Vinderola et al. (2002) assessed the interactions among lactic acid bacteria (LAB) and probiotic bacteria and found that the cell-free supernatant (CFS) obtained from skim milk cultures fermented by S. thermophilus and L. bulgaricus had no positive effect on probiotic L. casei, indicating that the extracellular products secreted by these yogurt bacteria were not conducive to the growth of L. casei.