RESUMEN
One hundred sixty strains of Lactococcus lactis were screened for bacteriocin production by well diffusion assay of GM17 agar. Fourteen (8.4 per cent) produced antimicrobial activity other than organic acids, bacteriophages or hydrogen peroxide. The frequency of bacteriocin production ranged from 2(per cent) in L.lactis subsp. cremoris up to 12(per cent) in L.lactis subsp.lactis. Antimicrobial activities were not observed in any strain of L.lactis subsp.lactis var.diacetylactis. Among thirteen bacteriocin-producing strains and two nisin-producing strains (L.lactis subsp.lactis ATCC 11454 and L.lactis subsp.lactis CNRZ 150), eight (53 per cent) were characterized as lactose-positive (Lac+) and proteinase-negative (Prt-). The bacteriocin-producing cultures were also characterized on the basis of plasmid content. All strains had 2 to 7 plasmids with molecular weights varying from 0.5 to 28.1 Mdal. Four strains (ITAL 435, ITAL 436, ITAL 437 and ITAL 438) showed identical profiles and the other were quite distinct.