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Twelve strains of bifidobacteria were identified which exhibited a broad spectrum of antagonistic activity against both Gram-positive and Gram-negative indicators, especially Pseudomonas species, using deferred antagonism spot plate assays. Inhibitory action was shown to be unrelated to hydrogen peroxide production and not solely dependent on acidity. However, attempts to detect inhibitory activity in cell-free supernatant fluids from these strains were unsuccessful. The production of inhibitory compound(s) by Bifidobacterium infantis NCFB 2255 was shown to be an unstable trait resulting in phenotypic alternation between production and non-production. Results from food trials using commercial cottage cheese which was inoculated with the inhibitor-producing strains of Bif. infantis NCFB 2255 and Bif. breve NCFB 2258 indicated that levels of Pseudomonas were reduced, but this observation was species-dependent. The viability of bifidobacteria themselves during storage in cottage cheese at refrigeration temperature was found to be a strain variable trait.