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Lactic acid bacteria play an important role in many food and feed fermentations. In recent years major advances have been made in unravelling the genetic and molecular basis of significant industrial traits of lactic acid bacteria. Bacteriophages which can infect and destroy lactic acid bacteria pose a particularly serious threat to dairy fermentations that can result in serious economic losses. Consequently, these organisms and the mechanisms by which they interact with their hosts have received much research attention. This paper reviews some of the key discoveries over the years that have led us to our current understanding of bacteriophages themselves and the means by which their disruptive influence may be minimized.

Recent examination of the relationships of the dry necrosis-inducing (necrogenic) erwinias using 16S rDNA sequences demonstrated that these bacteria comprise a polyphyletic group and, therefore, have been subdivided into three distinct genera, Erwinia, Brenneria and Pectobacterium, with the classical ' amylovora ' group species now being distributed nearly evenly among the first two. To further assess the molecular evolutionary relationships between current necrogenic Erwinia and Brenneria species, as well as between these genera and the exclusively soft-rotting genus Pectobacterium, the glyceraldehyde-3-phosphate dehydrogenase (gapDH) genes from 57 Erwinia and Brenneria isolates along with Pectobacterium type strains were PCRamplified, sequenced and subjected to phylogenetic analysis. Pairwise alignments of cloned gapDH genes revealed remarkably high interspecies genetic diversity among necrogenic isolates. Four evolutionary clades of necrogenic species were described that assorted more closely to known softrotting species than to each other. Interclade comparisons of gapDH nucleotide sequences revealed as much genetic divergence between these four necrogenic clades as existed between necrogenic and soft-rotting clades. An examination of the phylogenetic utility of the gapDH gene in light of current 16S rDNA clustering of these species revealed varying levels of taxonomic congruence between these genes for the structure of Erwinia, Brenneria and Pectobacterium. These analyses suggest that, while gapDH possesses sufficient genetic variation to fully differentiate Erwinia and Brenneria species, the gene may not accurately reflect interspecies taxonomic relatedness among all three phytopathogenic genera.

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Two genes from the total genomic DNA of dairy starter culture Lactococcus lactis subsp. crenoris UC503, encoding ScrFl modification enzymes, have been cloned and expressed in Escherichia coli. No homology between the two methylase genes was detected, and inverse polymerase chain reaction of flanking chromosomal DNA indicated that both were linked on the Lactococcus genome. Neither clone encoded the cognate endonuclease. The DNA sequence of one of the methylase genes (encoded by pCI931M) was determined and