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Lactacin B is a heat-stable bacteriocin produced by LactobaciUlus acidophilus N2 that is active against closely related lactobacilli, including Lactobacilus delbrueckii subsp. lactis (formerly Lactobacillus kichmannii) ATCC 4797. Pure producer cultures propagated in MRS broth (initial pH 6.5) contain no lactacin B; it is detected only in cultures maintained at pH 5.0 to 6.0 and produced optimally at pH 6.0 (S. F. Barefoot and T. R. Klaenhammer, Antimicrob. Agents Chemother. 26:328-334, 1984). Associative growth of producer and indicator, L. delbrueckii subsp. lactis ATCC 4797, resulted in production of an inhibitor identical to lactacin B.

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-Thirteen dairy propionibacteria were screened for catalase-insensitive, protease-sensitive inhibition of various Gram-positive and Gram-negative bacteria. Production of bacteriocin-like agents was identified in 8 ofthese cultures. The bacteriocin produced by Propionibacteriumjensenii B 1264 inhibited closely related propionibacteria and lactic acid bacteria and was bactericidal to Lactobacillus delbrueckii subsp. lactis ATCC 4797. It was produced in sodium lactate broth cultures during stationary phase; maximum activity was detected after 10 d of growth but only after IO-fold concentration of culture supematants. The bacteriocin was precipated by ammonium sulphate at 70 % saturation. The ammonium sulphate-concentrated, dialyzed bacteriocin was stable to 0.1-1.0 rnol-Lr' NaCI, 0.1-2.0 % SOS, 4 mol-L:' urea, 100 "C (60 min), pH 2-10 and organic sol vents (methanol, ethanol and isopropanol at 0-50 %). Batch anion exchange chromatography (pH 6.5) of the bacteriocin yielded 101 % activity and 15-fold increased purity. Flat bed analytical isoelectric focussing showed that the isoelectric point was between pH 3 and 3.5. SOS-PAGE yielded an estimated molecular mass between 6 and 9 kOa. The evidence suggests that this anionic bacteriocin is unique. © Inra/Eisevier, Paris.