Antagonism between yoghurt bacteria and probiotic bacteria isolated from four commercial starter cultures, three commercial yoghurts and a probiotic capsule was studied. Streptococcus thermophilus (7 strains), Lactobacillus delbrueckii subsp. bulgaricus (5 strains), Lactobacillus acidophilus (8 strains) and bifidobacteria (8 strains) were isolated and the isolates used as both producer and indicator organisms for inhibitory activity using modified spot on lawn and agar well diffusion assays. Seven out of 8 strains of L. acidophilus produced antimicrobial substances active against all 5 strains of L. delbrueckii subsp. bulgaricus; the inhibitory substances were sensitive to proteolytic enzymes and were active at neutral pH. At pH 5.5 and 6.0, there was increased production of a bacteriocin-like inhibitory substance (BLIS). Inhibition of 5 strains of S. thermophilus and 2 strains of bifidobacteria was due to organic acids not to BLIS.

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