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Organisms known to cause bovine mastitis, Enterococcus faecalis ssp. liquefaciens ATCC 27959, Staphylococcus aureus ATCC 29740, Streptococcus agalactiae ATCC 27956, Streptococcus equinus ATCC 27960, Streptococcus dysgalactiae ATCC 27957, Streptococcus uberis ATCC 27958, and the neotype Staphylococcus epidermidis ATCC 14990 were examined for their susceptibility to the small peptide antibiotic, nisin. Using a disc assay, minimum inhibitory concentrations of nisin ranged from 10 to 250 micrograms/ml among the strains. Examination of the antimicrobial effect of 50 micrograms/ml nisin in milk showed nisin inhibited all gram-positive pathogens tested.

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A small cryptic plasmid, pMBB1, isolated from Enterococcus faecium 226 was characterized. The plasmid contained an extremely stable replicon which has limited homology to the lactococcal plasmid pCI305. Sequence analysis of the replicon detected one open reading frame of 822 bp capable of encoding a 32-kDa protein. No detectable single-stranded intermediates were found for the replicon, suggesting that pMBB1 may be included in the same family as pCI305, although pCI305 exhibits a more narrow host range. A small stably maintained vector able to replicate in a variety of lactic acid bacteria, containing a large multiple cloning region, was constructed by using the pMBB1 replicon.