its constituents could have anticarcinogenic, antimutagenic, antiviral and antimicrobial properties (Garrote et al. 2000; Farnworth, 2005).
The interactions between different micro-organisms in kefir grains may contribute to the maintenance of grain structure and composition over time. In this process, ionic or Coulombic interactions, hydrogen bonding, hydrophobic effect or microbial surface macromolecules such as (glyco)proteins and polysaccharides could be involved.
Yeast surfaces have three major cell wall components, namely glucans, mannans, and chitin, all of which may play a role in coaggregation and coadhesion events (Chaffin et al. 1998; Millsap et al. 1998). Mannans form a capsule-like structure on the yeast cell surface and bacteria may associate with sugars in this capsule by means of a lectin-like activity (Millsap et al. 1998).
Our workgroup isolated several S-layer carrying Lactobacillus kefir (Lb. kefir) strains from kefir grains. These strains showed differences in surface and probiotic properties such as adhesion to Caco-2 cells, bile salts resistance and inhibitory power against intestinal pathogens in vitro assays (Garrote et al. 2001, 2004; Golowczyc et al. 2007).
To gain insight into the nature of the interactions between microorganisms present in kefir grains, we characterized surface components of Lb. kefir involved in the interaction with Saccharomyces lipolytica cells.
Materials and Methods
Strains and media
Yeast and lactobacilli used in this study were isolated from kefir grains (Garrote et al. 2001). Lactobacillus kefir strains (CIDCA 8310, 8314, 8315, 8317, 8319, 83110, 83111, 83113, 83115, 83116, 8321, 8325, 8326, 8332, 8335, 8343, 8344, 8345, 8347 and 8348) and Saccharomyces lipolytica CIDCA 812 were cultured in MRS broth (Biokar Diagnostics, Beauvais, France) at 30°C for 48 h.
Surface proteins extraction
Surface protein extraction from Lb. kefir with 5 m-LiCl was performed according to Golowczyc et al. (2007). Extraction with SDS was performed using the same experimental procedure with...