Structural characterization of the extracellular peptide metabolites of Luteococcus japonicus subsp. casei and their protective effect on probiotic bacteria

Protective effect of Luteococcus japonicus subsp. casei exometabolites on the cells of transient probiotic Propionibacterium strains, Enterococcus faecium, and the yeasts Saccharomyces cerevisiae Boulardii under exposure to bile salts (BS) and acid stress was studied. The extracellular peptide reactivating factor (RF) and the peptide component of the culture liquid (CL) after RF removal possessed a protective effect. Protective (preventive) and reactivating (after stress impact) application of RF resulted in 1.5- to 2.0-fold increased survival of the human transient probiotics P. freudenreichii, P. acidipropionici and E. faecium subjected to BS treatment of acid stress. The CL peptide fraction had a stronger protective effect. Its application for preincubation of P. acidipropionici cells resulted in 14-fold (BS treatment) and 8-fold (acid stress) increased survival compared to the control. Yeasts exhibited very high resistance to the stress factors used. Two active glycopeptide fractions with molecular masses of 1.8 and 2.4 kDa were found in RF. Analysis of their amino acid composition revealed the residues of glycine, leucine, proline, arginine, and aspartate/asparagine, while no residues of aromatic and sulfur-containing amino acids, or the disulfide bridge-type posttranslational modifications, were found. The possible mechanisms of reactivating and the protective effect of RF are discussed.