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Lactococcus lactis subsp. lactis strains isolated from various sprouted seed products were able to transfer the ability to ferment raffinose in conjugation experiments at frequencies between 10−4 and 10−7 per donor cell. There was no evidence of plasmid transfer, but pulsed-field gel electrophoresis analysis showed that all transconjugants had acquired large chromosomal insertions indicative of conjugative transposons. Raffinose transconjugants contained inserts of 45 or 60 kb at one of two chromosomal sites, and these inserts contained two copies of an element related to the lactococcal insertion sequence ISS1.

Four bacteriophages (phages) growing on the same Leuconostoc strain were characterised. Electron micrographs showed these phages to be similar in morphology to the commonly isolated lactococcal phages with head diameters ranging from 49–55 nm and tail lengths of 117–131 nm. A distinctive base plate and collar were also present. From restriction enzyme analysis of purified phage DNA, the genome sizes were 23–29 kb. All four phages showed one major structural protein (of approximately 24 kDa) on SDS polyacrylamide gels. Hybridization experiments confirmed that the phages belonged to the same homology group. There was no homology between DNA from these phages and DNA from a prolate or small isometric lactococcal phage.

This paper reports on the application of the ligase chain reaction (LCR) to the specific detection of variants of the nisin structural gene (nisinA and nisinZ) in nisin producing strains of Lactococcus lactis ssp lactis. The LCR assay was used to screen nisin producing strains to determine which form of the nisin structural gene they contained. This method of differentiating the nisin structural gene variants provides a useful alternative to the only other available genetic differentiation, that of sequencing the gene.

Lactococcus lactis strains isolated from vegetable products transferred the ability to ferment sucrose in conjugation experiments with the recipient strain L. lactis MG1614. Nisin production and sucrose fermentation were transferred together from two strains, but transfer also occurred from several other strains which did not produce nisin. Pulsed-field gel electrophoresis analysis showed that all transconjugants had acquired large chromosomal insertions at two main sites. Nisin–sucrose transconjugants had gained inserts of 70 kb, while those that fermented sucrose without nisin production contained inserts of between 50 and 110 kb. Transconjugants from one donor had acquired a separate insertion of 55 kb which correlated with enhanced bacteriophage resistance, but contained neither nisin nor sucrose fermentation genes.

Lactic acid bacteria isolated from minimally processed fresh fruit and vegetable products were identified as Lactococcus lactis subsp. lactis on the basis of phenotypic tests, presence of lactococcal IS elements, and partial sequence analysis of the 16S rRNA gene. Isolated bacteria were differentiated using pulsed-field gel electrophoresis of SmaI digests of genomic DNA. Sprouted seeds were the best source of strains, and lactococci appear to be the dominant microflora on these products during the period they are intended to be eaten. Although these plant strains showed many similarities to strains of L. lactis used as dairy starter cultures, their carbohydrate fermentation patterns were unusual and probably reflect their environmental origin. Most strains fermented sucrose and xylose, and some also fermented raffinose and melibiose. Most of the bacteriocin-producing strains produced nisin, and nisin genes could also be detected in strains that showed no bacteriocin activity, or that produced a different bacteriocin with a narrow spectrum of activity. One strain produced nisin but was unable to ferment sucrose, properties that have been generally regarded as linked. These strains may have uses as biopreservatives for minimally processed plant products.

The 16S ribosomal RNA gene sequences of Lactococcus lactis ssp. lactis and ssp. cremoris differ by 9–10 bp (depending on strain), within the first 200 bp of the sequence. These differences were used to develop two methods of genetically differentiating lactis and cremoris strains. Primers to conserved sequences in the 16S rRNA gene were used in a PCR reaction to amplify fragments of the 16S rRNA gene. A single base difference at position 180 of the sequence was utilised to develop a ligase chain reaction to differentiate lactis and cremoris sequences. The second method involved digestion of the amplified fragments with restriction endonucleases specific for either the lactis or cremoris sequence. Resolution of the digested fragments on an agarose gel allowed the strains to be identified as genetically lactis or cremoris. This method was used to examine lactococci isolated from raw milk. Of 31 raw milk strains examined, 21 contained the cremoris 16S rRNA sequence, however, all 31 strains exhibited the phenotypic characteristics of the lactis subspecies.

Lactococcal strains producing inhibitory material active against other lactic acid bacteria, including Lactobacillus delbrueckii subsp. bulgaricus, were first observed by Rogers in 1928. Further interest in this area was stimulated by the research of Whitehead and Riddet (1933), who isolated from cheese milk a Gram-positive Lactococcus strain. This culture produced an inhibitor which was active against other lactic cultures and was not completely destroyed in milk by heating at 100°C for 30 min. In the same year Whitehead (1933) reported on a Lactococcus lactis subsp. cremoris strain which, when grown in milk, prevented the growth of other lactic cultures subsequently added to this milk. This inhibitory agent was proteinaceous, sensitive to the action of trypsin (but not pepsin) and restricted in activity to lactic cultures. The production of a ‘toxin’ by lactococci was implicated by Meanwell (1943) in problems associated with cheese-making. This ‘toxin’ inhibited the growth of the cheese-making bacteria and was thermostable, being destroyed only by autoclaving at 103.3 kPa pressure and not by boiling.