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The sensitivity of nine strains of Listeria to nisin was determined as well as the minimum inhibitory concentration of nisin necessary to completely inhibit growth of these strains. All strains tested were variably sensitive to nisin and different MIC values were obtained, ranging from 740 to 10(5) IU/ml in trypticase soy agar and from 1.85 to 10(3) IU/ml in MRS agar. The inhibition of L. monocytogenes ATCC 7644 in TSB trypticase at different pH values and in sterilized and nonsterilized cottage cheese by nisin (37 X 10(2) IU/ml and 2.55 X 10(3) IU/g, respectively) also was investigated. This bacterium was completely inhibited after 24 h at pH 5.0 and above. At pH 4.5, 4.0, and 3.5, it was inhibited within 24 h. In cottage cheese no Listeria survivors were found at 24 h at 37 and 4 degrees C whether or not the cheese had been sterilized when as many as .35 X 10(6) cell/g were added at zero time.

The apparent instability of ,B-galactosidase in toluene-treated cells or cell-free extracts of lactic streptococci is explained by the fact that these organisms do not contain the expected enzyme. Instead, various strains of Streptococcus lactis, S. cremoris, and S. diacetilactis were shown to hydrolyze o-nitrophenyl-f3-D-galactoside-6-phosphate (ONPG-6-P), indicating the presence of a different enzyme. In addition, lactose metabolism in S. lactis C2F was found to involve enzyme I (El), enzyme II (EII), factor III (FIII), and a heat-stable protein (HPr) of a phosphoenolpyruvate (PEP)-dependent phosphotransferase system analogous to that of Staphylococcus aureus. Mutants of S. lactis C2F, defective in lactose metabolism, possessed the phenotype lac-gal-. These strains were unable to accumulate 4{Cthiomethyl-,3-D-galactoside, to hydrolyze ONPG, or to utilize lactose -when grown in lactose or galactose broth. In addition, these mutants contained EI and HPr, but lacked EII, FIII, and the ability to hydrolyze ONPG-6-P. This suggested that the defect was in the phosphorylation step. Lactose-negative mutants of S. lactis 7962, a strain containing j3-galactosidase, could be separated into several classes, which indicated that this organism is not dependent upon the PEP-phosphotransferase system for lactose metabolism.on August 4, 2020 by guest

No abstract

Bacteriocins in powders were produced from milk-based media and applied to food packaging films. Nisin and pediocin ''powders'' were retained in casings during dialysis. Antilisterial casings were prepared by internal coating with pediocin. Antilisterial activity applied in powdered form was retained during processing and retained on contact food packaging surfaces. Pediocin powder was applied to plastic packaging bags at 7.75 µg/cm 2 . Meats and poultry samples were inoculated with Listeria monocytogenes. The bags coated with pediocin powder completely inhibited growth of inoculated L. monocytogenes through 12 wk storage at 4ЊC. Applying bacteriocins to food packaging films is an effective approach to reduce L. monocytogenes contamination in meats and poultry.